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Object Oriented Programming using Java

1. Figure 6 9 Umbrello UML Modeller showing a Collaboration Diagram 138 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN State Diagram State Diagrams show the different states of an Object during its life and the stimuli that cause the Object to change its state State Diagrams view Objects as state machines or finite automata that can be in one of a set of finite states and that can change its state via one of a finite set of stimuli For example an Object of type NetServer can be in one of following states during its life e Ready e Listening e Working e Stopped and the events that can cause the Object to change states are e Object is created e Object receives message listen A Client requests a connection over the network A Client terminates a request The request is executed and terminated Object receives message stop etc Activity Diagram Activity Diagrams describe the sequence of activities in a system with the help of Activities Activity Diagrams are a special form of State Diagrams that only or mostly contains Activities Activity Diagrams are always associated to a Class an Operation or a Use Case Activity Diagrams support sequential as well as parallel Activities Parallel execution is represented via Fork Wait icons and for the Activities running in parallel it is not important the order in which they are carried out they can be executed at the same time or one after the other Com
2. end actionPerformed The complete source code for the applet can be found in the file SimpleCalculator java It contains very little in addition to the two methods shown above 8 4 Menus and Menubars ANY USER OF A GRAPHICAL USER INTERFACE is accustomed to selecting commands from menus which can be found in a menu bar at the top of a window or sometimes at the top of a screen In Java menu bars menus and the items in menus are JComponents just like all the other Swing components Java makes it easy to add a menu bar to a JApplet or to a JFrame Here is a sample applet that uses menus Applet Reference see Applet ShapeDrawWithMenus Add Color Options Rectangle Ctrl R Oval Ctrl 0 Round Rect Ctrl D Figure 8 3 ShapeDrawWithMenus 168 CHAPTER 8 ADVANCED GUI PROGRAMMING This is a much improved version of the ShapeDraw applet You can add shapes to the large white drawing area and drag them around To add a shape select one of the commands in the Add menu The other menus allow you to control the properties of the shapes and set the background color of the drawing area This applet illustrates many ideas related to menus There is a menu bar A menu bar serves as a container for menus In this case there are three menus in the menu bar The menus have titles Add Color and Options When you click on one of these titles the menu items in the menu appear Each menu has an associated
3. Any class that implements the Drawable interface defines a draw instance method Any object created from such a class includes a draw method We say that an object implements an interface if it belongs to a class that implements the interface For exam ple any object of type Line implements the Drawable interface Note that it is not enough for the object to include a draw method The class that it belongs to has to say that it implements Drawable While a class can extend only one other class it can implement any number of inter faces In fact a class can both extend another class and implement one or more interfaces So we can have things like class FilledCircle extends Circle implements Drawable Fillable The point of all this is that although interfaces are not classes they are something very similar An interface is very much like an abstract class that is a class that can never be used for constructing objects but can be used as a basis for making subclasses The subroutines in an interface are abstract methods which must be implemented in any concrete class that implements the interface And as with abstract classes even though you can t construct an object from an interface you can declare a variable whose type is given by the interface For example if Drawable is an interface and if Line and FilledCircle are classes that implement Drawable then you could say 4 1 INTERFACES 75 Drawable figure
4. heads System out println There were tails 2 9 Programming Exercises 1 In all versions of the PairOfDice class the instance variables die1 and die2 are declared to be public They really should be private so that they are protected from being changed from outside the class Write another version of the PairOfDice class in which the instance variables die1 and die2 are private Your class will need methods that can be used to find out the values of die1 and die2 The idea is to protect their values from being changed from outside the class but still to allow the values to be read Include other improvements in the class if you can think of any Test your class with a short program that counts how many times a pair of dice is rolled before the total of the two dice is equal to two 50 CHAPTER 2 OBJECT BASED PROGRAMMING 2 Acommon programming task is computing statistics of a set of numbers A statistic is a number that summarizes some property of a set of data Common statistics include the mean also known as the average and the standard deviation which tells how spread out the data are from the mean I have written a little class called StatCalc that can be used to compute these statistics as well as the sum of the items in the dataset and the number of items in the dataset You can read the source code for this class in the file StatCalc java If calc is a variable of type StatCalc then the following
5. o The result is formatted as an octal integer x X The result is formatted as a hexadecimal integer The following conversions apply to floating point types float double and BigDeci mal e EF The result is formatted as a decimal number in computerized scientific notation f The result is formatted as a decimal number In the example above f means that we want to format the first argument as a decimal number e The optional flags is a set of characters that modify the output format The set of valid flags depends on the conversion For example the flag means that the result will be left justified and the means that the result will always include a sign e The optional width is a non negative decimal integer indicating the minimum num ber of characters to be written to the output The width refers to how many charac ters will be used to write the argument If the width is larger than the length of the number then blanks will be used to fill up the space e The optional precision is a non negative decimal integer usually used to restrict the number of characters The specific behavior depends on the conversion For the floating point conversions e E and f the precision is the number of digits after the decimal separator Consider the following example where we read in a maximum mark and a student s mark and we then calculate and print the percentage
6. Figure 5 1 HelloWorldApplet2 and here is the source code for this applet including the init method import java awt import java applet public class HelloWorldApplet2 extends Applet public void init Initialize the applet by setting it to use blue and yellow as background and foreground colors setBackground Color blue setForeground Color yellow public void paint Graphics g g drawString Hello World 10 30 end of class HelloWorldApplet2 5 1 2 Using JApplet However JApplets have a lot of extra structure that plain Applets don t have Because of this structure the painting of a JApplet is a more complex affair and is handled by the system So when you make a subclass of JApplet you should not write a paint method for it As we will see if you want to draw on a JApplet you should add a component to the applet to be used for that purpose On the other hand you can and generally should write an init method for a subclass of JApplet Let s take a look at a simple JApplet that uses Swing This applet demonstrates some of the basic ideas of GUI programming Although you won t understand everything in it at this time it will give you a preliminary idea of how things work GUI programs use components such as buttons to allow interaction with the user Our sample applet contains a button In fact the button is the only thing in the applet and it fills the entire rat
7. new GridLayout 1 0 5 5 buttonBar add button1 buttonBar add button2 buttonBar add button3 8 1 3 An Example To finish this survey of layout managers here is an applet that demonstrates layout man agers of various types Applet Reference see Applet LayoutDemo BorderLayout x Center Button East Button West Button Panel BorderLayout was selected The applet itself uses a BorderLayout with vertical gaps of 3 pixels These gaps show up in blue The Center component of the applet is a JPanel which uses a CardLayout as its layout manager The layout contains eight cards Each card is itself another panel 8 2 BASIC COMPONENTS AND THEIR EVENTS 155 that contains several buttons Each card uses a different type of layout manager several of which are extremely stupid choices for laying out buttons The North component of the applet is a JComboBox which contains the names of the eight panels in the card layout The user can switch among the cards by selecting items from this menu The South component of the applet is a JLabel that displays an appropri ate message whenever the user clicks on a button or chooses an item from the JComboBox The source code for this applet is in the file LayoutDemo java It consists mainly of a long init method that creates all the buttons panels and other components and lays out the applet 8 2 Basic Components and Their Events
8. 2 2 2 0 2 ee ee ee 11 1 2 Formatted Output nb Se laa a a bs ha GIS Se Hdd Seas 11 2 Streams Readers and Writers 2 2 ee ee PAS BilGS 8 n aa a ue e at vat Ao eee al tte ina eh delat culate Ti a AY eh BA 11 3 1 File Names Directories and the File Class 11 3 2 File Dialog Boxes 2 ea a R eee 11 4 Programming With Files a i ak Bee ee aes cn amp ey 8 Se eee BS TES OW QUESOS x 6 aa aces Gane Rew de SOS ne eR EG AOS 11 6 Programming Exercises 2 ee ee 169 169 173 175 175 179 179 185 185 186 186 187 188 190 190 194 196 197 197 200 201 202 203 207 207 212 215 220 221 221 223 223 225 225 CONTENTS Preface These notes are intended for a Second course in Object Oriented Programming with Java It is assumed that students have taken a first year course in Programming and are famil iar with basic procedural programming and introductory object based programming in Java The student should be familiar with the various constuctur Arrays one and two dimensional the concepts of class and object input output and the concepts classes and objects Theses notes are in the most part taken from David J Eck s online book INTRO DUCTION TO PROGRAMMING USING JAVA VERSION 4 1 JUNE 2004 The online book is available at http math hws edu javanotes We ve refactored and used sections 5 and 6 of chapter 4 chapters 5 7 and 9 sections 1 3 of chapter 10 and s
9. ai 2 56 ga Gea eh a 207 SOUMMALY lt 5 ene ASe a RR Aa ada ae re e eee a e b eke hoe Ae 28 QUIZ QUCSEIONS y esea ner e tek lh oe a Bo ee Se EE el ee Ok ek 2 9 Programming Exercises 2 2 a E a aa ee ee 3 Object Oriented Programming I 3 1 Inheritance Polymorphism and Abstract Classes 3 1 1 Extending Existing Classes 4 es 2a ee RW OOS AS a aA 3 2 this AVS ICE gh aooe hoor tig de Bch era We odor Oy Re ee Ea yoke Berke a Yr Be Hosa k 3 2 1 The Special Variables this and super 3 2 2 Constructors in Subclasses eee ee he eee Ce ee eH 3 3 Summary eei SAF Wee EEE eek Bene ep deed a Seeger Be ll ae 3 3 1 Rules for Constructors avavungt Sak ew Gat amp Ate kl pap te eee 3 3 2 Overloaded and Overridden Methods 3 3 3 Inheritance Summary 4 16 4 16 aie bok o ahd eS eek Ra whe Soe Aaa IIA y Castine 2 na ete settee ieee Ee Gp eke tee Be a eS 3 3 5 Polymorphism ye eg A ey Ge ee Ha aes oie pee ANE wl 4 Object Oriented Programming II Meals SCOT ACES hoe a Niece Godt fe ae amp Sasa GE Be ade Aa a A vee gt BOE ween 4 1 1 Interface Summary 4 Je ep we be ae ae RN ROIS DE al 4 1 2 Interface Example Comparable and Comparator 5 11 11 11 12 18 21 23 23 29 35 36 36 37 38 43 44 46 48 49 53 53 62 64 64 67 68 68 69 70 70 71 4 2 Nested Classes 0 0000 ee eee eee 5 Applets HTML and GUI s
10. for int i wordCount i gt pos i words i words i 1 words pos w wordCount end insertWord This subroutine is called by the main routine of the program to process each word that it reads from the file If we ignore the possibility of errors an algorithm for the program is 248 CHAPTER 11 BASIC INPUT OUTPUT Get the file names from the user Create a Scanner for reading from the input file Create a PrintWriter for writing to the output file while there are more words in the input file Read a word from the input file Insert the word into the words array For i from O to wordCount 1 Write words i to the output file Most of these steps can generate IOExceptions and so they must be done inside try catch statements In this case we ll just print an error message and terminate the program when an error occurs public static void main String args Scanner in A stream for reading from the input file Scanner keyboard new Scanner System in PrintWriter out A stream for writing to the output file String inputFileName Input file name specified by the user String outputFileName Output file name specified by the user words new String 10 Start with space for 10 words wordCount 0 Currently there are no words in array Get the input file name from the user and try to create the input stream If there is a FileNotFoundException print a message and
11. 234 CHAPTER 11 BASIC INPUT OUTPUT public static void main String args Scanner keyboard new Scanner System in System out print Enter the max mark gt double max keyboard nextDouble System out print Enter your mark gt double mark keyboard nextDouble double percentage mark max 100 System out printf You scored 41 1f out of 2 5 1f which is 43 3 2f mark max percentage Notice that the printf statement consists of the format string and three arguments The format string specifes how the arguments are to be formatted For example 2 3 1f specifies that the 2 argument max must be formatted as a decimal number f using a width of 3 characters with one decimal place This outputs You scored 13 0 out of 30 0 which is 43 33 The is needed to print the Notice also the extra spaces in front of the 30 0 A final example Calculating and outputting compound interest on an initial input amount Calculates compound interest by determining final amount after investing an amount for a number years at R interest import java util public class CompInterest2 public static void main String args double amount the initial amount invested years the number of years invested rate the annual interest rate as a finalAmt the final amount the Scanner object to do input from the keyboard Scanner keyboard new Scanner System
12. Note that the dealer wins on a tie so if both players have Blackjack then the dealer wins Now if the game has not ended the user gets a chance to add some cards to her hand In this phase the user sees her own cards and sees one of the dealer s two cards In a casino the dealer deals himself one card face up and one card face down All the user s cards are dealt face up The user makes a decision whether to Hit which means to add another card to her hand or to Stand which means to stop taking cards If the user Hits there is a possibility that the user will go over 21 In that case the game is over and the user loses If not then the process continues The user gets to decide again whether to Hit or Stand If the user Stands the game will end but first the dealer gets a chance to draw cards The dealer only follows rules without any choice The rule is that as long as the value of the dealer s hand is less than or equal to 16 the dealer Hits that is takes another card The user should see all the dealer s cards at this point Now the winner can be determined If the dealer has gone over 21 the user wins Otherwise if the dealer s total is greater than or equal to the user s total then the dealer wins Otherwise the user wins Two notes on programming At any point in the subroutine as soon as you know who the winner is you can say return true or return false to end the subrout
13. Return the index of the first item in list for which the predicate is true if any If there is no such item return 1 In C methods similar to these are included as a standard part of the generic programming framework 5 One of the examples in Section 12 4 concerns the problem of making an index for a book A related problem is making a concordance for a document A concordance lists every word that occurs in the document and for each word it gives the line number of every line in the document where the word occurs All the subroutines for creating an index that were presented in Section12 4 can also be used to create a concordance The only real difference is that the integers in a concordance are line numbers rather than page numbers Write a program that can create a concordance The document should be read from an input file and the concordance data should be written to an output file The names of the input file and output file should be specified as command line argu ments when the program is run You can use the indexing subroutines from Sec tion12 4 modified to write the data to a file instead of to System out You will also need to make the subroutines static If you need some help with using files and command line arguments you can find an example in the sample program Word Count java which was also discussed in Section12 4 As you read the file you want to take each word that you encounter and add it to the c
14. method does not add the menu bar to the applet s content pane The menu bar appears in a separate area above the content pane A menu bar is a container for menus The type of menu that can appear in a menu bar is an object belonging to the class JMenu The constructor for a JMenu specifies a title for the menu which appears in the menu bar A menu is added to a menu bar using the menu bar s add method For example the following commands will create a menu with title Options and add it to the JMenuBar menubar JMenu optionsMenu new JMenu Options menubar add optionsMenu 8 5 Timers Animation and Threads JAVA IS A MULTI THREADED LANGUAGE which means that several different things can be going on in parallel A thread is the basic unit of program execution A thread executes a sequence of instructions one after the other When the system executes a stand alone program it creates a thread Threads are usually called processes in this context but the differences are not important here The commands of the program are executed sequentially from beginning to end by this thread The thread dies when the program ends In a typical computer system many threads can exist at the same time At a given time only one thread can actually be running since the computer s Central Processing Unit CPU can only do one thing at a time An exception to this is a multi processing computer which has several CPUs At a gi
15. return false else FullName other FullName obj return firstName equals other firstName amp amp lastName equals other lastName public void compareTo Object obj Fullname other FullName obj if lastName compareTo other lastName lt O If lastName comes before the last name of the other object then this FullName comes before the other FullName Return a negative value return 1 if lastName compareTo other lastName gt O If lastName comes after the last name of the other object then this FullName comes after the other FullName Return a positive value return 1 else Last names are the same so base the comparison on the first names return firstName compareTo other firstName other methods and constructors 196 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES There is another way to allow for comparison of objects in Java and that is to provide a separate object that is capable of making the comparison The object must implement the interface java util Comparator which defines the method public int compare Object obji Object obj2 This method compares two objects and returns a value that is negative or zero or positive depending on whether obj1 comes before obj2 or is the same as obj2 or comes after obj2 Comparators are useful for comparing objects that do not implement the Comparable interface and for defining several different or
16. than another we assume that the items in the list implement the Comparable interface and define the compareTo method This interface was discussed in the previous section Here is a method that will do this static void orderedInsert List list Comparable newlItem Precondition The items in list are sorted into ascending order according to the compareTo method newitem compareTo item must be defined for each item in the list Postcondition newItem has been added to the list in its correct position so that the list is still sorted into ascending order ListIterator iter list listIterator 200 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES Move the iterator so that it points to the position where newltem should be inserted into the list If newItem is bigger than all the items in the list then the while loop will end when iter hasNext becomes false that is when the iterator has reached the end of the list while iter hasNext Object item iter next if newItem compareTo item lt 0 newItem should come BEFORE item in the list Move the iterator back one space so that it points to the correct insertion point and end the loop iter previous break iter add newlItem end orderedIinsert Since the parameter in this method is of type List it can be applied to both ArrayLists and LinkedLists and it will be about equally
17. Declare a variable of type Drawable It can refer to any object that implements the Drawable interface figure new Line figure now refers to an object of class Line figure draw g calls draw method from class Line figure new FilledCircle Now figure refers to an object of class FilledCircle figure draw g calls draw method from class FilledCircle A variable of type Drawable can refer to any object of any class that implements the Drawable interface A statement like figure draw g above is legal because figure is of type Drawable and any Drawable object has a draw method Note that a type is something that can be used to declare variables A type can also be used to specify the type of a parameter in a subroutine or the return type of a function In Java a type can be either a class an interface or one of the eight built in primitive types These are the only possibilities Of these however only classes can be used to construct new objects You are not likely to need to write your own interfaces until you get to the point of writing fairly complex programs However there are a few interfaces that are used in important ways in Java s standard packages You ll learn about some of these standard interfaces in the next few chapters 4 1 1 Interface Summary An interface specifies the methods which a class should contain A class can then im plement an interface actually it can implement mo
18. Try to give a complete answer The computer does several things Question 5 What is meant by the terms instance variable and instance method Question 6 Explain what is meant by the terms subclass and superclass Question 7 Explain the term polymorphism Question 8 Java uses garbage collection for memory management Explain what is meant here by garbage collection What is the alternative to garbage collection Question 9 For this problem you should write a very simple but complete class The class represents a counter that counts 0 1 2 3 4 The name of the class should be Counter It has one private instance variable representing the value of the counter It has two instance methods increment adds one to the counter value and getValue returns the current counter value Write a complete definition for the class Counter Question 10 This problem uses the Counter class from Question 9 The following program segment is meant to simulate tossing a coin 100 times It should use two Counter objects headCount and tailCount to count the number of heads and the number of tails Fill in the blanks so that it will do so Counter headCount tailCount tailCount new Counter headCount new Counter for int flip 0 flip lt 100 flip if Math random lt 0 5 There s a 50 50 chance that this is true ea Ee ee ee Pee ee Count a head ee Count a tail System out println There were
19. and 2 the study of approaches as in 1 IEEE Standard 610 12 6 1 Object oriented Analysis and Design A large programming project goes through a number of stages starting with specification of the problem to be solved followed by analysis of the problem and design of a program to solve it Then comes coding in which the program s design is expressed in some actual programming language This is followed by testing and debugging of the program After that comes a long period of maintenance which means fixing any new problems that are found in the program and modifying it to adapt it to changing requirements Together these stages form what is called the software life cycle In the real world the ideal of consecutive stages is seldom if ever achieved During the analysis stage it might turn out that the specifications are incomplete or inconsistent A problem found during testing requires at least a brief return to the coding stage If the problem is serious enough it might even require a new design Maintenance usually involves redoing some of the work from previous stages 127 128 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN Large complex programming projects are only likely to succeed if a careful systematic approach is adopted during all stages of the software life cycle The systematic approach to programming using accepted principles of good design is called software engineering The software engineer tries to e
20. new JFrame frame setDefaultCloseOperation JFrame EXIT_ON_CLOSE frame setVisible true frame setSize 800 600 frame setTitle Solitaire Solitaire s new Solitaire frame add s frame validate s repaint public void mouseClicked MouseEvent e int x e getX int y e getY for int i 0 i lt 12 i if allPiles i includes x y allPiles i select repaint public void mousePressed MouseEvent e public void mouseReleased MouseEvent e public void mouseEntered MouseEvent e public void mouseExited MouseEvent e Completing the Implementation Write the classes TablePile SuitPile DiscardPile DeckPile I suggest that you create all the classes first and then work with them one at a time They all extend the CardPile class You must take care to consider situations when the pile is empty The following will guide you in writing these classes the DeckPile Class This class extends the CardPile class It must create a full deck of cards stored in its super class s pile attribute The cards should be shuffled after creation use Collections shuffle You never add cards to the DeckPile so its canTake method always returns false The select method removes a card from the deckPile and adds it to the discardPile In the Solitaire class 149 The DiscardPile Class This maintains a pile of cards that do not go into any of the other piles Override the addCar
21. public void print boolean b public void printin Output a carriage return to the stream public void println String s These methods are identical public void println char c to the previous set public void println int i except that the output public void println long 1 value is followed by public void println float f a carriage return public void println double d public void println boolean b Note that none of these methods will ever throw an IOException Instead the PrintWriter class includes the method public boolean checkError which will return true if any error has been encountered while writing to the stream The PrintWriter class catches any IOExceptions internally and sets the value of an in ternal error flag if one occurs The checkError method can be used to check the error 238 CHAPTER 11 BASIC INPUT OUTPUT flag This allows you to use PrintWriter methods without worrying about catching excep tions On the other hand to write a fully robust program you should call checkError to test for possible errors every time you use a PrintWriter method When you use PrintWriter methods to output data to a stream the data is converted into the sequence of characters that represents the data in human readable form Suppose you want to output the data in byte oriented machine formatted form The java io package includes a byte stream class DataOutputStream that can be used for writing data val
22. 0 0 null 8 7 PROGRAMMING EXERCISES 183 When the user clicks Undo just swap the values of OSI and undoBuffer and re paint The previous image will appear on the screen Clicking on Undo again will undo the undo As another improvement you could make it possible for the user to select a drawing color using a JColorChooser dialog box Here is a button that opens my program in its own window You don t have to write an applet to launch your frame Just create the frame in the program s main routine 184 CHAPTER 8 ADVANCED GUI PROGRAMMING Chapter 9 Generic Programming and Collection Classes HOW TO AVOID REINVENTING the wheel Many data structures and algorithms have been studied programmed and re programmed by generations of computer science stu dents This is a valuable learning experience Unfortunately they have also been pro grammed and re programmed by generations of working computer professionals taking up time that could be devoted to new more creative work A programmer who needs a list or a binary tree shouldn t have to re code these data structures from scratch They are well understood and have been programmed thousands of times before The problem is how to make pre written robust data structures available to programmers In this chapter we ll look at Java s attempt to address this problem 9 1 Generic Programming GENERIC PROGRAMMING refers to writing code that will work for m
23. Before the program terminates if the data has been changed while the program was running the file should be re written with the new data Designing a user interface for the program is part of the exercise
24. GENERIC PROGRAMMING AND COLLECTION CLASSES 9 1 1 Generic Programming in Smalltalk Smalltalk was one of the very first object oriented programming languages It is still used today Although it has not achieved anything like the popularity of Java or C it is the source of many ideas used in these languages In Smalltalk essentially all programming is generic because of two basic properties of the language First of all variables in Smalltalk are typeless A data value has a type such as integer or string but variables do not have types Any variable can hold data of any type Param eters are also typeless so a subroutine can be applied to parameter values of any type Similarly a data structure can hold data values of any type For example once you ve de fined a binary tree data structure in SmallTalk you can use it for binary trees of integers or strings or dates or data of any other type There is simply no need to write new code for each data type Secondly all data values are objects and all operations on objects are defined by methods in a class This is true even for types that are primitive in Java such as integers When the operator is used to add two integers the operation is performed by calling a method in the integer class When you define a new class you can define a operator and you will then be able to add objects belonging to that class by saying a b just as if you were adding numbers
25. Graphics g To create a drawing surface you should define a subclass of JPanel and provide a custom paintComponent method Create an object belonging to your new class and add it to your JApplet When the time comes for your component to be drawn on the screen the system will call its paintComponent to do the drawing All this is not really as complicated as it might sound We will go over this in more detail when the time comes Note that the paintComponent method has a parameter of type Graphics The Graphics object will be provided by the system when it calls your method You need this object to do the actual drawing To do any drawing at all in Java you need a graphics context A graphics context is an object belonging to the class java awt Graphics In stance methods are provided in this class for drawing shapes text and images Any given Graphics object can draw to only one location In this chapter that location will always be a GUI component belonging to some subclass of JComponent The Graphics class is an abstract class which means that it is impossible to create a graphics context directly with a constructor There are actually two ways to get a graphics context for drawing on a component First of all of course when the paintComponent method of a component is called by the system the parameter to that method is a graphics context for drawing on the component Second each component has an instance method called getGraph
26. Protecting a design decision involves providing a stable interface which shields the remainder of the program from the implementation the details that are most likely to change We strive for responsibility driven design each class should be responsible for its own data We strive for loose coupling each class is largely independent and communi cates with other classes via a small well defined interface We strive for cohesion each class performs one and only one task for readability reuse 130 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN Class Responsibility Collaboration cards Class Responsibility Collaboration cards CRC cards are a brainstorming tool used in the design of object oriented software They were proposed by Ward Cunningham They are typically used when first determining which classes are needed and how they will interact CRC cards are usually created from index cards on which are written 1 The class name 2 The package name if applicable 3 The responsibilities of the class 4 The names of other classes that the class will collaborate with to fulfill its responsi bilities For example consider the CRC Card for a Playing Card class Playing Card Know its rank Deck Know its suit Graphics Know if its face Up Flip itself Draw itself The responsibilities are listed on the left The classes that the Playing Carad class will collaborate with are listed on the right The idea is that clas
27. System out println Can t find file data dat return End the program by returning from main try Create the output stream result new PrintWriter new FileWriter result dat catch IOException e System out println Can t open file result dat System out println e toString data close Close the input file return End the program try Read the data from the input file numberCt 0 while data hasNext Read until end of file number numberCt data nextDouble numberCt t 242 CHAPTER 11 BASIC INPUT OUTPUT Output the numbers in reverse order for int i numberCt 1 i gt 0 i result println number i System out println Done catch IOException e Some problem reading the data from the input file System out println Input Error e getMessage catch IndexOutOfBoundsException e Must have tried to put too many numbers in the array System out println Too many numbers in data file System out println Processing has been aborted finally Finish by closing the files whatever else may have happened data close result close end of main end of class 11 3 1 File Names Directories and the File Class The subject of file names is actually more complicated than I ve let on so far To fully specify a file you have to give both the name of the file and the name of the directo
28. THIS SECTION DISCUSSES some of the GUI interface elements that are represented by subclasses of JComponent The treatment here is brief and covers only the basic uses of each component type After you become familiar with the basics you might want to consult a Java reference for more details I will give some examples of programming with components in the next section The exact appearance of a Swing component and the way that the user interacts with the component are not fixed They depend on the look and feel of the user interface While Swing supports a default look and feel which is probably the one that you will see most often it is possible to change the look and feel For example a Windows look and feel could be used to make a Java program that is running on a Windows computer look more like a standard Windows program While this can improve the user s experience it means that some of the details that I discuss will have to be qualified with the phrase depending on the look and feel The JComponent class itself defines many useful methods that can be used with com ponents of any type We ve already used some of these in examples Let comp be a variable that refers to any JComponent Then the following methods are available among many others e comp getSize is a function that returns a Dimension object This object contains two instance variables comp getSize width and comp getSize height that give the current size
29. and the system itself They represent the external interface of the system and specify a form of requirements of what the system has to do remember only what not how When working with Use Cases it is important to remember some simple rules e Each Use Case is related to at least one actor e Each Use Case has an initiator i e an actor e Each Use Case leads to a relevant result a result with a business value 6 2 THE UNIFIED MODELLING LANGUAGE 133 25 41 Umbrello UML Modeller 2 iyiii isin asians EN AE Eile Edit Diagram Code Settings Help L HSS XG UML Diagrams A w Views N Logical View a F Body Header incomming requests fre pec getCode int Ea setCode c int ki LA za Mer Fe a u E Z Documentation a hd af CEE e DP Ready Figure 6 2 Umbrello UML Modeller showing a Class Diagram An actor is an external entity outside of the system that interacts with the system by participating and often initiating a Use Case Actors can be in real life people for example users of the system other computer systems or external events Actors do not represent the physical people or systems but their role This means that when a person interacts with the system in different ways assuming different roles he will be represented by several actors For example a person that gives custo
30. but doesn t actually do anything useful It s just a blank area on the screen that doesn t respond to any events To cre ate a useful applet a programmer must define a subclass that extends the Applet class There are several methods in the Applet class that are defined to do nothing at all The programmer must override at least some of these methods and give them something to do When you first learned about Java programs you encountered the idea of a main routine which is not meant to be called by the programmer The main routine of a program is there to be called by the system when it needs to execute the program The programmer writes the main routine to say what happens when the system runs the program An applet needs no main routine since it is not a stand alone program However many of the methods in an applet are similar to main in that they are meant to be called by the system and the job of the programmer is to say what happens in response to the system s calls One of the methods that is defined in the Applet class to do nothing is the paint method The paint method is called by the system when the applet needs to be drawn In a subclass of Applet the paint method can be redefined to draw various graphical elements such as rectangles lines and text on the applet The definition of this method must have the form public void paint Graphics g draw some stuff The parameter g of type Gr
31. but will not change y Objects for Numbers For every primitive data type there is a corresponding object So there are Integer Double and Character classes etc These have several methods including e constructors which take the corresponding base type e accessor methods such as double doubleVal which return the base type and e static utilities such as Integer parseInt String str that translate a String into the base type Exceptions Java uses Exceptions to signal problems with execution such as an array index out of range or running out of memory A RuntimeException is called an unchecked exception since it does not require compiler check Other Exceptions are checked and require throws statements and or try catch construction Example usage if index lt 0 throw new RuntimeException Bad index 2 8 Quiz Questions Question 1 Object oriented programming uses classes and objects What are classes and what are objects What is the relationship between classes and objects Question 2 Explain carefully what null means in Java and why this special value is necessary Question 3 What is a constructor What is the purpose of a constructor in a class 2 9 PROGRAMMING EXERCISES 49 Question 4 Suppose that Kumquat is the name of a class and that fruit is a variable of type Kumquat What is the meaning of the statement fruit new Kumquat That is what does the computer do when it executes this statement
32. called awt Since awt is contained within java its full name is actually java awt This is the package that contains classes related to the AWT graphical user interface such as a Button class which represents push buttons on the screen and the Graphics class which provides routines for drawing on the screen Since these classes are contained in the pack age java awt their full names are actually java awt Button and java awt Graphics I hope that by now you ve gotten the hang of how this naming thing works in Java Sim ilarly javax contains a sub package named javax swing which includes such classes as javax swing JButton and javax swing JApplet The java package includes several other sub packages such as java io which pro vides facilities for input output java net which deals with network communication and java applet which implements the basic functionality of applets The most basic pack age is called java lang This package contains fundamental classes such as String and Math It might be helpful to look at a graphical representation of the levels of nesting in the java package its sub packages the classes in those sub packages and the subroutines in those classes This is not a complete picture since it shows only a few of the many items in each element Let s say that you want to use the class java awt Color in a program that you are writing One way to do this is to use the full name of the class For example you co
33. double operation2 Figure 6 3 Visual representation of a Class in UML the rectangle In UML Attributes are shown with at least their name and can also show their type initial value and other properties Attributes can also be displayed with their visibility e Stands for public attributes e Stands for protected attributes e Stands for private attributes Operations methods are also displayed with at least their name and can also show their parameters and return types Operations can just as Attributes display their visibil ity e Stands for public operations e Stands for protected operations e Stands for private operations Class Associations Classes can relate be associated with to each other in different ways Inheritance is one of the fundamental concepts of Object Orientated programming in which a class gains all of the attributes and operations of the class it inherits from and can override modify some of them as well as add more attributes and operations of its own e Generalization In UML a Generalization association between two classes puts them in a hierarchy representing the concept of inheritance of a derived class from a base class In UML Generalizations are represented by a line connecting the two classes with an arrow on the side of the base class e Association An association represents a relationship between classes and gives the common semantics and structure for many ty
34. g drawLine i 10 j 60 124 CHAPTER 5 APPLETS HTML AND GUI S Question 6 Suppose you would like an applet that displays a green square inside a red circle as illustrated Write a paint method that will draw the image Question 7 Suppose that you are writing an applet and you want the applet to respond in some way when the user clicks the mouse on the applet What are the four things you need to remember to put into the source code of your applet Question 8 Java has a standard class called MouseEventb What is the purpose of this class What does an object of type MouseEvent do Question 9 Explain what is meant by input focus How is the input focus managed in a Java GUI program Question 10 Java has a standard class called JPanel Discuss two ways in which JPanels can be used 5 8 Programming Exercises 1 Write an applet that shows a pair of dice When the user clicks on the applet the dice should be rolled that is the dice should be assigned newly computed random val ues Each die should be drawn as a square showing from 1 to 6 dots Since you have to draw two dice its a good idea to write a subroutine void drawDie Graphics g int val int x int y to draw a die at the specified x y coordinates The second parameter val specifes the value that is showing on the die Assume that the size of the applet is 100 by 100 pixels 2 Improve your dice applet from the previous exercise so that it also responds
35. if you know that all the items in coll are of type String then String coll toArray gives you an array of Strings containing all the strings in the collection Since these methods are part of the Collection interface they must be defined for every object that implements that interface There is a problem with this however For example the size of some kinds of Collection cannot be changed after they are created Methods that add or remove objects don t make sense for these collections While it is still legal to call the methods an exception will be thrown when the call is evaluated at run time The type of exception is UnsupportedOperationException There is also the question of efficiency Even when an operation is defined for several types of collections it might not be equally efficient in all cases Even a method as simple as size can vary greatly in efficiency For some collections computing the size might involve counting the items in the collection The number of steps in this process is equal to the number of items Other collections might have instance variables to keep track of the size so evaluating size just means returning the value of a variable In this case the computation takes only one step no matter how many items there are When working with collections it s good to have some idea of how efficient operations are and to choose a collection for which the operations you need can be implemented most efficiently
36. in the ArrayList If the object is found in the list then the position number where it is found is re turned If the object is not found then 1 is returned For example suppose again that players in a game are represented by objects of type Player The players currently in the game could be stored in an ArrayList named players This variable would be declared as ArrayList players and initialized to refer to a new empty ArrayList object with players new ArrayList If newPlayer is a variable that refers to a Player object the new player would be added to the ArrayList and to the game by saying players add newPlayer and if player number i leaves the game it is only necessary to say players remove i Or if player is a variable that refers to the Player that is to be removed you could say players remove player All this works very nicely The only slight difficulty arises when you use the function players get i to get the value stored at position i in the ArrayList The return type of this function is Object In this case the object that is returned by the function is actually of type Player In order to do anything useful with the returned value it s usually necessary to type cast it to type Player Player plr Player players get i For example if the Player class includes an instance method makeMove that is called to allow a player to make a move in the game then the code for letting all the players m
37. jack You can use the Card Hand and Deck classes developed previously However a hand in the game of Blackjack is a little different from a hand of cards in general since it must be possible to compute the value of a Blackjack hand according to the rules of the game The rules are as follows The value of a hand is obtained by adding up the values of the cards in the hand The value of a numeric card such as a three or a ten is its numerical value The value of a Jack Queen or King is 10 The value of an Ace can be either 1 or 11 An Ace should be counted as 11 unless doing so would put the total value of the hand over 21 Note that this means that the second third or fourth Ace in the hand will always be counted as 1 One way to handle this is to extend the existing Hand class by adding a method that computes the Blackjack value of the hand Here s the definition of such a class public class BlackjackHand extends Hand public int getBlackjackValue Returns the value of this hand for the game of Blackjack int val The value computed for the hand boolean ace Set to true if the hand contains an ace int cards Number of cards in the hand val 0 ace false cards getCardCount for int i 0 i lt cards it Add the value of the i th card in the hand Card card The i th card int cardVal The blackjack value of the i th card card getCard i cardVal card getValue The nor
38. obj1 and obj2 are consid ered to be equal if they are both nu11 or if they are both non nu11 and obj1 equals obj2 is true In the Object class obj1 equals obj2 is defined to be the same as obj 1 obj2 However for most sub classes of Object this definition is not reasonable and it should be overridden The String class for example overrides equals so that for a String str str equals obj if obj is also a String and obj contains the same sequence of characters as str If you write your own class you might want to define an equals method in that class to get the correct behavior when objects are tested for equality For example a Card class that will work correctly when used in collections could be defined as public class Card Class to represent playing cards int suit Number from 0 to 3 spades diamonds clubs or hearts int value Number from 1 to 13 that represents the value public boolean equals Object obj if obj null obj instanceof Card obj can t be equal to this Card if obj is not a Card or if it is null return false else Card other Card obj Type cast obj to a Card if suit other suit amp amp value other value The other card has the same suit and value as this card so they should be considered equal return true else return false other methods and constructors Without the equals method in this class methods such as
39. plementation of new classes it s important not to forget that the designers of Java have already provided a large number of reusable classes Some of these classes are meant to be extended to produce new classes while others can be used directly to create useful objects A true mastery of Java requires familiarity with the full range of built in classes something that takes a lot of time and experience to develop In the nextchapter we will begin the study of Java s GUI classes and you will encounter other built in classes 2 3 PROGRAMMING WITH OBJECTS 37 throughout the remainder of this book But let s take a moment to look at a few built in classes that you might find useful A string can be built up from smaller pieces using the operator but this is not very effi cient If str is a String and ch is a character then executing the command str str ch involves creating a whole new string that is a copy of str with the value of ch appended onto the end Copying the string takes some time Building up a long string letter by let ter would require a surprising amount of processing The class java lang StringBuffer makes it possible to be efficient about building up a long string from a number of smaller pieces Like a String a StringBuffer contains a sequence of characters However it is possible to add new characters onto the end of a StringBuffer without making a copy of the data that it already contains If buffer is a var
40. returns a boolean value which is true if the size of the collection isO e coll clear removes all objects from the collection 9 3 COLLECTIONS 191 e coll contains object returns a boolean value that is true if object is in the collection coll add object adds object to the collection The parameter can be any Object Some collections can contain the value null while others cannot This method returns a boolean value which tells you whether the operation actually mod ified the collection For example adding an object to a Set has no effect if that object was already in the set e coll remove object removes object from the collection if it occurs in the col lection and returns a boolean value that tells you whether the object was found e coll containsAll1 col112 returns a boolean value that is true if every object in col112 is also in the coll The parameter can be any Collection coll addAl1 col112 adds all the objects in the collection co112 to coll e coll removeAl1 col12 removes every object from coll that also occurs in the collection co112 e coll retainAll coll2 removes every object from coll that does not occur in the collection co112 It retains only the objects that do occur in co112 e coll toArray returns an array of type Object that contains all the items in the collection The return value can be type cast to another array type if appropri ate For example
41. that is the number showing on each of the dice The instance method represents the behaviour of a pair of dice that is the ability to be rolled This class would be reusable in many different programming projects On the other hand the Student class is not very reusable It seems to be crafted to represent students in a particular course where the grade will be based on three tests If there are more tests or quizzes or papers it s useless If there are two people in the class who have the same name we are in trouble one reason why numerical student ID s are often used Admittedly it s much more difficult to develop a general purpose student class than a general purpose pair of dice class But this particular Student class is good mostly as an example in a programming textbook Let s do another example in a domain that is simple enough that we have a chance of coming up with something reasonably reusable Consider card games that are played with a standard deck of playing cards a so called poker deck since it is used in the game of poker In a typical card game each player gets a hand of cards The deck is shuffled and cards are dealt one at a time from the deck and added to the players hands In some games cards can be removed from a hand and new cards can be added The game is won or lost depending on the value ace 2 king and suit spades diamonds clubs hearts of the cards that a player receives If we look f
42. this means adding it to its pile and making the new Card the topCard The Solitaire class The Solitaire class is the one that runs It creates and maintains the different piles of cards Notice that most of its attributes are static and visible to other classes in the package Study it carefully and make sure you understand it fully FULLY before you continue package solitaire import javax swing import java awt public class Solitaire extends JPanel implements MouseListener static DeckPile deckPile static DiscardPile discardPile static TablePile tableau static SuitPile suitPile static CardPile allPiles public Solitaire setBackground Color green addMouseListener this allPiles new CardPile 13 suitPile new SuitPile 4 tableau new TablePile 7 int deckPos 600 int suitPos 15 allPiles 0 deckPile new DeckPile deckPos 5 allPiles 1 discardPile new DiscardPile deckPos Card width 10 5 for int i 0 i lt 4 i allPiles 2 i suitPile i new SuitPile suitPos Card width 10 i 5 for int i 0 i lt 7 i allPiles 6 i tableau i new TablePile suitPos Card width 10 i Card height 20 i 1 repaint 148 CHAPTER 7 A SOLITAIRE GAME KLONDIKE public void paintComponent Graphics g super paintComponent g for int i 0 i lt 13 i allPiles i draw g public static void main String args JFrame frame
43. to obtain access to all the classes in the utilities package Unfortunately things are a little more complicated than this Remember that if a program uses a class then the class must be available when the program is compiled and when it is executed Exactly what this means depends on which Java environment you are using Most commonly classes in a package named utilities should be in a directory with the name utilities and that directory should be located in the same place as the program that uses the classes In projects that define large numbers of classes it makes sense to organize those classes into one or more packages It also makes sense for programmers to create new packages as toolboxes that provide functionality and API s for dealing with areas not covered in the standard Java API And in fact such toolmaking programmers often have more prestige than the applications programmers who use their tools You need to know about packages mainly so that you will be able to import the stan dard packages These packages are always available to the programs that you write You might wonder where the standard classes are actually located Again that depends to some extent on the version of Java that you are using But they are likely to be collected together into a large file named rt jar or classes zip which is located in some place where the Java compiler and the Java interpreter will know to look for it Every class is actuall
44. 1 0 the font names were TimesRoman Helvetica and Courier You can still use the older names if you want A serif is a little decoration on a character such as a short horizontal line at the bottom of the letter i SansSerif means without serifs Monospaced means that all the characters in the font have the same width The Dialog font is the one that is typically used in dialog boxes The style of a font is specified using named constants that are defined in the Font class You can specify the style as one of the four values e Font PLAIN e Font ITALIC e Font BOLD or e Font BOLD Font ITALIC The size of a font is an integer Size typically ranges from about 10 to 36 although larger sizes can also be used The size of a font is usually about equal to the height of the largest characters in the font in pixels but this is not a definite rule The size of the default font is 12 Java uses the class named java awt Font for representing fonts You can construct a new font by specifying its font name style and size in a constructor 96 CHAPTER 5 APPLETS HTML AND GUI S Font plainFont new Font Serif Font PLAIN 12 Font bigBoldFont new Font SansSerif Font BOLD 24 Every graphics context has a current font which is used for drawing text You can change the current font with the setFont method For example if g is a graphics context and bigBoldFont is a font then
45. Boxes In many programs you want the user to be able to select the file that is going to be used for input or output If your program lets the user type in the file name you will just have to assume that the user understands how to work with files and directories But in a graphical user interface the user expects to be able to select files using a file dialog box which is a special window that a program can open when it wants the user to select a file for input or output Swing includes a platform independent technique for using file dialog boxes in the form of a class called JFileChooser This class is part of the package javax swing We looked at some basic dialog boxes previously File dialog boxes are similar to those but are a little more complicated to use A file dialog box shows the user a list of files and sub directories in some directory and makes it easy for the user to specify a file in that directory The user can also navigate easily from one directory to another The most common constructors for JFileChooser specify the directory that is selected when the dialog box first appears new JFileChooser File startDirectory new JFileChooser String pathToStartDirectory There is also a constructor with no arguments that will set the user s home directory to be the starting directory in the dialog box The constructor call new JFileChooser produces a dialog box that has the current directory as its starting direc
46. Called by the constructor and called by actionPerformed when the user clicks the New Game button Start a new game if gameInProgress If the current game is not over it is an error to try to start a new game message You still have to finish this game repaint return deck new Deck Create a deck and hand to use for this game hand new Hand deck shuffle hand addCard deck dealCard Deal the first card message Is the next card higher or lower gameInProgress true State changes A game has started repaint The doHigher and doLower methods are almost identical to each other and could probably have been combined into one method with a parameter if I were more clever Let s look at the doHigher routine This is called when the user clicks the Higher button This only makes sense if a game is in progress so the first thing doHigher should do is check the value of the state variable gameInProgress If the value is false then doHigher should just set up an error message If a game is in progress a new card should be added to the hand and the user s prediction should be tested The user might win or lose at this time If so the value of the state variable gameInProgress must be set to false because the game is over In any case the board is repainted to show the new state Here is the doHigher method void doHigher Called by actionPerfor
47. Event handling class are defined here public class HelloSwing extends JApplet implements ActionListener public void init This method is called by the system before the applet appears It is used here to create the button and add it to the content pane of the JApplet The applet is also registered as an ActionListener for the button JButton bttn new JButton Click Me bttn addActionListener this getContentPane add bttn end init public void actionPerformed ActionEvent evt This method is called when an action event occurs In this case the only possible source of the event is the button So when this method is called we know that the button has been clicked Respond by showing an informational dialog box The dialog box will contain an OK button which the user must click to dismiss the dialog box String title Greetings Shown in title bar of dialog box String message Hello from the Swing User Interface Library JOptionPane showMessageDialog null message title JOptionPane INFORMATION_MESSAGE end actionPerformed end class HelloSwing In this source code I ve set up the applet itself to listen for action events from the button Some people don t consider this to be very good style They prefer to create a separate object to listen for and respond to events This is more object oriented in the sense that each object has its own
48. For example consider a program that is designed to read some numbers from the user and then print the same numbers in sorted order The program is correct if it works for any set of input numbers It is robust if it can also deal with non numeric input by for example printing an error message and ignoring the bad input A non robust program might crash or give nonsensical output in the same circumstance Every program should be correct A sorting program that doesn t sort correctly is pretty useless It s not the case that every program needs to be completely robust It depends on who will use it and how it will be used For example a small utility program that you write for your own use doesn t have to be at all robust The question of correctness is actually more subtle than it might appear A programmer works from a specification of what the program is supposed to do The programmer s work is correct if the program meets its specification But does that mean that the program itself is correct What if the specification is incorrect or incomplete A correct program should be a correct implementation of a complete and correct specification The question is whether the specification correctly expresses the intention and desires of the people for whom the program is being written This is a question that lies largely outside the domain of computer science Most computer users have personal experience with programs that don t work or that c
49. For example if you want to use a standard component you only have to add it to your applet You don t have to worry about painting it on the screen That will happen automatically Sometimes however you do want to draw on a component You will have to do this whenever you want to display something that is not included among the standard pre defined component classes When you want to do this you have to define your own component class and provide a method in that class for drawing the component When painting on a plain non Swing Applet the drawing is done in a paint method To do custom drawing you have to define a subclass of Applet and include a paint method to do the drawing However when it comes to Swing and JApplets things are a little more complicated You should not draw directly on JApplets or on other top level Swing components Instead you should make a separate component to use as a drawing surface and you should add that component to the JApplet You will have to write a class to represent the drawing surface so programming a JApplet that does custom drawing will always involve writing at least two classes a class for the applet itself and a class for the drawing surface Typically the class for the drawing surface will be defined as a sub class of javax swing JPanel which by default is nothing but a blank area on the screen A JPanel like any JComponent draws its content in the method public void paintComponent
50. It is still a top down approach because the top down analysis of the problem tells you what subroutines to write In the bottom up approach you start by writing or obtaining subroutines that are relevant to the problem domain and you build your solution to the problem on top of that foundation of subroutines 1 1 1 Preconditions and Postconditions When working with subroutines as building blocks it is important to be clear about how a subroutine interacts with the rest of the program This interaction is specified by the contract of the subroutine A convenient way to express the contract of a subroutine is in terms of preconditions and postconditions 11 12 CHAPTER 1 DESIGN I CLASS LEVEL DESIGN The precondition of a subroutine is something that must be true when the subrou tine is called if the subroutine is to work correctly For example for the built in function Math sqrt x a precondition is that the parameter x is greater than or equal to zero since it is not possible to take the square root of a negative number In terms of a contract a precondition represents an obligation of the caller of the subroutine If you call a subrou tine without meeting its precondition then there is no reason to expect it to work properly The program might crash or give incorrect results but you can only blame yourself not the subroutine A postcondition of a subroutine represents the other side of the contract It is some thing that will b
51. Selecta Color Java Applet Window Preview m E Sample Text Sample Text H 0O Sample Text Sample Text Cancel Reset Displaying color chooser dialog Message Dialog Confirm Dialog Input Dialog Color Chooser Figure 5 8 SimpleDialogDemo Applet The three simple dialogs in this applet are created by static methods in the class JOptionPane This class includes many methods for making dialog boxes but they are all variations on the three basic types shown here a message dialog a confirm dialog and an input dialog The variations allow you to provide a title for the dialog box to specify the icon that appears in the dialog and to add other components to the dialog box I will only cover the most basic forms here A message dialog simply displays a message string to the user The user hopefully reads the message and dismisses the dialog by clicking the OK button A message dialog can be shown by calling the method void JOptionPane showMessageDialog JFrame parent String message The parent as usual can be null The message can be more than one line long Lines in the message should be separated by newline characters n New lines will not be inserted automatically even if the message is very long An input dialog displays a question or request and lets the user type in a string as a response You can show an input dialog by calling String JOptionPane showInputDialo
52. TextArea 11 4 PROGRAMMING WITH FILES 253 private void doSave Carry out the Save command by letting the user specify an output file and writing the text from the TextArea to that file File file The file that the user wants to save JFileChooser fd File dialog that lets the user specify the file fd new JFileChooser fd setDialogTitle Save Text As int action fd showSaveDialog this if action JFileChooser APPROVE_OPTION User has canceled or an error occurred return file fd getSelectedFile if file exists If file already exists ask before replacing it action JOptionPane showConfirmDialog this Replace existing file if action JOptionPane YES_OPTION return try Create a PrintWriter for writing to the specified file and write the text from the window to that stream PrintWriter out new PrintWriter new FileWriter file String contents text getText out print contents if out checkError throw new I0Exception Error while writing to file out close catch IOException e Some error has occurred while trying to write Show an error message JOptionPane showMessageDialog this Sorry an error has occurred n e getMessage When the user selects the Open command a file dialog box allows the user to specify the file that is to be opened It is assumed that the file is a text file Since JTextAreas are no
53. The number of operations required to find an item in a sorted tree is the same as the distance from the root of the tree to the item Using a balanced tree ensures that all items are as close to the root as possible This makes finding an item very efficient Adding and removing elements are equally efficient 202 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES As an example suppose that coll is any Collection of Strings or any other type for which compareTo is properly defined We can use a TreeSet to sort the items of coll and remove the duplicates simply by saying TreeSet set new TreeSet set addAll coll The second statement adds all the elements of the collection to the set Since it s a Set duplicates are ignored Since it s a TreeSet the elements of the set are sorted If you would like to have the data in some other type of data structure it s easy to copy the data from the set For example to place the answer in an ArrayList you could say TreeSet set new TreeSet set addAll coll ArrayList list new ArrayList list addAll set Now in fact every one of Java s collection classes has a constructor that takes a Collection as an argument All the items in that Collection are added to the new collection when it is created So newTreeSet coll creates a TreeSet that contains the same elements as the Collection coll This means that we can abbreviate the four lines in the above example to the s
54. The determinant of M is determinant catch ArrayIndexOutOfBoundsException e System out println M is the wrong size to have a determinant catch NullPointerException e System out print Programming error M doesn t exist System out println e getMessage This example shows how to use multiple catch clauses in one try block It also shows what that little e is doing in the catch clauses The e is actually a variable name You can use any name you like Recall that when an exception occurs it is actually an object that is thrown Before executing a catch clause the computer sets this variable to refer to the exception object that is being caught This object contains information about the exception For example an error message describing the exception can be retrieved using the object s getMessage method as is done in the above example Another useful method in every exception object e is e printStackTrace This method will print out the list of subroutines that were being executed when the exception was thrown This information can help you to track down the part of your program that caused the error Note that both ArrayIndexOutOfBoundsException and NullPointerException are subclasses of RuntimeException It s possible to catch all RuntimeExceptions with a single catch clause For example try double determinant M 0 0 M 1 1 M 0 1 M 1 0 System out println T
55. Toolboxes APPs and Packages AS COMPUTERS AND THEIR USER INTERFACES have become easier to use they have also become more complex for programmers to deal with You can write programs for a simple console style user interface using just a few subroutines that write output to the console and read the user s typed replies A modern graphical user interface with windows buttons scroll bars menus text input boxes and so on might make things easier for the user But it forces the programmer to cope with a hugely expanded array of possibilities The programmer sees this increased complexity in the form of great numbers of subroutines that are provided for managing the user interface as well as for other purposes Someone who wants to program for Macintosh computers and to produce programs that look and behave the way users expect them to must deal with the Macintosh Tool box a collection of well over a thousand different subroutines There are routines for opening and closing windows for drawing geometric figures and text to windows for adding buttons to windows and for responding to mouse clicks on the window There are other routines for creating menus and for reacting to user selections from menus Aside from the user interface there are routines for opening files and reading data from them for communicating over a network for sending output to a printer for handling communication between programs and in general for doing all the
56. United States when a newly loaded computer program proved to contain a bug These are just a few examples Software problems are all too common As program mers we need to understand why that is true and what can be done about it Part of the problem according to the inventors of Java can be traced to programming languages themselves Java was designed to provide some protection against certain types of errors How can a language feature help prevent errors Let s look at a few examples Early programming languages did not require variables to be declared In such lan guages when a variable name is used in a program the variable is created automatically You might consider this more convenient than having to declare every variable explicitly But there is an unfortunate consequence An inadvertent spelling error might introduce an extra variable that you had no intention of creating This type of error was responsible according to one famous story for yet another lost spacecraft In the FORTRAN program ming language the command DO 20 I 1 5 is the first statement of a loop Now spaces are insignificant in FORTRAN so this is equivalent to DO20I 1 5 On the other hand the command D0O20I 1 5 with a period instead of a comma is an assignment statement that assigns the value 1 5 to the variable DO20I Supposedly the inadvertent 10 1 INTRODUCTION TO CORRECTNESS AND ROBUSTNESS 209 substitution of a period for a comm
57. We ll see specific examples of this in the next two sections The Collection interface defines a few basic generic algorithms but suppose you want to write your own generic algorithms Suppose for example you want to do some thing as simple as printing out every item in a collection To do this in a generic way you need some way of going through an arbitrary collection accessing each item in turn We have seen how to do this for specific data structures For an array you can use a for loop to iterate through all the array indices For a linked list you can use a while loop in which you advance a pointer along the list For a binary tree you can use a recursive 192 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES subroutine to do an infix traversal Collections can be represented in any of these forms and many others besides With such a variety of traversal mechanisms how can we hope to come up with a single generic method that will work for collections that are stored in wildly different forms This problem is solved by iterators An iterator is an object that can be used to traverse a collection Different types of collections have different types of iterators but all iterators are used in the same way An algorithm that uses an iterator to traverse a collection is generic because the same technique can be applied to any type of collection Iterators can seem rather strange to someone who is encountering generic pro gramming for t
58. What is the fundamental property that distinguishes Sets from other types of Collections 9 6 PROGRAMMING EXERCISES 203 Question 7 What is the essential difference in functionality between a TreeMap and a HashMap Question 8 Explain what is meant by a hash code Question 9 Modify the following Date class so that it implements the Comparable interface The ordering on objects of type Date should be the natural chronological or dering class Date int month Month number in range 1 to 12 int day Day number in range 1 to 31 int year Year number Date int m int d int y Convenience constructor month m day d year y Question 10 Suppose that syllabus is a variable of type TreeMap the keys in the map are objects belonging to the Date class from the previous problem and the values are of type String Write a code segment that will write out the value string for every key that is in the month of September 2002 9 6 Programming Exercises 1 In Section 12 2 I mentioned that a LinkedList can be used as a queue by using the addLast and removeFirst methods to enqueue and dequeue items But if we are going to work with queues it s better to have a Queue class The data for the queue could still be represented as a LinkedList but the LinkedList object would be hidden as a private instance variable in the Queue object Use this idea to write a generic Queue class for representing queues of
59. actual parameters In Java parameters to a subroutine must be of a specified type and the subroutine can only use operations that are defined for that type A subroutine with a parameter of type Object can be applied to objects of any type but the subroutine can only use operations that are defined in class Object and there aren t many of those For example there is no com parison operation defined in the Object class so it is not possible to write a completely generic sorting algorithm We ll see below how Java addresses this problem Because of problems like these some people including myself claim that Java does not really support true generic programming Other people disagree But whether it s true generic programming or not that doesn t prevent it from being very useful Java 1 5 Note Java 1 5 introduces templates that are similar to C class templates As in C this makes it possible to do generic programming in a more type safe way For example if Shape is a class then the type ArrayList lt Shape gt represents a list that can only hold values of type Shape The type name ArrayList lt Shape gt is used like any other type For example to declare a variable and create an object of this type you could say ArrayList lt Shape gt shapeList new ArrayList lt Shape gt The variable shapeList can then be used like any other ArrayList except that it can only hold values of type Shape Since the compiler knows this i
60. also be closed programmatically by calling the subroutine Mosaic close My idea is to use the Mosaic class as the basis for a neat animation I want to fill the window with randomly colored squares and then randomly change the colors in a loop that continues as long as the window is open Randomly change the colors could mean a lot of different things but after thinking for a while I decide it would be interesting to have a disturbance that wanders randomly around the window changing the color of each square that it encounters Here s an applet that shows what the program will do Applet Reference see Applet RandomMosaicWalkApplet 14 CHAPTER 1 DESIGN I CLASS LEVEL DESIGN Figure 1 1 Snapshot of the RandomMosaicWalkApplet With basic routines for manipulating the window as a foundation I can turn to the specific problem at hand A basic outline for my program is Open a Mosaic window Fill window with random colors Move around changing squares at random Filling the window with random colors seems like a nice coherent task that I can work on separately so let s decide to write a separate subroutine to do it The third step can be expanded a bit more into the steps Start in the middle of the window then keep moving to a new square and changing the color of that square This should continue as long as the mosaic window is still open Thus we can refine the algorithm to Open a Mosaic window Fill window
61. an error As another example consider a program that deals with shapes drawn on the screen Let s say that the shapes include rectangles ovals and roundrects of various colors _ CC SQ JQ Rectangles Ovals RoundRects LI Figure 3 4 Shapes Three classes Rectangle Oval and RoundRect could be used to represent the three types of shapes These three classes would have a common superclass Shape to represent features that all three shapes have in common The Shape class could include instance variables to represent the color position and size of a shape It could include instance methods for changing the color position and size of a shape Changing the color for example might involve changing the value of an instance variable and then redrawing the shape in its new color class Shape Color color Color of the shape Recall that class Color is defined in package java awt Assume that this class has been imported void setColor Color newColor Method to change the color of the shape color newColor change value of instance variable redraw redraw shape which will appear in new color void redraw method for drawing the shape what commands should go here more instance variables and methods end of class Shape Now you might see a problem here with the method redraw The problem is that each different type of shape is drawn differently The metho
62. an introduction to streams but it is not meant to cover them in full detail See the official Java documentation for more information When dealing with input output you have to keep in mind that there are two broad categories of data machine formatted data and human readable data Machine formatted data is represented in the same way that data is represented inside the computer that is as strings of zeros and ones Human readable data is in the form of characters When you read a number such as 3 141592654 you are reading a sequence of characters and interpreting them as a number The same number would be represented in the computer as a bit string that you would find unrecognizable To deal with the two broad categories of data representation Java has two broad categories of streams byte streams for machine formatted data and character streams for human readable data There are many predefined classes that represent streams of each type Every object that outputs data to a byte stream belongs to one of the subclasses of the abstract class OutputStream Objects that read data from a byte stream belong to subclasses of InputStream If you write numbers to an OutputStream you won t be able to read the resulting data yourself But the data can be read back into the computer with an InputStream The writing and reading of the data will be very efficient since there is no translation involved the bits that are used to represent the data inside the
63. and obj2 are considered to be the same if obj1 equals obj2 is true as discussed in the previous section Sets implement all the general Collection methods but do so in a way that ensures that no element occurs twice in the set For example if set is an object of type Set then set add obj will have no effect on the set if obj is already an element of the set Java has two classes that implement the Set interface java util TreeSet and java util HashSet In addition to being a Set a TreeSet has the property that the elements of the set are arranged into ascending sorted order An Iterator for a TreeSet will always visit the elements of the set in ascending order A TreeSet cannot hold arbitrary objects since there must be a way to determine the sorted order of the objects it contains Ordinarily this means that the objects in a TreeSet should implement the Comparable interface and that obj1 compareTo obj2 should be defined in a reasonable way for any two objects obj1 and obj2 in the set Alternatively a Comparator can be provided as a parameter to the constructor when the TreeSet is created In that case the Comparator will be used to compare objects that are added to the set In the implementation of a TreeSet the elements are stored in something like a binary sort tree See Section11 4 The actually type of tree that is used is balanced in the sense that all the leaves of the tree are at about the same distance from the root of the tree
64. are also a lot of tutorials on the Internet which you can take as a starting point 6 2 THE UNIFIED MODELLING LANGUAGE 131 The Unified Modelling Language UML is a diagramming language or notation to specify visualize and document models of Object Oriented software systems UML is not a development method that means it does not tell you what to do first and what to do next or how to design your system but it helps you to visualize your design and communicate with others UML is controlled by the Object Management Group OMG and is the industry standard for graphically describing software The OMG have recently completed version 2 of the UML standard known as UML2 UML is designed for Object Oriented software design and has limited use for other programming paradigms UML is not a method by itself however it was designed to be compatible with the leading object oriented software development methods of its time e g OMT Booch Ob jectory Since UML has evolved some of these methods have been recast to take advan tage of the new notation e g OMT and new methods have been created based on UML Most well known is the Rational Unified Process RUP created by the Rational Software Corporation Modelling There are three prominent parts of a system s model e Functional Model Showcases the functionality of the system from the user s Point of View Includes Use Case Diagrams e Object Model Showcases the structure and substru
65. been a while since we looked at a stand alone program with a main routine and we have never seen a stand alone GUI program It s easy for a stand alone program to use a graphical user interface all it has to do is open a frame Since a ShapeDrawFrame makes itself visible when it is created it is only necessary to create the frame object with the command newShapeDrawFrame The complete main routine in this case looks like public static void main String args new ShapeDrawFrame It might look a bit unusual to call a constructor without assigning the result to a vari able but it is perfectly legal and in this case we have no need to store the value The main routine ends as soon as the frame is created but the frame continues to exist and the pro gram will continue running Since the default close operation for the frame has been set to EXIT_ON_CLOSE the frame will close and the program will be terminated when the user clicks the close box of the window It might seem a bit odd to have this main routine in the same class that defines ShapeDrawFrame and in fact it could just as easily be in a separate class But there is no real need to create an extra class as long as you understand what is going on When you type java ShapeDrawFrame on the command line the system looks for a main routine in the ShapeDrawFrame class and executes it If the main routine happens to create an object belonging to the same class it
66. button This is a nice example of the sort of communication between objects that can be done with events You can check the source code to see exactly how it s done 5 6 1 Images in Applications Previously we saw how to load an image file into an applet The JApplet class has a method getImage that can be used for this purpose The JFrame class does not provide this method so some other technique is needed for using images in frames The standard class java awt Toolkit makes it possible to load an image into a stand alone application A Toolkit object has a getImage method for reading an Image from an image file To use this method you must first obtain a Toolkit and you have to do this by calling the static method Toolkit getDefaultToolkit Any running GUI program already has a toolkit which is used to perform various platform dependent functions You don t need to construct a new toolkit Toolkit getDefaultToolkit returns a reference to the toolkit that already exists Once you have a toolkit you can use its getImage method to create an Image object from a file This getImage method takes one parameter which specifies the name of the file For example Toolkit toolkit getDefaultToolkit Image cards toolkit getImage smallcards gif or in one line Image cards Toolkit getDefaultToolkit getImage smallcards gif Once the Image has been created it can be used in the same way whether it has bee
67. called MosaicCanvas that is used by Mosaic If you want to compile and run this program both of these classes must be available to the program public class RandomMosaicWalk This program shows a window full of randomly colored squares A disturbance moves randomly around in the window randomly changing the color of each square that it visits The program runs until the user closes the window static int currentRow row currently containing the disturbance static int currentColumn column currently containing disturbance public static void main String args Main program creates the window fills it with random colors then moves the disturbance in a random walk around the window for as long as the window is open Mosaic open 10 20 10 10 i11WithRandomColors currentRow 5 start at center of window currentColumn 10 while Mosaic isOpen changeToRandomColor currentRow currentColumn randomMove Mosaic delay 20 end of main static void fillWithRandomColors Precondition The mosaic window is open Postcondition Each rectangles have been set to a random color for int row 0 row lt 10 rowt for int column 0 column lt 20 column changeToRandomColor row column end of fillWithRandomColors 1 1 PROGRAM DESIGN 17 static void changeToRandomColor int rowNum int colNum Precondition rowNum and colN
68. can be represented as a Roman numeral made up of the following one and two letter combinations M 1000 X 10 CM 900 IX 9 D 500 V 5 CD 400 IV 4 C 100 I 1 XC 90 L 50 XL 40 Write a class to represent Roman numerals The class should have two constructors One constructs a Roman numeral from a string such as XVII or MCMXCV It should throw a NumberFormatException if the string is not a legal Roman numeral The other constructor constructs a Roman numeral from an int It should throw a NumberFormatException if the int is outside the range 1 to 3999 228 CHAPTER 10 CORRECTNESS AND ROBUSTNESS In addition the class should have two instance methods The method toString returns the string that represents the Roman numeral The method toInt returns the value of the Roman numeral as an int At some point in your class you will have to convert an int into the string that rep resents the corresponding Roman numeral One way to approach this is to gradually move value from the Arabic numeral to the Roman numeral Here is the beginning of a routine that will do this where number is the int that is to be converted String roman int N number while N gt 1000 Move 1000 from N to roman roman M N 1000 while N gt 900 Move 900 from N to roman roman CM N 900 Continue with other values from the above table You can save yourself a lot of typing in this routine if
69. code that makes up the definition of the subroutine and the postcondition of a sub routine is a postcondition of the same code In this section we have generalized these terms to make them more useful in talking about program correctness Let s see how this works by considering a longer program segment Scanner sc new Scanner System in do System out println Enter A B and C B B 4 A C must be gt 0 System out print A A sc nextDouble System out print B B sc nextDouble System out print C C sc nextDouble if A B B 4 A C lt 0 System out println Your input is illegal Try again while A B B 4 A C lt 0 disc B B 4 A C x B Math sqrt disc 2 A After the loop ends we can be sure that B B 4 A C gt 0 and that A 0 The precon ditions for the last two lines are fulfilled so the postcondition that x is a solution of the equation A x2 B x C Ois also valid This program segment correctly and provably computes a solution to the equation Actually because of problems with representing numbers on computers this is not 100 true The algorithm is correct but the program is not a perfect implementation of the algorithm See the discussion at the end of the previous section 214 CHAPTER 10 CORRECTNESS AND ROBUSTNESS Here is another variation in which the precondition is checked by an if statement In the first part of the if statemen
70. common to do some configuration of a JFileChooser before calling showOpenDialog or showSaveDialog For example the instance method setDialogTitle String can be used to specify a title to appear at the top of the window And setSelectedFile File can be used to set the file that is selected in the dialog box when it appears This can be used to provide a default file choice for the user We ll look at some more complete examples of using files and file dialogs in the next section 11 4 Programming With Files IN THIS SECTION we look at several programming examples that work with files The first example is a program that makes a list of all the words that occur in a specified file The user is asked to type in the name of the file The list of words is written to another file which the user also specifies A word here just means a sequence of letters The list of words will be output in alphabetical order with no repetitions All the words will be converted into lower case so that for example The and the will count as the same word Since we want to output the words in alphabetical order we can t just output the words as they are read from the input file We can store the words in an array but since there is no way to tell in advance how many words will be found in the file we need a dynamic array which can grow as large as necessary The data is represented in the program by two static variables static String wor
71. computer are simply copied to and from the streams For reading and writing human readable character data the main classes are Reader and Writer All character stream classes are subclasses of one of these If a number is 236 CHAPTER 11 BASIC INPUT OUTPUT to be written to a Writer stream the computer must translate it into a human readable sequence of characters that represents that number Reading a number from a Reader stream into a numeric variable also involves a translation from a character sequence into the appropriate bit string Even if the data you are working with consists of characters in the first place such as words from a text editor there might still be some translation Characters are stored in the computer as 16 bit Unicode values For people who use Western alphabets character data is generally stored in files in ASCII code which uses only 8 bits per character The Reader and Writer classes take care of this translation and can also handle non western alphabets in countries that use them It s usually easy to decide whether to use byte streams or character streams If you want the data to be human readable use character streams Otherwise use byte streams I should note that Java 1 0 did not have character streams and that for ASCII encoded character data byte streams are largely interchangeable with character streams In fact the standard input and output streams System in and System out are byte streams rather
72. events if there are any Note that the system can also call paintComponent for other reasons It is called when the component first appears on the screen It will also be called if the component is covered up by another window and then uncovered The system does not save a copy of the component s contents when it is covered When it is uncovered the component is responsible for redrawing itself As you will see some of our early examples will not be able to do this correctly This means that to work properly the paintComponent method must be smart enough to correctly redraw the component at any time To make this possible a pro gram should store data about the state of the component in its instance variables These variables should contain all the information necessary to redraw the component com pletely The paintComponent method should use the data in these variables to decide what to draw When the program wants to change the content of the component it should not simply draw the new content It should change the values of the relevant variables and call repaint When the system calls paintComponent this method will use the new values of the variables and will draw the component with the desired modifications This might seem a roundabout way of doing things Why not just draw the modifications directly There are at least two reasons First of all it really does turn out to be easier to get things right if all drawing is done
73. example the preferred size of the Center component is irrelevant but the preferred sizes of the North South East and West compo nents are used by the layout manager to decide how much space to allow for those components Standard components generally compute a correct preferred size au tomatically but it can be useful to set it in some cases For example if you use a JPanel as a drawing surface it might be a good idea to set a preferred size for it comp getParent is a function that returns a value of type java awt Container The return value is the container that directly contains the component if any For a top level component such as a JApplet the value will be null comp getLocation is a function that returns the location of the top left corner of the component The location is specified in the coordinate system of the component s parent The returned value is an object of type Point An object of type Point contains two instance variables x and y For the rest of this section we ll look at subclasses of Jcomponent that represent com mon GUI components Remember that using any component is a multi step process The component object must be created with a constructor It must be added to a container In many cases a listener must be registered to respond to events from the component And in some cases a reference to the component must be saved in an instance variable so that the component can be manipulated by the program aft
74. format javadoc can separate out the documentation and make it into a set of web pages The web pages are automati cally formatted and linked into an easily browseable Web site Sun Microsystem s on line documentation for the standard Java API was produced using javadoc Javadoc is actually very easy to use In a source code file javadoc documentation looks like a regular multi line comment except that it begins with instead of with 22 CHAPTER 1 DESIGN I CLASS LEVEL DESIGN Each such comment is associated with some class member variable or method The documentation for each item must be placed in a comment that precedes the item This is how javadoc knows which item the comment is for The comments can include certain special notations For example the notation return is used to begin the description of the return value of a function And param iparameter named marks the beginning of the description of a parameter of a method For example here is a short utility method with a javadoc comment x Return the real number represented by the string s or return Double NaN if s does not represent a legal real number param s String to interpret as real number return the real number represented by s public static double stringToReal String s try return Double parseDouble s catch NumberFormatException e return Double NalN Sun s Java Software Developmen
75. in display a heading System out println This program calculates compound interest input amount years and interest rate System out print Enter initial amount gt R amount keyboard nextDouble 11 2 STREAMS READERS AND WRITERS 235 System out print Enter years invested gt is years keyboard nextDouble System out print Enter interest rate gt rate keyboard nextDouble System out println calculate the final amount finalAmt amount Math pow 100 rate 100 years display the result System out printf R 1 5 2f invested for 2 0f years at 3 2f interest yields R 4 2f amount years rate finalAmt System out println 11 2 Streams Readers and Writers WITHOUT THE ABILITY TO INTERACT WITH the rest of the world a program would be useless The interaction of a program with the rest of the world is referred to as input output or I O Historically one of the hardest parts of programming language design has been coming up with good facilities for doing input and output A computer can be connected to many different types of input and output devices If a programming language had to deal with each type of device as a special case the complexity would be overwhelming One of the major achievements in the history of programming has been to come up with good abstractions for representing I O devices In Java the I O abstractions are called streams This section is
76. in a window an applet is a rectangular area that can contain other components such as buttons and text boxes It can display graphical elements such as images rectangles and lines And it can respond to certain events such as when the user clicks on the applet with a mouse The AWT Abstract Windowing Toolkit has been part of Java from the beginning It a toolkit API for creating graphical user interfaces It provides a hierarchy of classes useful for creating and displaying windows icons and enabling manipulation of the graphical user interface with a mouse However it has been clear that the AWT was not powerful or flexible enough for writing complex sophisticated applications The Swing graphical user interface library was created to address the problems with the AWT The Swing library is 81 82 CHAPTER 5 APPLETS HTML AND GUI S also a GUI toolkit and parts of it extensions of AWT library The classes that make up the Swing library can be found in the package javax swing You can use either the AWT or Swing libraries to create applets The AWT version of the Applet class defined in the package java applet is really only useful as a basis for making subclasses Swing includes the class javax swing JApplet to be used as a basis for writing applets JApplet is actually a subclass of Applet so JApplets are in fact Applets in the usual sense 5 1 1 Using the Applet class An object of type Applet has certain basic behaviors
77. in one method Second even if you did make modifications directly you would still have to make the paintComponent method aware of them in some way so that it will be able to redraw the component correctly when it is covered and uncovered You will see how all this works in practice as we work through examples in the rest of this chapter For now we will spend the rest of this section looking at how to get some actual drawing done 5 3 1 Coordinates The screen of a computer is a grid of little squares called pixels The color of each pixel can be set individually and drawing on the screen just means setting the colors of individual pixels A graphics context draws in a rectangle made up of pixels A position in the rectangle is specified by a pair of integer coordinates x y The upper left corner has coordinates 0 0 The x coordinate increases from left to right and the y coordinate increases from top to bottom The illustration on the right shows a 12 by 8 pixel component with very large pixels A small line rectangle and oval are shown as they would be drawn by coloring individual pixels Note that properly speaking the coordinates don t belong to the pixels but to the grid lines between them For any component you can find out the size of the rectangle that it occupies by calling the instance method getSize This method returns an object that belongs to the class java awt Dimension A Dimension object has two integer in
78. in parallel with the rest of the program When the subroutine ends the thread will die and it cannot be restarted or reused There is no stop method for stopping a thread Actually there is one but it is deprecated meaning that you are not supposed to call it If you want to be able to stop the thread you need to provide some way of telling the thread to stop itself I often do this with an instance variable named running that is visible both in the run method and elsewhere in the program When the program wants the thread to stop it just sets the value of running to false In the run method the thread checks the value of running regularly If it sees that the value of running has become false then the run method should end Here for example are the methods that the Mandelbrot applet uses to start and to stop the thread void startRunning A simple method that starts the computational thread unless it is already running This should be impossible since this method is only called when the user clicks the Start button and that button is disabled when the thread is running if running return runner new Thread this Creates a thread that will execute the run method in this class which implements Runnable running true runner start void stopRunning A simple method that is called to stop the computational thread This is done by setting the value of the variable running T
79. in the subclasses of Shape There is no reason for the abstract redraw in class Shape to contain any code at all Shape and its redraw method are semantically abstract You can also tell the com puter syntactically that they are abstract by adding the modifier abstract to their definitions For an abstract method the block of code that gives the implementation of an ordinary method is replaced by a semicolon An implementation must be provided for the abstract method in any concrete subclass of the abstract class Here s what the Shape class would look like as an abstract class abstract class Shape Color color color of shape void setColor Color newColor method to change the color of the shape color newColor change value of instance variable redraw redraw shape which will appear in new color abstract void redraw abstract method must be defined in concrete subclasses more instance variables and methods end of class Shape Once you have done this it becomes illegal to try to create actual objects of type Shape and the computer will report an error if you try to do so In Java every class that you declare has a superclass If you don t specify a superclass then the superclass is automatically taken to be Object a predefined class that is part of 3 1 INHERITANCE POLYMORPHISM AND ABSTRACT CLASSES 61 the package java lang The class Object itself has no superclass bu
80. in this way However my opinion is that making high quality Web pages still requires some work with raw HTML and serious Web authors still need to learn the HTML language The mark up commands used by HTML are called tags An HTML tag takes the form lt tag name optional modifiers gt Where the tag name is a word that specifies the command and the optional modifiers if present are used to provide additional information for the command much like param eters in subroutines A modifier takes the form modifier name value Usually the value is enclosed in quotes and it must be if it is more than one word long or if it contains certain special characters There are a few modifiers which have no value in which case only the name of the modifier is present HTML is case insensitive which means that you can use uppercase and lowercase letters interchangeably in tags and modifiers A simple example of a tag is lt HR gt which draws a line also called a horizontal rule across the page The HR tag can take several possible modifiers such as WIDTH and ALIGN For example the short line just after the heading of this page was produced by the HTML command lt HR align center width 33 gt The WIDTH here is specified as 33 of the available space It could also be given as a fixed number of pixels The value for ALIGN could be CENTER LEFT or RIGHT A LEFT alignment would shove the line to the left side of the page
81. int x 5 10 width b int 5 y 10 height where a and b are the horizontal and vertical coordinates of the pixel and width and height are the width and height of the canvas 230 CHAPTER 10 CORRECTNESS AND ROBUSTNESS Chapter 11 Basic Input Output COMPUTER PROGRAMS ARE ONLY USEFUL if they interact with the rest of the world in some way This interaction is referred to as input output or I O Up until now the only type of interaction that has been covered in this textbook is interaction with the user through either a graphical user interface or a command line interface But the user is only one possible source of information and only one possible destination for information In this chapter we ll look at others including files and network connections In Java input output involving files and networks is based on streams which are objects that support the same sort of I O commands that you have already used to communicate with the user in a command line interface In fact standard output System out and standard input System in are examples of streams Working with files and networks requires familiarity with exceptions which were intro duced in the previous chapter Many of the subroutines that are used can throw exceptions that require mandatory exception handling This generally means calling the subroutine in atry catch statement that can deal with the exception if one occurs 11 1 Input and Ouput in Ja
82. is a little more complicated than I ve indicated so far The syntax can be described as try statements optional catch clauses optional finally clause Note that this is a case where a block of statements enclosed between and is required You need the and even if they enclose just one statement The try statement can include zero or more catch clauses and optionally a finally clause The try statement must include either a finally clause or at least one catch clause The syntax for a catch clause is catch exception class name variable name statements and the syntax for a finally clause is finally statements The semantics of the finally clause is that the block of statements in the finally clause is guaranteed to be executed as the last step in the execution of the try statement whether or not any exception occurs and whether or not any exception that does occur is caught and handled The finally clause is meant for doing essential cleanup that under no circumstances should be omitted There are times when it makes sense for a program to deliberately throw an exception This is the case when the program discovers some sort of exceptional or error condition but there is no reasonable way to handle the error at the point where the problem is discovered The program can throw an exception in the hope that some other part of the program will catch and handle the exception To throw an exception u
83. is declared to throw FileNotFoundException For example suppose you have a file named data txt and you want your program to read data from that file You could do the following to create an input stream for the file FileReader data Declare the variable before the try statement or else the variable is local to the try block and you won t be able to use it later in the program try data new FileReader data txt create the stream catch FileNotFoundException e do something to handle the error maybe end the program 240 CHAPTER 11 BASIC INPUT OUTPUT The FileNotFoundException class is a subclass of IOException so it would be ac ceptable to catch IOExceptions in the above try catch statement More generally just about any error that can occur during input output operations can be caught by a catch clause that handles IOException Once you have successfully created a FileReader you can start reading data from it But since FileReaders have only the primitive input methods inherited from the basic Reader class you will probably want to wrap your FileReader in a Scanner object or in some other wrapper class Scanner data try data new Scanner new FileReader data dat catch FileNotFoundException e handle the exception Once you have a Scanner named data you can read from it using such methods as data nextInt and data next exactly as you would fr
84. it at this time Even the definitions of the shape classes are somewhat different from those I described in this section For example the draw method used in the applet has a parameter of type Graphics This parameter is required because of the way Java handles all drawing I ll return to this example in later chapters when you know more about applets However it would still be worthwhile to look at the definition of the Shape class and its subclasses in the source code for the applet You might also check how an ArrayList is used to hold a list of shapes 3 1 1 Extending Existing Classes We have been discussing subclasses but so far we have dealt mostly with the theory In the remainder of this section I want to emphasize the practical matter of Java syntax by giving an example In day to day programming especially for programmers who are just beginning to work with objects subclassing is used mainly in one situation There is an existing class that can be adapted with a few changes or additions This is much more common than designing groups of classes and subclasses from scratch The existing class can be extended to make a subclass The syntax for this is class subclass name extends existing class name Changes and additions 3 1 INHERITANCE POLYMORPHISM AND ABSTRACT CLASSES 63 Of course the class can optionally be declared to be public As an example suppose you want to write a program that plays the card game Black
85. it is for any other non static part of a class The non static members of a class specify what will be contained in objects that are created from that class The same is true at least logically for non static nested classes It s as if each object that belongs to the containing class has its own copy of the nested class This copy has access to all the instance methods and instance variables of the object Two copies of the nested class in different objects differ because the instance variables and methods they refer to are in different objects In fact the rule for deciding whether a nested class should be static or non static is simple If the class needs to use any instance variable or instance method make it non static Otherwise it might as well be static From outside the containing class a non static nested class has to be referred to as variableName NestedClassName where variableName is a variable that refers to the object that contains the class This is actually rather rare however A non static nested class is generally used only inside the class in which it is nested and there it can be referred to by its simple name In order to create an object that belongs to a non static nested class you must first have an object that belongs to the containing class When working inside the class the object this is used implicitly The nested class object is permanently associated with the containing class object and it has comp
86. list of formal parameters In fact the ability to include parameters is one of the main reasons for using constructors The parameters can provide data to be used in the construction of the object For example a constructor for the PairOfDice class could provide the values that are initially showing on the dice Here is what the class would look like in that case public class PairOfDice public int die1 Number showing on the first die public int die2 Number showing on the second die public PairOfDice int val1 int val2 Constructor Creates a pair of dice that are initially showing the values vali and val2 diet vali Assign specified values die2 val2 to the instance variables public void roll Roll the dice by setting each of the dice to be a random number between 1 and 6 diel int Math random 6 1 die2 int Math random 6 1 end class PairOfDice The constructor is declared as public PairOfDice int vall int val2 with no return type and with the same name as the name of the class This is how the Java compiler recognizes a constructor The constructor has two parameters and values for these parameters must be provided when the constructor is called For example the ex pression new PairOfDice 3 4 would create a PairOfDice object in which the values of the instance variables die1 and die 2 are initially 3 and 4 Of course in a program the value return
87. mnemonic which is a character that is underlined in the name Instead of clicking on the menu you can select it by pressing the mnemonic key while holding down the ALT key This assumes that the applet has the keyboard focus Once the menu has appeared you can select an item in the menu by clicking on it or by using the arrow keys to select the item and then pressing return It is possible to assign mnemonics to individual items in a menu but I haven t done that in this example The commands in the Add menu and the Clear command do have accelerators An accelerator is a key or combination of keys that can be pressed to invoke a menu item without ever opening the menu The accelerator is shown in the menu next to the name of the item For example the accelerator for the Rectangle command in the Add menu is Ctrl R This means that you can invoke the command by holding down the Control key and pressing the R key The commands in the Color menu act like a set of radio buttons Only one item in the menu can be selected at a given time The selected item in this menu determines the color of newly added shapes Similarly two of the commands in the Options menu act just like checkboxes The first of these items determines whether newly added shapes will be large or small The second determines whether newly added shapes will have a black border drawn around them The last item in the Options menu is ac
88. object is returned as the value of the constructor call The end result of this is that you have a reference to a newly constructed object You can use this reference to get at the instance variables in that object or to call its instance methods For another example consider the Student class Pll add a constructor and I ll also take the opportunity to make the instance variable name private 2 2 CONSTRUCTORS AND OBJECT INITIALIZATION 35 public class Student private String name Student s name public double test1 test2 test3 Grades on three tests Student String theName Constructor for Student objects provides a name for the Student name theName public String getName Accessor method for reading value of private instance variable name return name public double getAverage Compute average test grade return testi test2 test3 3 end of class Student An object of type Student contains information about some particular student The constructor in this class has a parameter of type String which specifies the name of that student Objects of type Student can be created with statements such as std new Student John Smith stdi new Student Mary Jones In the original version of this class the value of name had to be assigned by a program after it created the object of type Student There was no guarantee that the programmer would always remember to set t
89. oe fot 5 Use Case View saatan 5 DEG 5 Book flight flight booker p4 Book flight Book flight ts i tas l Call center Book flight via phone Book flight at Travel Agency 77 a 2 Customer 4 amp flight booker Ye i Quse case dia Call center agent Customer Documentation bi 4 a gt Ready p Figure 6 1 Umbrello UML Modeller showing a Use Case Diagram e Component Diagrams show the high level programming components such as KParts or Java Beans e Deployment Diagrams show the instances of the components and their relationships Use Case Diagrams Use Case Diagrams describe the relationships and dependencies between a group of Use Cases and the Actors participating in the process It is important to notice that Use Case Diagrams are not suited to represent the design and cannot describe the internals of a system Use Case Diagrams are meant to facilitate the communication with the future users of the system and with the customer and are specially helpful to determine the required features the system is to have Use Case Di agrams tell what the system should do but do not and cannot specify how this is to be achieved A Use Case describes from the point of view of the actors a group of activities in a system that produces a concrete tangible result Use Cases are descriptions of the typical interactions between the users of a system
90. of selection void setEditable boolean canBeEdited specify whether or not the text in the component can be edited by the user The requestFocus method inherited from JComponent is also useful for text com ponents The constructor for a JTextField takes the form JTextField int columns where columns specifies the number of characters that should be visible in the text field This is used to determine the preferred width of the text field Because characters can be of different sizes the number of characters visible in the text field might not be exactly equal to columns You don t have to specify the number of columns for example you might use the text field in a context where it will expand to the maximum size avail able In that case you can use the constructor JTextField with no parameters You can also use the following constructors which specify the initial contents of the text field JTextField String contents JTextField String contents int columns JTextField has a subclass JPasswordField which is identical except that it does not reveal the text that it contains The characters in a JPasswordField are all displayed as asterisks or some other fixed character A password field is obviously designed to let the user enter a password without showing that password on the screen The constructors for a JTextArea are JTextArea JTextArea int lines int columns JTextArea String contents JTe
91. of the component You can also get the height and width more directly by calling comp getHeight and comp getWidth One warning When a component is first created its size is zero The size will be set later probably by a layout manager A common mistake is to check the size of a component before that size has been set such as in a constructor e comp setEnabled true and comp setEnabled false can be used to enable and disable the component When a component is disabled its appearance changes and the user cannot do anything with it There is a boolean function comp getEnabled that you can call to discover whether the component is enabled e comp setVisible true and comp setVisible false can be called to hide or show the component e comp setBackground color and comp setForeground color set the background and foreground colors for the component If no colors are set for a component the colors are determined by the look and feel 156 CHAPTER 8 ADVANCED GUI PROGRAMMING comp setOpaque true tells the component that the area occupied by the compo nent should be filled with the component s background color before the content of the component is painted In the default look and feel only JLabels are non opaque A non opaque or transparent component ignores its background color and simply paints its content over the content of its container This usually means that it inherits the background color from its cont
92. of writing empty method definitions in cases like this Java provides adapter classes An adapter class implements a listener interface by providing empty definitions for all the methods in the interface An adapter class is only useful as a basis for making subclasses In the subclass you can define just those methods that you actually want to use For the remaining methods the empty definitions that are provided by the adapter class will be used The adapter class for the MouseListener interface is named MouseAdapter For example if you want a mouse listener that only responds to mouse pressed events you can use a command of the form component addMouseListener new MouseAdapter public void mousePressed MouseEvent evt To see how this works in a real example let s write another version of the applet ClickableRandomStrings that uses an anonymous class based on MouseAdapter to han dle mouse events 106 CHAPTER 5 APPLETS HTML AND GUI S import java awt event public class ClickableRandomStrings2 extends RandomStrings public void init When the applet is created do the initialization of the superclass RandomStrings Then add a mouse listener to listen for mouse events on the drawingSurface drawingSurface is defined in the superclass RandomStrings super init drawingSurface addMouseListener new MouseAdapter public void mousePressed MouseEvent evt When user press
93. other hand an actual object has a fixed type The type of the object that a variable references is sometimes called the run time type of that variable A cast tells the compiler that it may assume that the object has the type that the cast says If at run time the object doesn t have the claimed type you will get a ClassCastException Suppose class Rectangle extends class Shape 3 3 SUMMARY 71 Shape X Rectangle Y Y new Rectangle Y okay new Rectangle okay Rectangle X cast needed X X Y Casting is also used for primitive data types but this is more like a conversion For example System out print char A 2 prints out C without the char cast it prints out 67 the code for C For example producing an equals method for the Coord class mentioned earlier public boolean equals Object other if other instanceof Coord Coord otherPoint Coord other return this x otherPoint x amp amp this y otherPoint y else return false 3 3 5 Polymorphism Polymorphism multiple forms is achieved in Java through over riding methods in a sub class For example suppose we have a class Sound that has a method play We might have sub classes Flute and Drum that extend Sound and over ride this method to provide effects specific to that instrument Thus objects which are Sounds can come in many forms Java ensures that the appropriate form of the method is used Hence we can obta
94. produce the release version The assertion facility in Java 1 4 and later takes all this into account A new assert statement is introduced into the language that has the syntax assert condition error message The condition in this statement is a boolean valued expression The idea is that this condition is something that is supposed to be true at that point in the program if the program is correct The error message is generally a string though in fact it can be an expression of any type When an assert statement is executed the expression in the statement is evaluated If the condition is true the assertion has no effect and the program proceeds with the next statement If the condition is false then an error of type AssertionError is thrown and this will cause the program to crash The error message is passed to the AssertionError object and becomes part of the error message that is printed when the program is terminated Of course it s possible to catch the AssertionError to stop the program from crashing but the whole point of an assertion is to make the program crash if it has gotten into some state where a necessary condition is false By default however assert statements are not executed Remember that assertions should only be executed during testing and debugging so there has to be some way to turn them on and off In C C this is done at compile time in Java it is done at run time When you run a program in the ordinary
95. range In Java any attempt to do so is detected automatically by the system In some other languages such as C and C it s up to the programmer to make sure that the index is within the legal range Suppose that an array A has three locations A 0 A 1 and A 2 Then A 3 A 4 and so on refer to memory locations beyond the end of the array In Java an attempt to store data in A 3 will be detected The program will be terminated unless the error is caught In C or C the computer will just go ahead and store the data in memory that is not part of the array Since there is no telling what that memory location is being used for the result will be unpredictable The consequences could be much more serious than a terminated program See for example the discussion of buffer overflow errors later in this section Pointers are a notorious source of programming errors In Java a variable of object type holds either a pointer to an object or the special value null Any attempt to use a null value as if it were a pointer to an actual object will be detected by the system In some other languages again it s up to the programmer to avoid such null pointer errors In my old Macintosh computer a null pointer was actually implemented as if it were a pointer to memory location zero A program could use a null pointer to change values stored in memory near location zero Unfortunately the Macintosh stored important system data in those
96. representation of the object For example if card refers to an object of type Card then System out println card is equivalent to System out println card toString Similarly if an object is appended to a string using the operator the object s toSring method is used Thus System out println Your card is the card is equivalent to System out println Your card is the card toString If the card is the queen of hearts either of these will print out Your card is the Queen of Hearts Here is the complete Card class It is general enough to be highly reusable so the work that went into designing writing and testing it pays off handsomely in the long run CHAPTER 2 OBJECT BASED PROGRAMMING An object of class card represents one of the 52 cards ina standard deck of playing cards Each card has a suit and a value public class Card public final static int SPADES 0 Codes for the 4 suits HEARTS 1 DIAMONDS 2 CLUBS 3 public final static int ACE 1 Codes for non numeric cards JACK 11 Cards 2 through 10 have QUEEN 12 their numerical values KING 13 for their codes private final int suit The suit of this card one of the four constants SPADES HEARTS DIAMONDS CLUBS private final int value The value of this card from 1 to 13 public Card int theValue int theSuit Construct a card with the specified value and suit Value m
97. s not a problem It s just a command to be executed The source code for the frame class is in the file ShapeDrawFrame java The applet shown above which opens a frame of this type is in ShapeDrawLauncherjava We ll look at a few more fine points of programming with frames by looking at another example In this case it s a frame version of the HighLowGUI2 applet Click on the button to open the frame Applet Reference see Applet HighLowLauncher The frame in this example is defined in the file HighLowFrame java In many ways this example is similar to the previous one but there are several differences You can resize the frame in the first example by dragging its border or corner but if you try to resize the High Low frame you will find that it is impossible A frame is resizable by default You can make it non resizable by calling setResizable false I do this in the constructor of the HighLowFrame class Another difference shows up if you click the frame s close box This will not simply close the window Instead a new window will open to ask you whether 5 6 FRAMES AND DIALOGS 119 Lower New Game Close HighLow Figure 5 7 HighLowFrame Applet you really want to close the HighLow frame The new window is an example of a dialog box You have to click a button in the dialog box If you click on Yes the HighLow frame will be closed if not the frame will remain open This would be more useful i
98. sorting methods which take arrays of Objects that implement Comparable A more general idea is a Comparator Suppose for example one wanted to sort Strings in a different way to the standard way The idea is to define a comparator which specifies how two Strings are to be compared with methods compare and equals The comparator is then a parameter to the constructor of the Collection 4 2 Nested Classes A class seems like it should be a pretty important thing A class is a high level building block of a program representing a potentially complex idea and its associated data and behaviors I ve always felt a bit silly writing tiny little classes that exist only to group a few scraps of data together However such trivial classes are often useful and even essential Fortunately in Java I can ease the embarrassment because one class can be nested inside another class My trivial little class doesn t have to stand on its own It becomes part of a larger more respectable class This is particularly useful when you want to create a little class specifically to support the work of a larger class And more seriously there are other good reasons for nesting the definition of one class inside another class In Java a nested class or inner class is any class whose definition is inside the definition of another class Inner classes can be either named or anonymous I will come back to the topic of anonymous classes later in this section A named i
99. standard things that a computer has to do Windows 98 and Windows 2000 provide their own sets of subrou tines for programmers to use and they are quite a bit different from the subroutines used on the Mac The analogy of a toolbox is a good one to keep in mind Every programming project involves a mixture of innovation and reuse of existing tools A programmer is given a set of tools to work with starting with the set of basic tools that are built into the lan guage things like variables assignment statements if statements and loops To these the programmer can add existing toolboxes full of routines that have already been written for performing certain tasks These tools if they are well designed can be used as true black boxes They can be called to perform their assigned tasks without worrying about the particular steps they go through to accomplish those tasks The innovative part of pro gramming is to take all these tools and apply them to some particular project or problem word processing keeping track of bank accounts processing image data from a space probe Web browsing computer games This is called applications programming A software toolbox is a kind of black box and it presents a certain interface to the programmer This interface is a specification of what routines are in the toolbox what parameters they use and what tasks they perform This information constitutes the API or Applications Programming Interfa
100. that it is the interface that is platform independent the imple mentation varies from one platform to another A Java system on a particular computer includes implementations of all the standard API routines A Java program includes only calls to those routines When the Java interpreter executes a program and encounters a call to one of the standard routines it will pull up and execute the implementation of that routine which is appropriate for the particular platform on which it is running This is a very powerful idea It means that you only need to learn one API to program for a wide variety of platforms Like all subroutines in Java the routines in the standard API are grouped into classes To provide larger scale organization classes in Java can be grouped into packages You can have even higher levels of grouping since packages can also contain other packages In fact the entire standard Java API is implemented in several packages One of these which is named java contains the non GUI packages as well as the original AWT graphics user interface classes Another package javax was added in Java version 1 2 and contains the classes used by the Swing graphical user interface A package can contain both classes and other packages A package that is contained in another package is sometimes called a sub package Both the java package and the javax package contain sub packages One of the sub packages of java for example is
101. the applet in the so called North South East and West positions A component in the Center position fills in all the remaining space This type of layout is called a BorderLayout In our example the button bar occupies the South position and the drawing area fills the Center position When you add a component to a BorderLayout you have to specify its position using a constant such as BorderLayout SOUTH or BorderLayout CENTER In this example buttonBar is added to the applet with the command getContentPane add buttonBar BorderLayout SQUTH The display area of the applet is a drawing surface like those we have seen in other examples A nested class named Display is created as a subclass of JPanel and the display area is created as an object belonging to that class The applet class has an instance variable named display of type Display to represent the drawing surface The display object is simply created and added to the applet with the commands 5 5 INTRODUCTION TO LAYOUTS AND COMPONENTS 109 display new Display getContentPane add display BorderLayout CENTER Putting this all together the complete init method for the applet becomes public void init display new Display Q The component that displays Hello World getContentPane add display BorderLayout CENTER Adds the display panel to the CENTER position of the JApplet s content pane JPan
102. the command g setFont bigBoldFont will set the current font of g to bigBoldFont You can find out the current font of g by calling the method g getFont which returns an object of type Font Every component has an associated font It can be set with the instance method setFont font which is defined in the Component class When a graphics context is created for drawing on a component the graphic context s current font is set equal to the font of the component 5 3 4 Shapes The Graphics class includes a large number of instance methods for drawing various shapes such as lines rectangles and ovals The shapes are specified using the x y coordinate system described above They are drawn in the current drawing color of the graphics context The current drawing color is set to the foreground color of the compo nent when the graphics context is created but it can be changed at any time using the setColor method Here is a list of some of the most important drawing methods With all these com mands any drawing that is done outside the boundaries of the component is ignored Note that all these methods are in the Graphics class so they all must be called through an object of type Graphics e drawString String str int x int y Draws the text given by the string str The string is drawn using the current color and font of the graphics context x specifies the position of the left end of the string y is the y coordinate of the basel
103. the file Write two lists to the output file The first list contains the words in alphabetical order The number of times that the word occurred in the file should be listed along with the word Then write a second list to the output file in which the words are sorted according to the number of times that they occurred in the files The word that occurred most often should be listed first 2 Write a program that will count the number of lines in each file that is specified on the command line Assume that the files are text files Note that multiple files can be specified as in java LineCounts file1 txt file2 txt file3 txt Write each file name along with the number of lines in that file to standard output If an error occurs while trying to read from one of the files you should print an error message for that file but you should still process all the remaining files 3 APhoneDirectory holds a list of names and associated phone numbers But a phone directory is pretty useless unless the data in the directory can be saved permanently that is in a file Write a phone directory program that keeps its list of names and phone numbers in a file The user of the program should be able to look up a name 256 CHAPTER 11 BASIC INPUT OUTPUT in the directory to find the associated phone number The user should also be able to make changes to the data in the directory Every time the program starts up it should read the data from the file
104. the user clicked on the square Keep track of and display the number of times that the user hits the square and the number of times that the user misses it Don t assume that you know the size of the applet in advance 8 Write a Blackjack applet that lets the user play a game of Blackjack with the com puter as the dealer The applet should draw the user s cards and the dealer s cards just as was done for the graphical HighLow card game You can use the source code for that game HighLowGUl java for some ideas about how to write your Black jack game The structures of the HighLow applet and the Blackjack applet are very similar You will certainly want to use the drawCard method from that applet You can find a description of the game of Blackjack in Exercise 5 5 Add the following rule to that description If a player takes five cards without going over 21 that player wins immediately This rule is used in some casinos For your applet it means that you only have to allow room for five cards You should assume that your applet is just wide enough to show five cards and that it is tall enough to show the user s hand and the dealer s hand Note that the design of a GUI Blackjack game is very different from the design of the text oriented program that you wrote for Exercise 5 5 The user should play the game by clicking on Hit and Stand buttons There should be a New Game button that can be used to start another gam
105. the variables colorNum and textFont int colorNum Keeps track of which color is displayed 1 for red 2 for green 3 for blue Font textFont The font in which the message is displayed A font object represents a certain size and style of text drawn on the screen Display Constructor for the Display class Set the background color and assign initial values to the instance variables colorNum and textFont setBackground Color black colorNum 1 The color of the message is set to red textFont new Font Serif Font BOLD 36 Create a font object representing a big bold font void setColor int code This method is provided to be called by the main class when it wants to set the color of the message The parameter value should be 1 2 or 3 to indicate the desired color colorNum code repaint Tell the system to repaint this component 5 5 INTRODUCTION TO LAYOUTS AND COMPONENTS 111 public void paintComponent Graphics g This routine is called by the system whenever this panel needs to be drawn or redrawn It first calls super paintComponent to fill the panel with the background color It then displays the message Hello World in the proper color and font super paintComponent g switch colorNum Set the color case 1 g setColor Color red break case 2 g setColor Color green break case 3 g setColor Color blue br
106. time that the user presses a key or moves the mouse and the time that a subroutine in your program is called to respond to the event Fortu nately you don t need to know much about that processing But you should understand this much Even though your GUI program doesn t have a main routine there is a sort of main routine running somewhere that executes a loop of the form while the program is still running Wait for the next event to occur Call a subroutine to handle the event This loop is called an event loop Every GUI program has an event loop In Java you don t have to write the loop It s part of the system If you write a GUI program in some other language you might have to provide a main routine that runs an event loop For an event to have any effect a program must detect the event and react to it In order to detect an event the program must listen for it Listening for events is something that is done by an object called an event listener An event listener object must contain instance methods for handling the events for which it listens For example if an object is to serve as a listener for events of type MouseEvent then it must contain the following method among several others public void mousePressed MouseEvent evt The body of the method defines how the object responds when it is notified that a mouse button has been pressed The parameter evt contains information about the event This inf
107. was destroyed int where_x where_y Location where ship was destroyed ShipDestroyed String message Ship s int x int y Constructor Create a ShipDestroyed object carrying an error message and the information that the ship s was destroyed at location x y on the screen super message ship s where_x x where_y y Here a ShipDestroyed object contains an error message and some information about a ship that was destroyed This could be used for example in a statement if userShip isHit throw new ShipDestroyed You ve been hit userShip xPos yPos Note that the condition represented by a ShipDestroyed object might not even be considered an error It could be just an expected interruption to the normal flow of a game Exceptions can sometimes be used to handle such interruptions neatly 10 4 2 Exceptions in Subroutines and Classes The ability to throw exceptions is particularly useful in writing general purpose subrou tines and classes that are meant to be used in more than one program In this case the person writing the subroutine or class often has no reasonable way of handling the error since that person has no way of knowing exactly how the subroutine or class will be used In such circumstances a novice programmer is often tempted to print an error message and forge ahead but this is almost never satisfactory since it can lead to unpredictable results down the line Printing an erro
108. way using the java command assertions in the program are ignored To have an effect they must be enabled This is done by adding an option to the java command The form of the option is enableassertions class name 10 5 QUIZ QUESTIONS 225 to enable all the assertions in a specified class or enableassertions package name to enable all the assertions in a package and in its sub packages To enable assertions in the default package that is classes that are not specified to belong to a package like almost all the classes in this book use enableassertions You can abbreviate enableassertions as ea and you can use this option several times in the same com mand For example to run a Java program named MegaPaint with assertions enabled for the packages named paintutils and drawing you would use the command java ea paintutils ea drawing MegaPaint Remember that you would use the ea options during development of the program but your customers would not have to use them when they run your program 10 5 Quiz Questions THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 9 of this on line Java textbook You should be able to answer these questions after studying that chapter Question 1 What does it mean to say that a program is robust Question 2 Why do programming languages require that variables be declared before they are used What does this have to
109. when it is inactive In particular an applet should probably not leave a timer running when it is inactive The HelloWorldSpectrum applet defines the stop method to call stopAnimation which in turn will stop the timer if it is running When the applet is about to become inactive the system will call stop and the animation if it was running will be stopped You can try it if you are reading this page in a Web browser Start the animation running in the above applet go to a different page and then come back to this page You should see that the animation has been stopped The animation in the HelloWorldSpectrum applet is started and stopped under the control of the user In many cases we want an animation to run for the entire time that an applet is active In that case the animation can be started in the applet s start method and stopped in the applet s stop method It would also be possible to start the animation in the init method and stop it in the destroy method but that would leave the animation running uselessly while the applet is inactive In the following example a message is scrolled across the page It uses a timer which churns out events for the whole time the applet is active Hello World for abs Figure 8 5 HelloWorldApplet2 You can find the source code in the file ScrollingHelloWorld java but the relevant part here is the start and stop methods 8 5 TIMERS ANIMATION AND THREADS 1
110. where a programmer neglects to delete objects that are no longer in use This can lead to filling memory with objects that are completely inaccessible and the program might run out of memory even though in fact large amounts of memory are being wasted Because Java uses garbage collection such errors are simply impossible Garbage col lection is an old idea and has been used in some programming languages since the 1960s You might wonder why all languages don t use garbage collection In the past it was considered too slow and wasteful However research into garbage collection techniques combined with the incredible speed of modern computers have combined to make garbage collection feasible Programmers should rejoice 2 3 Programming with Objects THERE ARE SEVERAL WAYS in which object oriented concepts can be applied to the pro cess of designing and writing programs The broadest of these is object oriented analysis and design which applies an object oriented methodology to the earliest stages of program development during which the overall design of a program is created Here the idea is to identify things in the problem domain that can be modeled as objects On another level object oriented programming encourages programmers to produce generalized software components that can be used in a wide variety of programming projects 2 3 1 Built in Classes Although the focus of object oriented programming is generally on the design and im
111. with random colors Set the current position to the middle square in the window As long as the mosaic window is open Randomly change color of current square Move current position up down left or right at random I need to represent the current position in some way That can be done with two int variables named currentRow and currentColumn I ll use 10 rows and 20 columns of squares in my mosaic so setting the current position to be in the center means setting currentRow to 5 and currentColumn to 10 I already have a subroutine Mosaic open to open the window and I have a function Mosaic isOpen to test whether the window is open To keep the main routine simple I decide that I will write two more subroutines of my own to carry out the two tasks in the while loop The algorithm can then be written in Java as Mosaic open 10 20 10 10 il1WithRandomColors currentRow 5 Middle row halfway down the window currentColumn 10 Middle column while Mosaic isOpen changeToRandomColor currentRow currentColumn randomMove 1 1 PROGRAM DESIGN 15 With the proper wrapper this is essentially the main routine of my program It turns out I have to make one small modification To prevent the animation from running too fast the line Mosaic delay 20 is added to the while loop The main routine is taken care of but to complete the program I still have to write the subroutines f illWithRandomColo
112. working with templates which were mentioned in the first Java 1 5 Note on this page For example if integerList is a variable of type ArrayList lt Integer gt you can say integerList add 42 and it will be automatically interpreted by the compiler as integerList add newInteger 42 9 4 List Classes IN THE PREVIOUS SECTION we looked at the general properties of collection classes in Java In this section we look at some specific collection classes and how to use them These classes can be divided into two categories lists and sets A list consists of a sequence of items arranged in a linear order A list has a definite order but is not necessarily sorted into ascending order A set is a collection that has no duplicate entries The elements of a set might or might not be arranged into some definite order As with all of Java s collection classes the items in a list or set are of type Object 9 4 1 The ArrayList and LinkedList Classes There are two obvious ways to represent a list as a dynamic array and as a linked list We ve encountered these already in Sections 8 3 and 11 2 Both of these options are avail able in generic form as the collection s java util ArrayList and java util LinkedList That is a ArrayList represents an ordered sequence of objects stored in an array that will grow in size as new items are added and a LinkedList represents an ordered sequence of objects stored in nodes that are linked togeth
113. you use arrays in a clever way to represent the data in the above table Once you ve written your class use it in a main program that will read both Arabic numerals and Roman numerals entered by the user If the user enters an Arabic numeral print the corresponding Roman numeral If the user enters a Roman nu meral print the corresponding Arabic numeral The program should end when the user inputs an empty line The file Expr java defines a class Expr that can be used to represent mathematical expressions involving the variable x The expression can use the operators and where represents the operation of raising a number to a power It can use mathematical functions such as sin cos abs and 1n See the source code file for full details The Expr class uses some advanced techniques which have not yet been covered in this textbook However the interface is easy to understand It contains only a constructor and two public methods The constructor new Expr def creates an Expr object defined by a given expres sion The parameter def is a string that contains the definition For example new Expr x72 or new Expr sin x 3 x If the parameter in the con structor call does not represent a legal expression then the constructor throws an IllegalArgumentException The message in the exception describes the error If func is a variable of type Expr and nun is of type double then func value num
114. 150 gt lt APPLET gt lt P gt This assumes that the file HelloWorldApplet class is located in the same directory with the HTML document If this is not the case you can use another modifier CODEBASE to give the URL of the directory that contains the class file The value of CODE itself is always just a file name not a URL If an applet uses a lot of class files it s a good idea to collect all the class files into a single zip or jar file Zip and jar files are archive files which hold a number of smaller files Your Java development system is probably capable of creating them in some way If your class files are in an archive then you have to specify the name of the archive file in an ARCHIVE modifier in the lt APPLET gt tag Archive files won t work on older browsers but they should work for any browser that understands Java version 1 1 or later Applets can use applet parameters to customize their behavior Applet parameters are specified by using lt PARAM gt tags which can only occur between an lt APPLET gt tag and the closing lt APPLET gt The PARAM tag has required modifiers named NAME and VALUE and it takes the form lt PARAM NAME param name VALUE param value gt The parameters are available to the applet when it runs An applet can use the pre defined method getParameter to check for parameters specified in PARAM tags The getParameter method has the following interface String
115. 5 1 The Basic Java Applet and JApplet 5 1 1 Using the Applet class 24 305 4 404 4 cgcesara amp amp 8839 bal 2 Using JApplet r re Aig a ea a eR ey ae 4 5 2 SHTME Basics 24 ese 2 ie gente asad ee Eee Be pe ee 5 2 1 Overall Document Structure 5 2 2 The Applet tag and Applet Parameters 5 3 Graphics and Painting ern oy eka aie ae HOS OE ere oe 5 3 1 Coordinates 5 d 6 8 eo hele tee on a 5 3 2 COLPS 424m de 6 Sate hs Sie Acetate oer e bik a DO FONTS eo AE are ds e ace Nt dale aw ubr gh Golde AE Gets A Bio Shapes s ni Ace e Ge Orde Rak Ao Se eee g BA Mous Events n aed ets Gi ies a dee eg a ce So E T e 5 4 1 MouseEvent and MouseListener 5 4 2 Anonymous Event Handlers and Adapter Classes 5 5 Introduction to Layouts and Components 5 6 Frames and Dialogs gaa 4S 4 ee ea ek re eR 5 6 1 Images in Applications 5 0 2 Dialogs pis at Parge gk eed wae Be he ee 5 7 QUIZ QUCSHONS hos ese eh Gch ai ial he RO RANE A 5 8 Programming Exercises 22000 6 Design II Object Oriented Design 6 1 Object oriented Analysis and Design 6 1 1 Software Engineering Life Cycles 6 1 2 Object Oriented design 0 6 2 The Unified Modelling Language Class Associations i 4 4 6M bod ed Re we 7 A Solitaire Game Klondike 8 Advanced GUI Programming 8 1 More about Layouts a
116. 5 4 HelloWorldJApplet In the previous JApplets that we ve looked at the entire applet was filled with a JPanel that served as a drawing surface In this example there are two JPanels the large black area at the top that displays the message and the smaller area at the bottom that holds the three buttons Let s first consider the panel that contains the buttons This panel is created in the apple s init method as a variable named buttonBar of type JPanel JPanel buttonBar new JPanel 108 CHAPTER 5 APPLETS HTML AND GUI S When a panel is to be used as a drawing surface it is necessary to create a subclass of the JPanel class and include a paintComponent method to do the drawing However when a JPanel is just being used as a container there is no need to create a subclass A standard JPanel is already capable of holding components of any type Once the panel has been created the three buttons are created and are added to the panel A button is just an object belonging to the class javax swing JButton When a button is created the text that will be shown on the button is provided as a parameter to the constructor The first button in the panel is created with the command JButton redButton new JButton Red This button is added to the buttonBar panel with the command buttonBar add redButton Every JPanel comes automatically with a layout manager This default layout manager will simply line up the componen
117. 75 public void start Called when the applet is being started or restarted Create a new timer or restart the existing timer if timer null This method is being called for the first time since the timer does not yet exist timer new Timer 300 this Applet listens for events timer start else timer restart public void stop Called when the applet is about to be stopped Stop the timer timer stop These methods can be called several times during the lifetime of an applet The first time start is called it creates and starts a timer If start is called again the timer already exists and the existing timer is simply restarted 8 5 3 Other Useful Timer Methods Although timers are most often used to generate a sequence of events a timer can also be configured to generate a single event after a specified amount of time In this case the timer is being used as an alarm which will signal the program after a specified amount of time has passed To use a Timer object in this way call setRepeats false after constructing it For example Timer alarm new Timer 5000 listener alarm setRepeats false alarm start The timer will send one action event to the listener five seconds 5000 milliseconds after it is started You can cancel the alarm before it goes off by calling its stop method Here s one final way to control the timing of events When you star
118. Data class and name and age are instance variables in the object It is important to remember that the class of an object determines the types of the instance variables however the actual data is contained inside the individual objects not the class Thus each object has its own set of data An applet that scrolls a message across a Web page might include a subroutine named scroll Since the applet is an object this subroutine is an instance method of the applet The source code for the method is in the class that is used to create the applet 2 1 OBJECTS INSTANCE METHODS AND INSTANCE VARIABLES 25 Still it s better to think of the instance method as belonging to the object not to the class The non static subroutines in the class merely specify the instance methods that every object created from the class will contain The scrol1 methods in two different applets do the same thing in the sense that they both scroll messages across the screen But there is a real difference between the two scro11 methods The messages that they scroll can be different You might say that the subroutine definition in the class specifies what type of behavior the objects will have but the specific behavior can vary from object to object depending on the values of their instance variables As you can see the static and the non static portions of a class are very different things and serve very different purposes Many classes contain only static memb
119. JButton divide new JButton divide addActionListener this buttonPanel add divide Create the label for displaying the answer in red on a white background The label is set to be opaque to make sure that the white background is painted answer new JLabel x y 0 JLabel CENTER answer setForeground Color red answer setBackground Color white answer setOpaque true Set up the layout for the applet using a GridLayout and add all the components that have been created content setLayout new GridLayout 4 1 2 2 content add xPanel content add yPanel content add buttonPanel content add answer Try to give the input focus to xInput which is the natural place for the user to start xInput requestFocus end init The action of the applet takes place in the actionPerformed method The algorithm for this method is simple get the number from the input box xInput get the number from the input box yInput get the action command the name of the button if the command is add the numbers and display the result in the answer label else if the command is subtract the numbers and display the result in the label else if the command is multiply the numbers and display the result in the label else if the command is divide the numbers and display the result in the label There is one problem with this When we call xInput getText and yInput getText to g
120. M AND ABSTRACT CLASSES 59 BeveledRects Figure 3 5 BeveledRects One of the most beautiful things about polymorphism is that it lets code that you write do things that you didn t even conceive of at the time you wrote it If for some reason I decide that I want to add beveled rectangles to the types of shapes my program can deal with I can write a new subclass BeveledRect of class Shape and give it its own redraw method Automatically code that I wrote previously such as the statement oneShape redraw can now suddenly start drawing beveled rectangles even though the beveled rectangle class didn t exist when I wrote the statement In the statement oneShape redraw the redraw message is sent to the object oneShape Look back at the method from the Shape class for changing the color of a shape void setColor Color newColor color newColor change value of instance variable redraw redraw shape which will appear in new color A redraw message is sent here but which object is it sent to Well the setColor method is itself a message that was sent to some object The answer is that the redraw message is sent to that same object the one that received the setColor message If that object is a rectangle then it is the redraw method from the Rectangle class that is executed If the object is an oval then it is the redraw method from the Oval class This is what you should expect but it means that
121. MOUSE EVENTS 105 5 4 2 Anonymous Event Handlers and Adapter Classes As I mentioned above it is a fairly common practice to use anonymous nested classes to define listener objects A special form of the new operator is used to create an object that belongs to an anonymous class For example a mouse listener object can be created with an expression of the form new MouseListener public void mousePressed MouseEvent evt public void mouseReleased MouseEvent evt public void mouseClicked MouseEvent evt public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt This is all just one long expression that both defines an un named class and creates an object that belongs to that class To use the object as a mouse listener it should be passed as the parameter to some component s addMouseListener method in a command of the form component addMouseListener new MouseListener public void mousePressed MouseEvent evt public void mouseReleased MouseEvent evt public void mouseClicked MouseEvent evt public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt Now in a typical application most of the method definitions in this class will be empty A class that implements an interface must provide definitions for all the methods in that interface even if the definitions are empty To avoid the tedium
122. NCED GUI PROGRAMMING system at various times in the applet s life cycle We have already seen that init is called when the applet is first created before it appears on the screen Another method destroy is called just before the applet is destroyed to give it a chance to clean things up Two other applet methods start and stop are called by the system between init and destroy The start method is always called by the system just after init and stop is always called just before destroy However start and stop can also be called at other times The reason is that an applet is not necessarily active for the whole time that it exists Suppose that you are viewing a Web page that contains an applet and suppose you follow a link to another page The applet still exists It will be there if you hit your browser s Back button to return to the page that contains the applet However the page containing the applet is not visible so the user can t see the applet or interact with it The applet will not receive any events from the user and since it is not visible it will not be asked to paint itself The system calls the applet s stop method when user leaves the page that contains the applet The system will call the applet s start method if the user returns to that page This lets the applet keep track of when it is active and when it is inactive It might want to do this so that it can avoid using system resources
123. Now suppose that you write a subroutine that uses the operator to add up the items in a list The subroutine can be applied to a list of integers but it can also be applied automatically to any other data type for which is defined Similarly a subroutine that uses the lt operator to sort a list can be applied to lists containing any type of data for which lt is defined There is no need to write a different sorting subroutine for each type of data Put these two features together and you have a language where data structures and algorithms will work for any type of data for which they make sense that is for which the appropriate operations are defined This is real generic programming This might sound pretty good and you might be asking yourself why all programming languages don t work this way This type of freedom makes it easier to write programs but unfortunately it makes it harder to write programs that are correct and robust See Chapter9 Once you have a data structure that can contain data of any type it becomes hard to ensure that it only holds the type of data that you want it to hold If you have a subroutine that can sort any type of data it s hard to ensure that it will only be applied to data for which the lt operator is defined More particularly there is no way for a compiler to ensure these things The problem will show up at run time when an attempt is made to apply some operation t
124. Object Oriented Programming using Java School of Computer Science University of KwaZulu Natal January 31 2006 Object Oriented Programming using Java Notes for the Computer Science Module Object Oriented Programming COMP200 Adapted from Introduction to Programming Using Java Version 4 1 June 2004 by David J Eck http math hws edu javanotes Adapted by Anban Pillay School of Computer Science University of KwaZulu Natal Durban February 2006 Contents 1 Design I Class Level Design dd Program Designs snes als Eick ada OR rie ae OO a E S 1 1 1 Preconditions and Postconditions 000 1 1 2 ADesignExample 0 2002 ee eee eee ae 1 2 Toolboxes APIs and Packages lt 15 2 400 4 sdsanara we GSR Ra as 4 Rs 1 3 Javadoc acs oe ike ees Blk ete ea aa ee ates Boe ie ie dead 2 Object Based Programming 2 1 Objects Instance Methods and Instance Variables 2 2 Constructors and Object Initialization 0000 000s 2 2 1 Garbage Collection cens of tu ae ata ee Get amp Ate kl pap te eee 2 3 Programming with Objects a ss cde on Bee gd Ge Se ae A iw a eh 2 3 T B iltin Classes as unane gb pote Gk aaa Se Que Ra whe Soe Aaa 2 3 2 Generalized Software Components 0 2 4 Programming Exampless o aisd amp ae Gooch id ee SES OR Ae ea 2 5 More about Access Modifiers ae Sg kh acetal amp Ave ak gle BS Re 2 6 Mixing Static and Non Statie
125. Objects Also write a generic Stack class that uses either a LinkedList or an ArrayList to store its data Stacks and queues were introduced in Section11 3 2 In mathematics several operations are defined on sets The union of two sets A and B is a set that contains all the elements that are in A together with all the elements that are in B The intersection of A and B is the set that contains elements that are in both A and B The difference of A and B is the set that contains all the elements of A except for those elements that are also inB Suppose that A and B are variables of type set in Java The mathematical operations on A and B can be computed using methods from the Set interface In particular The set A addA11 B is the union of A and B A retainA11 B is the intersection of A and B and A removeA11 B is the difference of A and B These operations change the contents of the set A while the mathematical operations create a new set without changing A but that difference is not relevant to this exercise For this exercise you should write a program that can be used as a set calculator for simple operations on sets of non negative integers Negative integers are not allowed A set of such integers will be represented as a list of integers separated by commas and optionally spaces and enclosed in square brackets For example 204 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES 1 2 3 or 17 42 9 53 108 The c
126. Often the size is set in the frame s constructor If you forget to do this the frame will have size zero and all you will see on the screen is a tiny border There are two methods in the JFrame class that can be used to set the size of a frame void setSize int width int height and void pack Use setSize if you know exactly what size the frame should be The pack method is more interesting It should only be called after all GUI components have been added to the frame Calling pack will make the frame just big enough to hold all the compo nents It will determine the size of the frame by checking the preferred size of each of the components that it contains When you create your own drawing surface or custom 5 6 FRAMES AND DIALOGS 117 component you can set its preferred size by calling its setPreferredSize method or by definining a getPreferredSize method to compute the size You can also set the location of the frame This can be done with by calling the JFrame method void setLocation int x int y The parameters x and y give the position of the upper left corner of the frame on the screen They are given in terms of pixel coordinates on the screen as a whole where the upper left corner of the screen is 0 0 Creating a frame object does not automatically make a window appear on the screen Initially the window is invisible You must make it visible by calling its method void show This can be called a
127. Scroll BorderLayout SOUTH Assume vScroll is a vertical scroll bar component that already exists panel add vScroll BorderLayout EAST Sometimes you want to leave space between the components in a container You can specify horizontal and vertical gaps in the constructor of a BorderLayout object For example if you say panel setLayout new BorderLayout 5 7 then the layout manager will insert horizontal gaps of 5 pixels between components and vertical gaps of 7 pixels between components The horizontal gap is inserted between the center and west components and between the center and east components the vertical gap is inserted between the center and north components and between the center and south components The default layout for a JApplet s content pane is a BorderLayout with no horizontal or vertical gap 8 1 2 GridLayout A GridLayout lays out components in a grid of equal sized rectangles The illustration shows how the components would be arranged in a grid layout with 3 rows and 2 columns If a container uses a GridLayout the appropriate add method takes a single parameter of type Component for example add myButton Components are added to the grid in the order shown that is each row is filled from left to right before going on the next row 154 CHAPTER 8 ADVANCED GUI PROGRAMMING The constructor for a GridLayout with R rows and C columns takes the form new GridLayout R C If you want to lea
128. The amount of time between events and an ActionListener that will be notified of each event Timer int delayTime ActionListener listener The listener should be programmed to respond to events from the Timer in its actionPerformed method The delay time between events is specified in milliseconds where one second equals 1000 milliseconds The actual delay time between two events can be longer than the requested delay time depending on how long it takes to process the events and how busy the computer is with other things In a typical animation somewhere between ten and thirty frames should be displayed every second These rates correspond to delay times between 100 and 33 A Timer does not start running automatically when it is created To make it run you must call its start method A timer also has a stop method which you can call to make it stop generating events If you have stopped a timer and want to start it up again you can call its restart method The start method should be called only once None of these methods have parameters Let s look at an example In the following applet you can start an animation running by clicking the Start button When you do this the text on the button changes to Stop and you can stop the animation by clicking the button again This is yet another applet that says Hello World The animation simply cycles the color of the message through all possible hues Applet Refer
129. a in a statement of this type caused a rocket to blow up on take off Because FORTRAN doesn t require variables to be declared the compiler would be happy to accept the statement DO20I 1 5 It would just create a new variable named DO201 If FORTRAN required variables to be declared the compiler would have complained that the variable D0201 was undeclared While most programming languages today do require variables to be declared there are other features in common programming languages that can cause problems Java has eliminated some of these features Some people complain that this makes Java less efficient and less powerful While there is some justice in this criticism the increase in security and robustness is probably worth the cost in most circumstances The best defense against some types of errors is to design a programming language in which the errors are impossible In other cases where the error can t be completely eliminated the language can be designed so that when the error does occur it will automatically be detected This will at least prevent the error from causing further harm and it will alert the programmer that there is a bug that needs fixing Let s look at a few cases where the designers of Java have taken these approaches An array is created with a certain number of locations numbered from zero up to some specified maximum index It is an error to try to use an array location that is outside of the specified
130. ainer comp setFont font sets the font that is used for text displayed on the component The parameter is an object of type java awt Font comp setToolTipText string sets the specified string as a tool tip for the com ponent The tool tip is displayed if mouse cursor is in the component and the mouse is not moved for a few seconds The tool tip should give some information about the meaning of the component or how to use it comp setCursor cursor sets the cursor image that represents the mouse position when the mouse cursor is inside this component The parameter is an object be longing to the class java awt Cursor Generally this parameter takes the form Cursor getPredefinedCursor id where id is one of several constants defined in the Cursor class The most useful values are probably Cursor HAND_CURSOR Cursor CROSSHAIR_CURSOR Cursor WAIT_CURSOR and Cursor DEFAULT_CURSOR For example if you would like the cursor to appear as a little pointing hand when the mouse is in the component comp you can use comp setCursor Cursor getPredefinedCursor Cursor HAND_CURSOR comp setPreferredSize size sets the size at which the component should be dis played if possible The parameter is of type Dimension and a call to this method usually looks something like setPreferredSize newDimension 100 50 The preferred size is used as a hint by layout managers but will not be respected in all cases In a BorderLayout for
131. alled brighten If row and col are the row and column numbers of a square then brighten row col increases the brightness of that square All we need is a subclass of RandomBrighten with a modified brighten routine Instead of just brightening one square the modified routine will also brighten the horizontal and vertical reflections of that square But how will it brighten each of the four individual squares By calling the brighten method from the original class It can do this by calling super brighten There is still the problem of computing the row and column numbers of the horizontal and vertical reflections To do this you need to know the number of rows and the number of columns The RandomBrighten class has instance variables named ROWS and COLUMNS to represent these quantities Using these variables it s possible to come up with formulas for the reflections as shown in the definition of the brighten method below Here s the complete definition of the new class public class SymmetricBrighten extends RandomBrighten void brighten int row int col Brighten the specified square and its horizontal and vertical reflections This overrides the brighten method from the RandomBrighten class which just brightens one square super brighten row col super brighten ROWS 1 row col super brighten row COLUMNS 1 col super brighten ROWS 1 row COLUMNS 1 col end class Sym
132. alue returned by this function is a new BigInteger object that is equal to the sum of N and M The BigInteger class has a constructor new BigInteger str where str is a string The string must represent an integer such as 3 or 39849823783783283733 If the string does not represent a legal integer then the constructor throws a NumberFormatException There are many instance methods in the BigInteger class Here are a few that you will find useful for this exercise Assume that N and M are variables of type BigInteger N add M a function that returns a BigInteger representing the sum of N and M N multiply M a function that returns a BigInteger representing the result of multiplying N times M N divide M a function that returns a BigInteger representing the result of di viding N by M N signum a function that returns an ordinary int The returned value represents the sign of the integer N The returned value is 1 if N is greater than zero It is 1 if N is less than zero And it is O if N is zero N equals M a function that returns a boolean value that is true if N and M have the same integer value 10 6 PROGRAMMING EXERCISES 227 N toString a function that returns a String representing the value of N N testBit k a function that returns a boolean value The parameter k is an integer The return value is true if the k th bit in N is 1 and it is false if the k th bit is 0 Bits are nu
133. an modeless dialogs All the examples we will look at are modal Aside from having a parent a JDialog can be created and used in the same way as a JFrame However we will not be using JDialog directly Swing has many con venient methods for creating many common types of dialog boxes For example the JColorChooser class has the static method Color JColorChooser showDialog JFrame parent Color initialColor When you call this method a dialog box appears that allows the user to select a color The first parameter specifies the parent of the dialog or it can be null When the dialog first appears initialColor is selected The dialog has a sophisticated interface that allows the user to change the selection When the user presses an OK button the dialog box closes and the selected color is returned as the value of the method The user can also click a Cancel button or close the dialog box in some other way in that case null is 122 CHAPTER 5 APPLETS HTML AND GUI S returned as the value of the method By using this predefined color chooser dialog you can write one line of code that will let the user select an arbitrary color The following applet demonstrates a JColorChooser dialog and three other simpler standard dialog boxes When you click one of the buttons a dialog box appears The label at the top of the applet gives you some feedback about what is happening Applet Reference see Applet SimpleDialogDemo
134. and a RIGHT alignment to the right side WIDTH and ALIGN are optional modifiers If you leave them out then their default values will be used The default for WIDTH is 100 and the default for ALIGN is LEFT Many tags require matching closing tags which take the form lt tag name gt For example the tag lt PRE gt must always have a matching closing tag lt PRE gt later in the document The tag applies to everything that comes between the opening tag and the closing tag The lt PRE gt tag tells a Web browser to display everything between the lt PRE gt and the lt PRE gt just as it is formatted in the original HTML source code including all the spaces and carriage returns But tags between lt PRE gt and lt PRE gt are still interpreted by the browser PRE stands for preformatted text All of the sample programs in these notes are formatted using the lt PRE gt command It is important for you to understand that when you don t use PRE the computer will completely ignore the formatting of the text in the HTML source code The only thing it pays attention to is the tags Five blank lines in the source code have no more effect than one blank line or even a single blank space Outside of lt PRE gt if you want to force a new line on the Web page you can use the tag lt BR gt which stands for break For example I might give my address as David Eck lt BR gt Department of Mathematics and Computer Science lt BR
135. andom numbers An object of type Random can generate random integers as well as random real numbers If randGen is created with the command Random randGen new Random and if N is a positive integer then randGen nextInt N generates a random integer in the range from 0 to N 1 For example this makes it a little easier to roll a pair of dice Instead of saying die1 int 6 Math random 1 one can say diel randGen nextInt 6 1 Since you also have to import the class java util Random and create the Random object you might not agree that it is actually easier The main point here again is that many problems have already been solved and the solutions are available in Java s standard classes If you are faced with a task that looks like it should be fairly common it might be worth looking through a Java reference to see whether someone has already written a subroutine that you can use 2 3 2 Generalized Software Components Every programmer builds up a stock of techniques and expertise expressed as snippets of code that can be reused in new programs using the tried and true method of cut and paste The old code is physically copied into the new program and then edited to cus 38 CHAPTER 2 OBJECT BASED PROGRAMMING tomize it as necessary The problem is that the editing is error prone and time consuming and the whole enterprise is dependent on the programmer s ability to pull out that partic ular piece of code f
136. any types of data An ArrayList is essentially a dynamic array of values of type Object Since every class is a sub class of Object objects belonging to any class can be stored in an ArrayList This is an example of generic programming The source code for the ArrayList class was written once and it works for objects of any type It does not however work for data belonging to the primitive types such as int and double The ArrayList class is just one of several classes and interfaces that are used for generic programming in Java We will spend this chapter looking at these classes and how they are used All the classes discussed in this chapter are defined in the package java util and you will need an import statement at the beginning of your program to get access to them Before you start putting import java util at the beginning of every program you should know that some things in java util have names that are the same as things in other packages For example both java util List and java awt List exist It is no easy task to design a library for generic programming Java s solution has many nice features but is certainly not the only possible approach It is almost certainly not the best but in the context of the overall design of Java it might be close to optimal To get some perspective on generic programming in general it might be useful to look very briefly at generic programming in two other languages 185 186 CHAPTER 9
137. apabilities The wrapper object is also a stream so you can read from or write to it but you can do so using fancier operations than those available for basic streams For example PrintWriter is a subclass of Writer that provides convenient methods for outputting human readable character representations of all of Java s basic data types If you have an object belonging to the Writer class or any of its subclasses and you would like to use PrintWriter methods to output data to that Writer all you have to do is wrap the Writer in a PrintWriter object You do this by constructing a new PrintWriter object using the Writer as input to the constructor For example if charSink is of type Writer then you could say PrintWriter printableCharSink new PrintWriter charSink When you output data to printableCharSink using PrintWriter s advanced data output methods that data will go to exactly the same place as data written directly to charSink You ve just provided a better interface to the same output stream For example this allows you to use PrintWriter methods to send data to a file or over a network connection For the record the output methods of the PrintWriter class include public void print String s Methods for outputting public void print char c standard data types public void print int i to the stream in public void print long 1 human readable form public void print float f public void print double d
138. aphics is provided by the system when it calls the paint method In Java all drawing of any kind is done using methods provided by a Graphics object There are many such methods As a first example of an applet let s go the traditional route and look at an applet that displays the string Hello World We ll use the paint method to display this string The import statements at the beginning make it possible to use the short names Applet and Graphics instead of the full names of the classes java applet Applet and java awt Graphics import java awt import java applet An applet that simply displays the string Hello World public class HelloWorldApplet extends Applet public void paint Graphics g g drawString Hello World 10 30 end of class HelloWorldApplet 5 1 THE BASIC JAVA APPLET AND JAPPLET 83 The drawString method defined in the Graphics class actually does the drawing The parameters of this method specify the string to be drawn and the point in the applet where the string is to be placed More about this later Now an applet is an object not a class So far we have only defined a class Where does an actual applet object come from It is possible of course to create such objects Applet hw new HelloWorldApplet This might even be useful if you are writing a program and would like to add an applet to a window you ve created Most often however applet objects are created by the s
139. arison about 2 3 of all civil engineering architecture and construction work is maintenance in a similar way 6 1 2 Object Oriented design OOP design rests on three principles e Abstraction Ignore the details In philosophical terminology abstraction is the thought process wherein ideas are distanced from objects In computer science abstraction is a mechanism and practice to reduce and factor out details so that one can focus on few concepts at a time Abstraction uses a strategy of simplification wherein formerly concrete details are left ambiguous vague or undefined wikipedia Abstraction e Modularization break into pieces A module can be defined variously but generally must be a component of a larger system and operate within that system indepen dently from the operations of the other components Modularity is the property of computer programs that measures the extent to which they have been composed out of separate parts called modules Programs that have many direct interrelationships between any two random parts of the program code are less modular than programs where those relationships occur mainly at well defined interfaces between modules e Information hiding separate the implementation and the function The principle of information hiding is the hiding of design decisions in a computer program that are most likely to change thus protecting other parts of the program from change if the design decision is changed
140. ass I need a listener to respond to the windowClosing event that is generated when the user clicks the close box The listener is created in the constructor using an anonymous subclass of WindowAdapter with a command of the form 120 CHAPTER 5 APPLETS HTML AND GUI S addWindowListener new WindowAdapter This window listener responds when the user clicks the window s close box by giving the user a chance to change his mind public void windowClosing WindowEvent evt Show the dialog box and respond to it 3 Another window listener is used in the little applet that appears on this page as the Launch HighLow button above This applet is defined in the file HighLowLauncher java Note that when you click on the button to open the frame the name of the button changes to Close HighLow You can close the frame by clicking the button again as well as by clicking the frame s close box When the frame is closed for either reason the name of the button changes back to Launch HighLow The question is how does the applet know to change the button s name when the user closes the frame by clicking its close box That doesn t seem to have anything to do with the applet The trick is to use an event listener When the applet creates the frame it also creates a WindowListener and registers it with the frame This WindowListener is programmed to respond to the windowClosed event by changing the name of the
141. atError which occurs when the Java virtual machine finds some kind of illegal data in a file that is supposed to contain a compiled Java class If that class was being loaded as part of the program then there is really no way for the program to proceed On the other hand subclasses of the class Exception represent exceptions that are meant to be caught In many cases these are exceptions that might naturally be called errors but they are errors in the program or in input data that a programmer can an ticipate and possibly respond to in some reasonable way However you should avoid the temptation of saying Well IIl just put a thing here to catch all the errors that might occur so my program won t crash If you don t have a reasonable way to respond to the error it s usually best just to terminate the program because trying to go on will probably only lead to worse things down the road in the worst case a program that gives an incorrect answer without giving you any indication that the answer might be wrong The class Exception has its own subclass RuntimeException This class groups to gether many common exceptions such as ArithmeticException which occurs for exam ple when there is an attempt to divide an integer by zero ArrayIndexOutOfBoundsException which occurs when an out of bounds index is used in an array and NullPointerException which occurs when there is an attempt to use a null reference in a context
142. ava s primitive data types But what happens when you want to read and write objects Traditionally you would have to come up with some way of encoding your object as a sequence of data values belonging to the primitive types which can then be output as bytes or characters This is called serializing the object On input you have to read the serialized data and somehow reconstitute a copy of the original ob ject For complex objects this can all be a major chore However you can get Java to do all the work for you by using the classes ObjectInputStream and ObjectOutputStream These are subclasses of InputStream and Outputstream that can be used for writing and reading serialized objects ObjectInputStream and ObjectOutputStream are wrapper classes that can be wrapped around arbitrary InputStreams and OutputStreams This makes it possible to do object input and output on any byte stream The methods for object I O are readObject in ObjectInputStream and writeObject Object obj in ObjectOutputStream Both of these methods can throw IOExceptions Note that readObject returns a value of type Object which generally has to be type cast to a more useful type ObjectInputStream and ObjectOutputStream only work with objects that implement an interface named Serializable Furthermore all of the instance variables in the object must be serializable However there is little work involved in making an object serializ able since the Serializable in
143. beginning of the list and moving from node to node along the list for n steps Operations that can be done efficiently for both types of lists include sorting and adding an item at the end of the list All lists implement the Collection methods discussed in the previous section includ ing size isEmpty add Object remove Object and clear The add Object method adds the object at the end of the list The remove Object method involves first finding the object which is not very efficient for any list since it involves going through the items in the list from beginning to end until the object is found The List interface 198 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES adds some methods for accessing list items according to their numerical positions in the list For an object list of type List these methods include list get index returns the Object at position index in the list where index is an integer Items are numbered 0 1 2 list size 1 The parameter must be in this range or an IndexOutOfBoundsException is thrown list set index obj stores an object obj at position number index in the list replacing the object that was there previously This does not change the number of elements in the list or move any of the other elements list add index obj inserts an object obj into the list at position number index The number of items in the list increases by one and items that come after po
144. bjects belong to classes but this might not be much clearer There is a real shortage of English words to properly distinguish all the concepts 23 24 CHAPTER 2 OBJECT BASED PROGRAMMING involved An object certainly doesn t belong to a class in the same way that a member variable belongs to a class From the point of view of programming it is more exact to say that classes are used to create objects A class is a kind of factory for constructing ob jects The non static parts of the class specify or describe what variables and subroutines the objects will contain This is part of the explanation of how objects differ from classes Objects are created and destroyed as the program runs and there can be many objects with the same structure if they are created using the same class Consider a simple class whose job is to group together a few static member variables For example the following class could be used to store information about the person who is using the program class UserData static String name static int age In a program that uses this class there is only one copy of each of the variables UserData name and UserData age There can only be one user since we only have memory space to store data about one user The class UserData and the variables it contains exist as long as the program runs Now consider a similar class that includes non static variables class PlayerData String name i
145. blic A public class can be accessed from anywhere Certain classes have to be public A class that defines a stand alone application must be public so that the system will be able to get at its main routine A class that defines an applet must be public so that it can be used by a Web browser If a class is not declared to be public then it can only be used by other classes in the same package as the class Classes that are not explicitly declared to be in any package are put into something called the default package All the examples in this textbook are in the default package so they are all accessible to one another whether or not they are declared public So except for applications and applets which must be public it makes no practical difference whether our classes are declared to be public or not However once you start writing packages it does make a difference A package should contain a set of related classes Some of those classes are meant to be public for access from outside the package Others can be part of the internal workings of the package 44 CHAPTER 2 OBJECT BASED PROGRAMMING and they should not be made public A package is a kind of black box The public classes in the package are the interface More exactly the public variables and subroutines in the public classes are the interface The non public classes are part of the non public implementation Of course all the classes in the package have unrestricted acc
146. by calling the setSelected method no ActionEvent is generated However there is another method in the JCheckBox class doClick which simulates a user click on the checkbox and does generate an ActionEvent When handling an ActionEvent you call evt getSource in the actionPerformed method to find out which object generated the event Of course if you are only listening for events from one component you don t even have to do this The returned value is of type Object but you can type cast it to another type if you want Once you know the object that generated the event you can ask the object to tell you its current state For example if you know that the event had to come from one of two checkboxes cb1 or cb2 then your actionPerformed method might look like this 8 2 BASIC COMPONENTS AND THEIR EVENTS 159 public void actionPerformed ActionEvent evt Object source evt getSource if source cb1 boolean newState JCheckBox cb1 getSelected respond to the change of state else if source cb2 boolean newState JCheckBox cb2 getSelected respond to the change of state Alternatively you can use evt getActionCommand to retrieve the action command associated with the source For a JCheckBox the action command is by default the label of the checkbox 8 2 4 The JRadioButton and ButtonGroup Classes Closely related to checkboxes are radio buttons Radio buttons occur in gro
147. c static final int red 0 public static final int black 1 Its a convenience class that allows us to use these names in a consistent manner Thus we can use the name CardNames ace throughout the program consistently i e Different parts of the program will mean the same thing when they say CardNames ace The Deck class This class is meant to represent a deck of 52 cards A Deck is composed of 52 Cards Its responsibilities are Create a deck of 52 cards Know the cards in the deck Shuffle a deck Deal a card from the deck Know how many cards are in the deck Design write and test the Deck class Implementation of Klondike To program the game we notice that we basically need to keep track of several piles of cards The piles have similar functionality so inheritance is strongly suggested What 145 CardPile CardPiled addCard canTakeQ draw emptyd includesd popo select topCardd TablePile SuitPile DiscardPile O DeckPile TablePiled SuitPileg DiscaraPileg DeckPiled canTaked canTake addCardo canTaked draw select canTaked select includesd select shuffled select Solitaire Card 2 allPiles CardPile 28 deckPile DeckPile 28 discardPile DiscardPi 28 suitPile SuitPile 28 tableau TablePile amp Solitaireg maing mouseClickedO mouseEnter
148. cAngle degrees from a starting angle at startAngle degrees Angles are measured with 0 degrees at the 3 o clock position the positive direction of the horizontal axis Positive angles are measured counterclockwise from zero and negative angles are measured clockwise To get an arc of a circle make sure that width is equal to height e fillRect int x int y int width int height Draws a filled in rectangle This fills in the interior of the rectangle that would be drawn by drawRect x y width height The extra pixel along the bottom and right edges is not included The width and height parameters give the exact width and height of the rectangle For example if you wanted to fill in the entire compo nent you could say g fillRect 0 0 getSize width getSize height e fillOval int x int y int width int height Draws a filled in oval e fillRoundRect int x int y int width int height int xdiam int ydiam Draws a filled in rounded rectangle e i113DRect int x int y int width int height boolean raised Draws a filled in three dimensional rectangle e fillArc int x int y int width int height int startAngle int arcAngle Draw a filled in arc This looks like a wedge of pie whose crust is the arc that would be drawn by the drawArc method Let s use some of the material covered in this section to write a JApplet Since we will be drawing on the applet we will need to create a drawing surface The drawing su
149. can look at the source code Deck java if you want And given the source code you can use the class in your programs without understanding the implementation We can do a similar analysis for the Hand class When a hand object is first created it has no cards in it An addCard instance method will add a card to the hand This method needs a parameter of type Card to specify which card is being added For the removeCard method a parameter is needed to specify which card to remove But should we specify the card itself Remove the ace of spades or should we specify the card by its position in the hand Remove the third card in the hand Actually we don t have to decide since we can allow for both options We ll have two removeCard instance methods one with a parameter of type Card specifying the card to be removed and one 40 CHAPTER 2 OBJECT BASED PROGRAMMING with a parameter of type int specifying the position of the card in the hand Remember that you can have two methods in a class with the same name provided they have different types of parameters Since a hand can contain a variable number of cards it s convenient to be able to ask a hand object how many cards it contains So we need an instance method getCardCount that returns the number of cards in the hand When I play cards I like to arrange the cards in my hand so that cards of the same value are next to each other Since this is a generally useful thin
150. ce associated with the toolbox The Macintosh API is a specification of all the routines available in the Macintosh Toolbox A company that makes some hardware device say a card for connecting a computer to a network might publish an API for that device consisting of a list of routines that programmers can call in order to communicate with and control the device Scientists who write a set of routines for doing some kind of complex computation such as solving differential equations say would provide an API to allow others to use those routines without understanding the details of the computations they perform The Java programming language is supplemented by a large standard API You ve seen part of this API already in the form of mathematical subroutines such as Math sqrt the String data type and its associated routines and the System out print routines The standard Java API includes routines for working with graphical user interfaces for 1 2 TOOLBOXES API S AND PACKAGES 19 network communication for reading and writing files and more It s tempting to think of these routines as being built into the Java language but they are technically subroutines that have been written and made available for use in Java programs Java is platform independent That is the same program can run on platforms as diverse as Macintosh Windows UNIX and others The same Java API must work on all these platforms But notice
151. ce until totals are the same firstDice roll Roll the first pair of dice totall firstDice diel firstDice die2 Get total System out println First pair comes up total1 secondDice roll Roll the second pair of dice total2 secondDice diei secondDice die2 Get total System out println Second pair comes up total2 countRollst Count this roll System out println Blank line while totall total2 System out println It took countRolls rolls until the totals were the same end main end class RollTwoPairs This applet simulates this program Applet Reference see Applet RollTwoPairsConsole Constructors are subroutines but they are subroutines of a special type They are cer tainly not instance methods since they don t belong to objects Since they are responsible for creating objects they exist before any objects have been created They are more like static member subroutines but they are not and cannot be declared to be static In fact according to the Java language specification they are technically not members of the class at all 34 CHAPTER 2 OBJECT BASED PROGRAMMING Sample program RollTwoPairs Second pair comes up 6 First pair comes up Second pair comes up First pair comes up Second pair comes up First pair comes up Second pair comes up It took 6 rolls until the tetals were the same Figure 2 2 Applet RollTwoPairsC
152. ck Drawing the cards requires some care and computation I wrote a method void drawCard Graphics g Card card int x int y which draws a card with its upper left corner at the point x y The paintComponent routine decides where to draw each card and calls this routine to do the drawing You can check out all the details in the source code HighLowGUL java 5 6 Frames and Dialogs APPLETS ARE A FINE IDEA It s nice to be able to put a complete program in a rectangle on a Web page But more serious large scale programs have to run in their own windows independently of a Web browser In Java s Swing GUI library an independent window is represented by an object of type JFrame A stand alone GUI application can open one or more JFrames to provide the user interface It is even possible for an applet to open a frame The frame will be a separate window from the Web browser window in which the applet is running Any frame created by an applet includes a warning message such as Warning Insecure Applet Window The warning is there so that you can always recognize windows created by applets This is just one of the security restrictions on applets which after all are programs that can be downloaded automatically from Web sites that you happen to stumble across without knowing anything about them Here is an applet that contains just one small button When you click this Launch ShapeDraw button a JFrame will be opened Ap
153. classes form an is a hierarchy The advantage of inheritance is that it avoids code duplication promotes code reuse and improves maintenance and extendability In fact every class automatically extends Object Substitution subtyping A variable may reference objects of its declared type or any subtype of its declared type subtype objects may be used whenever supertype objects are expected There are times an objects may need to be cast back to original type Note that the actual type of an object is forever fixed To create a class Rectangle which extends a class Shape one starts with class Rectangle extends Shape The class Rectangle then automatically has all the methods of class Shape and you can add some of your own The default constructor for Rectangle is automatically called at the start of the constructor for Shape The methods and the instance variables of the superclass can be accessed if not de clared as private This can be done with the prefix super The methods and instance variables of a class may be declared as protected to allow direct access only to extensions Overriding is providing a method with the same signature as the superclass but dif ferent body This method takes precedence on the subclass object 3 3 4 Casting Recall that the type of a variable specifies what it can reference it can reference any object that is of that type or a subtype of that type It is an informational message to the compiler On the
154. clearly defined area of responsibility The most convenient way to make a separate event handling object is to use a nested anonymous class We will see more examples of this in the future but here for the record is a version of HelloSwing that uses an anonymous class for event handling This applet has exactly the same behavior as the original version 5 2 HTML BASICS 87 import javax swing Swing GUI classes are defined here import java awt event Event handling class are defined here public class HelloSwing2 extends JApplet public void init This method is called by the system before the applet appears It is used here to create the button and add it to the content pane of the JApplet An anonymous class is used to create an ActionListener for the button JButton bttn new JButton Click Me bttn addActionListener new ActionListener The action listener for the button is defined by this nested anonymous class public void actionPerformed ActionEvent evt This method is called to respond when the user presses the button It displays a message in a dialog box along with an OK button which the user can click to dismiss the dialog box String title Greetings Shown in box s title bar String message Another hello from Swing JOptionPane showMessageDialog null message title JOptionPane INFORMATION_MESSAGE end actionPerformed H getContentPa
155. compute the distance between two points public static double distance Coord pl Coord p2 It is then called as follows Coord X new Coord 0 0 0 0 Coord Y new Coord 3 0 4 0 System out println Coord distance X Y Assignment and Equality Testing If A and B are primitive data types then A B sets A to the value of B changing B will have no impact on A If A and B are objects then A B sets A to refer to the same object that B references until either A or B is re assigned they will point to the same object The test A B for objects only compares whether they reference the SAME object in memory It is good practice to override the equals method if one will need to test two objects of one s class for equality This will be discussed in the section on casting 48 CHAPTER 2 OBJECT BASED PROGRAMMING Objects as Return Values and Parameters An object even an array can be returned from a method but only one An object can also be a parameter to a method Note that the method receives a reference to an object this is called pass by ref erence This means that changes to the object so referenced ARE reflected outside the class On the other hand changing a primitive data type parameter does NOT affect the value outside the class void clear int x int A x 0 for int i 0 i lt A length it A i 0 is called from int y 42 int B 1 2 3 4 5 clear y B will cause all the values of B to be zero
156. conditions The design of Java makes it impossible for programmers to ignore such conditions Among the exceptions that require mandatory handling are several that can occur when using Java s input output routines This means that you can t even use these rou tines unless you understand something about exception handling The next chapter deals with input output and uses exception handling extensively 10 4 Programming with Exceptions EXCEPTIONS CAN BE USED to help write robust programs They provide an organized and structured approach to robustness Without exceptions a program can become clut tered with if statements that test for various possible error conditions With exceptions it becomes possible to write a clean implementation of an algorithm that will handle all the normal cases The exceptional cases can be handled elsewhere in a catch clause of a try statement 10 4 1 Writing New Exception Classes When a program encounters an exceptional condition and has no way of handling it imme diately the program can throw an exception In some cases it makes sense to throw an ex ception belonging to one of Java s predefined classes such as IllegalArgumentException or IOException However if there is no standard class that adequately represents the ex ceptional condition the programmer can define a new exception class The new class must extend the standard class Throwable or one of its subclasses In general the new class will
157. contains and remove from the Collection interface will not work as expected for values of type Card 9 3 COLLECTIONS 195 A similar concern arises when items in a collection are sorted Sorting refers to ar ranging a sequence of items in ascending order according to some criterion The problem is that there is no natural notion of ascending order for arbitrary objects Before objects can be sorted some method must be defined for comparing them Objects that are meant to be compared should implement the interface java lang Comparable This interface defines one method public int compareTo Object obj The value returned by obj1 compareTo obj2 should be zero if and only if the objects are equal that is if obj1 equals obj2 is true It should be negative if and only if obj1 comes before obj2 when the objects are arranged in ascending order And it should be positive if and only if obj1 comes after obj2 In general it should be considered an error to call obj1 compareTo obj2 if obj2 is not of the same type as obj1 The String class implements the Comparable interface and defines compareTo in a reasonable way If you define your own class and want to be able to sort objects belonging to that class you should do the same For example class FullName implements Comparable Represents a full name a first name and a last name String firstName lastName public boolean equals Object obj if obj null obj instanceof FullName
158. content pane and in a case like this where a single component fills the applet it makes sense to replace the content pane with the setContentPane method paintComponent in the Display class makes a call to super paintComponent g The command super paintComponent g simply calls the paintComponent method that is defined in the superclass JPanel The effect of this is to fill the component with its background color Most paintComponent methods call super paintComponent g as the first statement but this is not necessary if the drawing commands in the method cover the background of the component completely 5 3 GRAPHICS AND PAINTING 99 Here is the complete source code for the RandomStrings applet This applet displays 25 copies of a message The color and position of each message is selected at random The font of each message is randomly chosen from among five possible fonts The messages are displayed on a black background Note This applet uses bad style because every time the paintComponent method is called new random values are used This means that a different picture will be drawn each time This is particularly bad if only part of the applet needs to be redrawn since then the applet will contain cut off pieces of messages When this file is compiled it produces two classes RandomStrings class and RandomStrings Display class Both classes are required to use the applet import java awt import ja
159. correct or poorly written software Let s look at a few incidents recounted in the book Computer Ethics by Tom Forester and Perry Morrison This book covers various ethical issues in computing It or something like it is essential reading for any student of computer science In 1985 and 1986 one person was killed and several were injured by excess radi ation while undergoing radiation treatments by a mis programmed computerized radi ation machine In another case over a ten year period ending in 1992 almost 1 000 cancer patients received radiation dosages that were 30 less than prescribed because of a programming error In 1985 a computer at the Bank of New York started destroying records of on going security transactions because of an error in a program It took less than 24 hours to fix the program but by that time the bank was out 5 000 000 in overnight interest payments on funds that it had to borrow to cover the problem The programming of the inertial guidance system of the F 16 fighter plane would have turned the plane upside down when it crossed the equator if the problem had not been discovered in simulation The Mariner 18 space probe was lost because of an error in one line of a program The Gemini V space capsule missed its scheduled landing target by a hundred miles because a programmer forgot to take into account the rotation of the Earth In 1990 AT amp T s long distance telephone service was disrupted throughout the
160. cture of the system using objects attributes operations and associations Includes Class Diagrams e Dynamic Model Showcases the internal behavior of the system Includes Sequence Diagrams Activ ity Diagrams and State Machine Diagrams UML is composed of many model elements that represent the different parts of a soft ware system The UML elements are used to create diagrams which represent a certain part or a point of view of the system In UML 2 0 there are 13 types of diagrams Some of the more important diagrams are e Use Case Diagrams show actors people or other users of the system use cases the scenarios when they use the system and their relationships Class Diagrams show classes and the relationships between them Sequence Diagrams show objects and a sequence of method calls they make to other objects Collaboration Diagrams show objects and their relationship putting emphasis on the objects that participate in the message exchange State Diagrams show states state changes and events in an object or a part of the system e Activity Diagrams show activities and the changes from one activity to another with the events occurring in some part of the system 132 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN 34 4 Umbrello UML Modeller 22 522 EEE TEE EE Ta Eile Edit Diagram Code Settings Help L ASaASB xno UML Diagrams ali k g X Logical View SE class diagram
161. cult problem especially if you don t assume a fixed size for the applet Java has a solution for this as well Components are the visible objects that make up a GUI Some components are con tainers which can hold other components An applet s content pane is an example of a container The standard class JPanel which we have only used as a drawing surface up till now is another example of a container Because a JPanel object is a container it can hold other components So JPanels are dual purpose You can draw on them and you can add other components to them Because a JPanel is itself a component you can add a JPanel to an applet s content pane or even to another JPanel This makes complex nesting of components possible JPanels can be used to organize complicated user interfaces The components in a container must be laid out which means setting their sizes and positions It s possible to program the layout yourself but ordinarily layout is done by a layout manager A layout manager is an object associated with a container that implements some policy for laying out the components in that container Different types of layout manager implement different policies Our first example is rather simple It s another Hello World applet in which the color of the message can be changed by clicking one of the buttons at the bottom of the applet Applet Reference see Applet HelloWorldJApplet rea J creen J ome Figure
162. cussion of threads is that it is a realistic example of a long computation that requires a separate thread The example is based on the Mandelbrot set a mathematical curiosity that has become familiar because it can be used to produce a lot of pretty pictures The Mandelbrot set has to do with the following simple algorithm Start with a point x y in the plane where x and y are real numbers Let zx x and let zy y Repeat the following Replace zx zy with zx zx zy zy x 2 zx zy y The question is what will happen to the point zx zy as the loop is repeated over and over The answer depends on the initial point x y For some initial points x y the point zx zy will sooner or later move arbitrarily far away from the origin 0 0 For other starting points the point zx zy will stay close to 0 0 no matter how many times you repeat the loop The Mandelbrot set consists of the x y points for which zx zy stays close to 0 0 forever This would probably not be very interesting except that the Mandelbrot set turns out to have an incredibly intricate and quite pretty structure To get a pretty picture from the Mandelbrot set we change the question just a bit Given a starting point x y we ask how many steps does it take up to some specified maximum number before the point zx zy moves some set distance away from 0 0 We then assign the point a color depending on the number of steps If we do this for each
163. d listOfShapes add oneShape The shape could be removed from the list with listOfShapes remove oneShape The number of shapes in the list is given by the function listOfShapes size And it is possible to retrieve the i th object from the list with the function call listOfShapes get i Items in the list are numbered from 0 to listOfShapes size 1 However note that this method returns an Object not a Shape Of course the people who wrote the ArrayList class didn t even know about Shapes so the method they wrote could hardly have a return type of Shape Since you know that the items in the list are in fact Shapes and not just Objects you can type cast the Object returned by listOfShapes get i to be a value of type Shape oneShape Shape listOfShapes get i Let s say for example that you want to redraw all the shapes in the list You could do this with a simple for loop which is lovely example of object oriented programming and polymorphism for int i 0 i lt listOfShapes size i Shape s i th element of the list considered as a Shape Shape listOfShapes get i s redraw It might be worthwhile to look at an applet that actually uses an abstract Shape class and an ArrayList to hold a list of shapes Applet Reference see Applet ShapeDraw If you click one of the buttons along the bottom of this applet a shape will be added to the screen in the up
164. d This is to make sure that the user won t accidently overwrite an important file However if the command line has three arguments then the first argument must be f while the second and third arguments are file names The f is a command line option which is meant to modify the behavior of the program The program interprets the f to mean that it s OK to overwrite an existing program The f stands for force since it forces the file to be copied in spite of what would otherwise have been considered an error You can see in the source code how the command line arguments are interpreted by the program import java io public class CopyFile public static void main String args String sourceName Name of the source file as specified on the command line String copyName Name of the copy as specified on the command line InputStream source Stream for reading from the source file OutputStream copy Stream for writing the copy boolean force This is set to true if the f option is specified on the command line int byteCount Number of bytes copied from the source file 11 4 PROGRAMMING WITH FILES 251 Get file names from the command line and check for the presence of the f option If the command line is not one of the two possible legal forms print an error message and end this program if args length 3 amp amp args 0 equalsIgnoreCase f sourceNam
165. d always check the return value to make sure that the user has in fact selected a file If that is the case then you can find out which file was selected by calling the JFileChooser s getFile method which returns an object of type File that represents the selected file Putting all this together typical code for using a JFileChooser to read character data from a file looks like this 246 CHAPTER 11 BASIC INPUT OUTPUT JFileChooser fileDialog new JFileChooser int option fileDialog show0penDialog this if option JFileChooser APPROVE_OPTION File selectedFile fileDialog getFile try Scanner data new Scanner new FileReader selectedFile F catch FileNotFoundException e Handle the error Read data from the file The first line creates a new JFileChooser object in which the current directory is initially selected The second line shows the file dialog box on the screen and waits for the user to select a file or close the dialog box in some other way The third line tests whether the user has actually selected a file Only in that case do we proceed to get the selected file open it and use it Writing data to a file would be similar but showSaveDialog would replace showOpenDialog There is nothing to stop you by the way from using the same JFileChooser object over and over This would have the advantage that the selected directory would be re membered from one use to the next It s
166. d which de tects actions taken by the user and calls appropriate subroutines in the program to handle each event An animation however is not driven by user actions It is something that happens by itself as an independent process In Java the most natural way to accomplish this is to create a separate thread to run the animation Before the introduction of the Swing GUI a Java programmer would have to deal with this thread directly This made 170 CHAPTER 8 ADVANCED GUI PROGRAMMING animation much more complicated than it should have been The good news in Swing is that it is no longer necessary to program directly with threads in order to do simple animations The neat idea in Swing is to integrate animation with event handling so that you can program animations using the same techniques that are used for the rest of the program In Swing an animation can be programmed using an object belonging to the class javax swing Timer A Timer object can generate a sequence of events on its own with out any action on the part of the user To program an animation all your program has to do is create a Timer and respond to each event from the timer by displaying another frame in the animation Behind the scenes the Timer runs a separate thread which is responsible for generating the events but you never need to deal with the thread directly The events generated by a Timer are of type ActionEvent The constructor for a Timer specifies two things
167. d Student object is stored in the variable std But in the next line the value of std is changed and the reference to the Student object is gone In fact there are now no references whatsoever to that object stored in any variable So there is no way for the program ever to use the object again It might as well not exist In fact the memory occupied by the object should be reclaimed to be used for another purpose Java uses a procedure called garbage collection to reclaim memory occupied by objects that are no longer accessible to a program It is the responsibility of the system not the programmer to keep track of which objects are garbage In the above example it was very easy to see that the Student object had become garbage Usually it s much harder If an object has been used for a while there might be several references to the object stored in several variables The object doesn t become garbage until all those references have been dropped In many other programming languages it s the programmer s responsibility to delete the garbage Unfortunately keeping track of memory usage is very error prone and many serious program bugs are caused by such errors A programmer might accidently delete an object even though there are still references to that object This is called a dangling pointer error and it leads to problems when the program tries to access an object that is no longer there Another type of error is a memory leak
168. d method to check first if the card is faceUp and flip it if its not Then add the card to the pile You never add cards to the DiscardPile so its canTake method always returns false The select method requires careful thought Remember that this method runs when the user selects this pile Now what happens when the user clicks on the topCard in the discardPile We must check if any SuitPile 4 of them or any TablePile 7 of them all in the Solitaire class can take the card If any of these piles can take the card we add the Card to that pile If not we leave it on the discardPile The SuitPile Class The select method is empty Cards are never removed from this pile The canTake method should return true if the Card is the same suit as all others in the pile and if its rank is one more that its topCard The TablePile Class Write the constructor to initialize the table pile The construc tor accepts three parameters the x and y coordinates of the pile and an integer that tell it how many cards it contains remember that the first tablePile contains 1 card the second 2 Cards etc It takes Cards from the deckPile The table pile is displayed differently from the other piles the cards overlap We thus need to override the includes the method and the draw method The canTake method is also different The table piles will accept a card if its suit is opposite in color and its rank is one less than the pile s topCard The select method i
169. d setColor can be called for any type of shape How does the computer know which shape to draw when it executes the redraw Informally we can answer the question like this The computer executes redraw by asking the shape to redraw itself Every shape object knows what it has to do to redraw itself 58 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I In practice this means that each of the specific shape classes has its own redraw method class Rectangle extends Shape void redraw commands for drawing a rectangle possibly more methods and variables class Oval extends Shape void redraw commands for drawing an oval possibly more methods and variables class RoundRect extends Shape void redraw commands for drawing a rounded rectangle possibly more methods and variables If oneShape is a variable of type Shape it could refer to an object of any of the types Rectangle Oval or RoundRect As a program executes and the value of oneShape changes it could even refer to objects of different types at different times Whenever the statement oneShape redraw is executed the redraw method that is actually called is the one appropriate for the type of object to which oneShape actually refers There may be no way of telling from looking at the text of the program what shape this statement will draw since it depends on the value that oneShape happens to have when the program is executed Ev
170. data is stored A list could be represented as an array as a linked list or for that matter as a map that associates the elements of the list to the numbers 0 1 2 In fact these terms are represented in Java not by classes but by interfaces The interfaces Collection List Set and Map specify the basic operations on data structures of these types but do not specify how the data structures are to be represented or how the operations are to be implemented That will be specified in the classes that implement the interfaces Even when you use these classes you might not know what the implementation is unless you go look at the source code Java s generic data structures are abstract data types They are defined by the operations that can be performed on them not by the physical layout of the data in the computer 9 3 1 Generic Algorithms and Iterators The Collection interface includes methods for performing some basic operations on col lections of objects Since collection is a very general concept operations that can be applied to all collections are also very general They are generic operations in the sense that they can be applied to various types of collections containing various types of objects Suppose that co11 is any object that implements the Collection interface Here are some of the operations that are defined e coll size returns an int that gives the number of objects in the collection e coll isEmpty
171. derings on the same collection of objects In the next two sections we ll see how Comparable and Comparator are used in the context of collections and maps 9 3 3 Wrapper Classes As noted above Java s generic programming does not apply to the primitive types Before leaving this section we should try to address this problem You can t store an integer in a generic data structure designed to hold Objects On the other hand there is nothing to stop you from making an object that contains an integer and putting that object into the data structure In the simplest case you could define a class that does nothing but contain an integer public class IntContainer public int value In fact Java already has a class similar to this one An object belonging to the class java lang Integer contains a single int It is called a wrapper for that int The int value is provided as a parameter to the constructor For example Integer val new Integer 17 creates an Integer object that wraps the number 17 The Integer object can be used in generic data structures and in other situations where an object is required The int value is stored in a private final instance variable of the Integer object If val refers to an object of type Integer you can find out what int it contains by calling the instance method val intValue There is no way to change that value We say that an Integer is an immutable object That is after it has been co
172. developed Specifications are most important for external interfaces that must remain stable e Design and Architecture Design and architecture refer to determining how software is to function in a general way without being involved in details Usually this phase is divided into two sub phases 6 1 OBJECT ORIENTED ANALYSIS AND DESIGN 129 Coding Reducing a design to code may be the most obvious part of the software engineering job but it is not necessarily the largest portion Testing Testing of parts of software especially where code by two different engineers must work together falls to the software engineer Documentation An important and often overlooked task is documenting the in ternal design of software for the purpose of future maintenance and enhancement Documentation is most important for external interfaces Maintenance Maintaining and enhancing software to cope with newly discovered problems or new requirements can take far more time than the initial development of the software Not only may it be necessary to add code that does not fit the original design but just determining how software works at some point after it is completed may require significant effort by a software engineer About 2 3 of all software engineering work is maintenance but this statistic can be misleading A small part of that is fixing bugs Most maintenance is extending systems to do new things which in many ways can be considered new work In comp
173. dialog to ask whether the user really wants to quit The frame will close if the return value is JOptionPane YES_OPTION By the way it is possible to omit the Cancel button from a confirm dialog by calling one of the other methods in the JOptionPane class Just call JOptionPane showConfirmDialog parent question title JOptionPane YES_NO_OPTION The final parameter is a constant which specifies that only a Yes button and a No button should be used The third parameter is a string that will be displayed as the title of the dialog box window If you would like to see how dialogs are created and used in the sample applet you can find the source code in the file SimpleDialogDemo java 5 7 Quiz Questions THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 6 of this on line Java textbook You should be able to answer these questions after studying that chapter Question 1 Programs written for a graphical user interface have to deal with events Explain what is meant by the term event Give at least two different examples of events and discuss how a program might respond to those events Question 2 What is an event loop Question 3 Explain carefully what the repaint method does Question 4 What is HTML Question 5 Draw the picture that will be produced by the following paint method public static void paint Graphics g for int i 10 i lt 210 i i 50 for int j 10 j lt 210 j j 50
174. display Instance variable message to be displayed String fontName Instance variable font to use for display public void init String value value getParameter message Get message PARAM if any if value null display Hello World default value else display value Value from PARAM tag value getParameter font if value null fontName SansSerif else fontName value Dealing with the size parameter would be just a little harder since a parameter value is always a String and the size is supposed to be an int This means that the String value must somehow be converted to an int We ll worry about how to do that later 5 3 Graphics and Painting EVERYTHING YOU SEE ON A COMPUTER SCREEN has to be drawn there even the text The Java API includes a range of classes and methods that are devoted to drawing In this section Pll look at some of the most basic of these An applet is an example of a GUI component The term component refers to a visual element in a GUI including buttons menus text input boxes scroll bars check boxes and 92 CHAPTER 5 APPLETS HTML AND GUI S so on In Java GUI components are represented by objects belonging to subclasses of the class java awt Component Most components in the Swing GUI although not top level components like JApplet belong to subclasses of the class javax swing JComponent Every component is responsible for drawing itself
175. do with correctness and robustness Question 3 What is Double NaN Question 4 What is a precondition Give an example Question 5 Explain how preconditions can be used as an aid in writing correct pro grams Question 6 Java has a predefined class called Throwable What does this class repre sent Why does it exist Question 7 Write a subroutine that prints out a 3N 1 sequence starting from a given integer N The starting value should be a parameter to the subroutine If the parame ter is less than or equal to zero throw an IllegalArgumentException If the number in the sequence becomes too large to be represented as a value of type int throw an ArithmeticException Question 8 Some classes of exceptions require mandatory exception handling Explain what this means Question 9 Consider a subroutine processData that has the header static void processData throws IOException Write a try catch statement that calls this subroutine and prints an error message if an IOException occurs Question 10 Why should a subroutine throw an exception when it encounters an error Why not just terminate the program 10 6 Programming Exercises 1 Write a program that uses the following subroutine from Section 3 to solve equa tions specified by the user static double root double A double B double C throws IllegalArgumentException Returns the larger of the two roots of 226 CHAPTER 10 CORRECTNESS AND ROBUSTNESS
176. does not have the expected form A throw statement can be used in a program to throw an error of type ParseError The constructor for the ParseError object must specify an error message For example throw new ParseError Encountered an illegal negative number or throw new ParseFrror The word word is not a valid file name If the throw statement does not occur in a try statement that catches the error then the subroutine that contains the throw statement must declare that it can throw a ParseError It does this by adding the clause throws ParseError to the subroutine heading For example void getUserData throws ParseError This would not be required if ParseError were defined as a subclass of RuntimeException instead of Exception since in that case exception handling for ParseErrors would not be mandatory A routine that wants to handle ParseErrors can use a try statement with a catch clause that catches ParseErrors For example try getUserData processUserData catch ParseError pe Handle the error 10 4 PROGRAMMING WITH EXCEPTIONS 223 Note that since ParseError is a subclass of Exception a catch clause of the form catch Exception e would also catch ParseErrors along with any other object of type Exception Sometimes it s useful to store extra data in an exception object For example class ShipDestroyed extends RuntimeException Ship ship Which ship
177. drawing The graphics context continues to use the color until you explic itly change it with another setColor command If you want to know what the current drawing color is you can call the function g getColor which returns an object of type Color This can be useful if you want to change to another drawing color temporarily and then restore the previous drawing color Every component has an associated foreground color and background color Gener ally the component is filled with the background color before anything else is drawn although some components are transparent meaning that the background color is ig nored When a new graphics context is created for a component the current drawing color is set to the foreground color Note that the foreground color and background color are properties of the component not of a graphics context The foreground and background colors can be set by instance methods setForeground c and setBackground c which are defined in the Component class and therefore are avail able for use with any component 5 3 3 Fonts A font represents a particular size and style of text The same character will appear differ ent in different fonts In Java a font is characterized by a font name a style and a size The available font names are system dependent but you can always use the following four strings as font names Serif SansSerif Monospaced and Dialog In the original Java
178. ds An array that holds the words static int wordCount The number of words currently stored in the array 11 4 PROGRAMMING WITH FILES 247 The program starts with an empty array Every time a word is read from the file it is inserted into the array if it is not already there The array is kept at all times in alphabetical order so the new word has to be inserted into its proper position in that order The insertion is done by the following subroutine static void insertWord String w int pos 0 This will be the position in the array where the word w belongs w w toLowerCase Convert word to lower case Find the position in the array where w belongs after all the words that precede w alphabetically If a copy of w already occupies that position then it is not necessary to insert w so return immediately while pos lt wordCount amp amp words pos compareTo w lt 0 post if pos lt wordCount amp amp words pos equals w return If the array is full make a new array that is twice as big copy all the words from the old array to the new and set the variable words to refer to the new array if wordCount words length String newWords new String words length 2 System arraycopy words 0 newWords 0 wordCount words newWords Put w into its correct position in the array Move any words that come after w up one space in the array to make room for w
179. e args 1 copyName args 2 force true else if args length 2 sourceName args 0 copyName args 1 force false else System out print1n Usage java CopyFile lt source file gt lt copy name gt System out println s or java CopyFile f lt source file gt lt copy name gt return Create the input stream If an error occurs end the program try source new FileInputStream sourceName catch FileNotFoundException e System out println Can t find file sourceName return If the output file already exists and the f option was not specified print an error message and end the program File file new File copyName if file exists amp amp force false System out println Output file exists Use the f option to replace it return Create the output stream If an error occurs end the program try copy new FileOutputStream copyName catch IOException e System out println Can t open output file copyName return 252 CHAPTER 11 BASIC INPUT OUTPUT Copy one byte at a time from the input stream to the output stream ending when the read method returns 1 which is the signal that the end of the stream has been reached error occurs print an error message the file has been copied successfully If any Also print a message if byteCount 0 try while true i
180. e access modifier e A method can be overloaded in the same class or in a subclass Overloading Example public void changeSize int size String name float pattern The following methods are legal overloads of the changeSize method public void changeSize int size String name public int changeSize int size float pattern Deciding which of the matching methods to invoke is based on the arguments Over riding lets you redefine a method in a subclass when you need new subclass specific behavior Rules for overridden methods 1 The argument list must exactly match that of the overridden method 2 The return type must exactly match that of the overridden method 3 The access level must not be more restrictive than that of the overridden method 4 The access level can be less restrictive than that of the overridden method 70 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I Illegal Override Code and their problems Consider the following example public class Animal public void eat Illegal Override Problem with Override private void eat public void eat String food A legal overload not an override because the argument list changed public String eatQ Not an override because of the return type but not an overload either because there s no change in the argument list 3 3 3 Inheritance Summary In Java you can make one class an extension of another This is called inheritance The
181. e after one game ends You have to decide what happens when each of these buttons is pressed You don t have much chance of getting this right unless you think in terms of the states that the game can be in and how the state can change Your program will need the classes defined in Card java Hand java BlackjackHand_java and Deck java 126 CHAPTER 5 APPLETS HTML AND GUI S Chapter 6 Design II Object Oriented Design THE DIFFICULTIES INHERENT WITH THE DEVELOPMENT OF SOFTWARE has led many computer scientists to suggest that software development should be treated as an engi neering activity They argue for a disciplined approach where the software engineer uses carefully thought out methods and processes The term software engineering has several meanings from wikipedia As the broad term for all aspects of the practice of computer programming as op posed to the theory of computer programming which is called computer science As the term embodying the advocacy of a specific approach to computer program ming one that urges that it be treated as an engineering profession rather than an art or a craft and advocates the codification of recommended practices in the form of software engineering methodologies Software engineering is 1 the application of a systematic disciplined quantifiable approach to the de velopment operation and maintenance of software that is the application of engineering to software
182. e can throw such an exception that fact must be announced in a throws clause in the routine definition Failing to do so is a syntax error that will be reported by the compiler On the other hand suppose that some statement in a program can generate an excep tion that requires mandatory handling The statement could be a throw statement which throws the exception directly or it could be a call to a subroutine that can throw the ex ception In either case the exception must be handled This can be done in one of two ways The first way is to place the statement in a try statement that has a catch clause that handles the exception The second way is to declare that the subroutine that contains the statement can throw the exception This is done by adding a throws clause to the subroutine heading If the throws clause is used then any other routine that calls the subroutine will be responsible for handling the exception If you don t handle the possible exception in one of these two ways it will be considered a syntax error and the compiler will not accept your program Exception handling is mandatory for any exception class that is not a subclass of either Error or RuntimeException Exceptions that require mandatory handling generally rep resent conditions that are outside the control of the programmer For example they might represent bad input or an illegal action taken by the user A robust program has to be pre pared to handle such
183. e less formal in my commenting style 1 1 2 A Design Example Let s work through an example of program design using subroutines In this example we will both use prewritten subroutines as building blocks and design new subroutines that we need to complete the project Suppose that we have an already written class called Mosaic This class allows a program to work with a window that displays little colored rectangles arranged in rows and columns The window can be opened closed and otherwise manipulated with static member subroutines defined in the Mosaic class Here are some of the available routines void Mosaic open int rows int cols int w int h Precondition The parameters rows cols w and h are greater than zero Postcondition A mosaic window is opened on the screen that can display rows and columns of colored rectangles Each rectangle is w pixels wide and h pixels high The number of rows is given by the first parameter and the number of columns by the second Initially all the rectangles are black Note The rows are numbered from 0 to rows 1 and the columns are numbered from 0 to cols 1 1 1 PROGRAM DESIGN 13 void Mosaic setColor int row int col int r int g int b Precondition row and col are in the valid ranges of row numbers and column numbers r g and b are in the range 0 to 255 Also the mosaic window should be open Postcondition The color of the rectangle in row number row and column number c
184. e of code defines a subroutine template If you were to remove the first line template lt class ItemType gt and substitute the word int for the word ItemType in the rest of the template you get a subroutine for sorting arrays of ints Even though it says class ItemType you can actually substitute any type for ItemType including the primitive types If you substitute string for ItemType you get a subroutine for sorting arrays of strings This is pretty much what the compiler does with the template If your program says sort list 10 where list is an array of ints the compiler uses the template to generate a subroutine for sorting arrays of ints If you say sort cards 10 where cards is an array of objects of type Card then the compiler generates a subroutine for sorting arrays of Cards At least it tries to The template uses the gt operator to compare values If this operator is defined for values of type Card then the compiler will successfully use the template to generate a subroutine for sorting Cards If gt is not defined for Cards then the compiler will fail but this will happen at compile time not as in Smalltalk at run time where it would make the program crash C also has templates for making classes If you write a template for binary trees you can use it to generate classes for binary trees of ints binary trees of strings binary trees of dates and so on al
185. e suit and next higher card or if the foundation is empty and the card is an ace Spaces in the tableau that arise during play can be filled only by kings The topmost card of each tableau pile and the topmost card of the discard pile are always available for play The only time more than one card is moved is when an entire collection of face up cards from a tableau called a build is moved to another tableau pile This can be done if the bottommost card of the build can be legally played on the topmost card of the destination Our initial game will not support the transfer of a build The topmost card of a tableau is always face up If a card is moved from a tableau leaving a face down card on the top the latter card can be turned face up 141 142 CHAPTER 7 A SOLITAIRE GAME KLONDIKE Suit Piles nfo e OE Figure 7 1 Layout of the Solitaire Game Card Games In this section and the next we will explore games that employ playing cards and use them to build our simplified game of Klondike Solitaire To start off we will program two classes a Card class and a Deck class These two classes will be useful in almost all card games Create and new project CardGames is good name and write these classes in a package called cardGames The Card class The aim is to build an ABSTRACTION of a playing card Objects of type Card represent a single playing card The class has the following responsibilites Know its suit rank and whet
186. e true after the subroutine has run assuming that its preconditions were met and that there are no bugs in the subroutine The postcondition of the function Math sqrt is that the square of the value that is returned by this function is equal to the parameter that is provided when the subroutine is called Of course this will only be true if the preconditiion that the parameter is greater than or equal to zero is met A postcondition of the built in subroutine System out print is that the value of the parameter has been displayed on the screen Preconditions most often give restrictions on the acceptable values of parameters as in the example of Math sqrt x However they can also refer to global variables that are used in the subroutine The postcondition of a subroutine specifies the task that it performs For a function the postcondition should specify the value that the function returns Subroutines are often described by comments that explicitly specify their preconditions and postconditions When you are given a pre written subroutine a statement of its preconditions and postcondtions tells you how to use it and what it does When you are assigned to write a subroutine the preconditions and postconditions give you an exact specification of what the subroutine is expected to do I will use this approach in the example that constitutes the rest of this section I will also use it occasionally later in the book although I will generally b
187. e used to register a listener with a JTextField In the actionPerformed method the evt getActionCommand method will return a copy of the text from the JTextField It is also common to use a JTextField by checking its contents when needed with the getText method JTextAreas do not generate action events 8 2 6 Other Components This section has introduced many but not all Swing components Some Swing compo nents will not be covered at all These include e JSlider e JScrollbar and JScrollPane e JList displays a list of items to the user and allows the user to select one or several items from the list e JTable displays a two dimensional table of items and possibly allows the user to edit them e JTree displays hierarchical data in a tree like structure e JToolBar holds a row of tools such as icons and buttons The user can drag the tool bar away from the window that contains it and it becomes a separate floating tool window e JSplitPane a container that displays two components The user can drag the divid ing line between the components to adjust their relative sizes e JIabbedPane a container that displays one of a set of panels The user selects which panel is visible by clicking on a tab at the top of the pane 8 3 Programming with Components THE TWO PREVIOUS SECTIONS described some raw materials that are available in the form of layout managers and standard GUI components This secti
188. eak g setFont textFont Set the font g drawString Hello World 25 50 Draw the message end paintComponent end nested class Display The main class has an instance variable named display of type Display When the user clicks one of the buttons in the applet this variable is used to call the setColor method in the drawing surface object This is done in the applet s actionPerformed method This method is called when the user clicks any one of the three buttons so it needs some way to tell which button was pressed This information is provided in the parameter to the actionPerformed method This parameter contains an action command which in the case of a button is just the string that is displayed on the button public void actionPerformed ActionEvent evt This routine is called by the system when the user clicks on one of the buttons The response is to set the display s color accordingly String command evt getActionCommand The action command associated with the event is the text on the button that was clicked if command equals Red Set the color display setColor 1 else if command equals Green display setColor 2 else if command equals Blue display setColor 3 end actionPerformed We have now looked at all the pieces of the sample applet You can find the entire 112 CHAPTER 5 APPLETS HTML AND GUI S source code in the fi
189. ections 1 2 and 4 of chapter 12 Subsections of some these capters were also deleted A notable change has been the use of the Scanner class and the printf method for input and output The summary sections of chapters 2 3 and 4 were taken from the notes of Prof Wayne Goddard of Clemson University The sections on UML chapter 6 were adapted from the user manual of the UML tool Umbrello http docs kde org stable en_GB kdesdk umbrello The defini tions of various software engineering terms and concepts were adapted from wikipedia http wikipedia org These notes are released under the following terms and conditions This is a free textbook There are no restrictions on using or redistributing or posting on the web a complete unmodified copy of this material There are some restrictions on modified copies To be precise Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with no invariant sections front cover text or back cover text The BIEX source for these notes are available on request 10 CONTENTS Chapter 1 Design I Class Level Design 1 1 Program Design UNDERSTANDING HOW PROGRAMS WORK IS ONE THING Designing a program to per form some particular task is another thing altogether Stepwise refinement can be used to methodically develop an algorithm Stepwise refi
190. ed by the constructor should be used in some way as in PairOfDice dice Declare a variable of type PairOfDice dice new PairOfDice 1 1 Let dice refer to a new PairOfDice object that initially shows 1 1 32 CHAPTER 2 OBJECT BASED PROGRAMMING Now that we ve added a constructor to the PairOfDice class we can no longer create an object by saying new PairOfDice The system provides a default constructor for a class only if the class definition does not already include a constructor However this is not a big problem since we can add a second constructor to the class one that has no parameters In fact you can have as many different constructors as you want as long as their signatures are different that is as long as they have different numbers or types of formal parameters In the PairOfDice class we might have a constructor with no parameters which produces a pair of dice showing random numbers public class PairOfDice public int die1 Number showing on the first die public int die2 Number showing on the second die public PairOfDice Constructor Rolls the dice so that they initially show some random values roll Call the roll method to roll the dice public PairOfDice int val1 int val2 Constructor Creates a pair of dice that are initially showing the values vali and val2 diet vali Assign specified values die2 val2 to the instance variables public
191. ed for a particular student the method getAverage will compute an average using that student s test grades Different students can have different averages Again this is what it means to say that an instance method belongs to an individual object not to the class In Java a class is a type similar to the built in types such as int and boolean So a class name can be used to specify the type of a variable in a declaration statement the type of a formal parameter or the return type of a function For example a program could define a variable named std of type Student with the statement Student std However declaring a variable does not create an object This is an important point which is related to this Very Important Fact In Java no variable can ever hold an object A variable can only hold a reference to an object You should think of objects as floating around independently in the computer s mem ory In fact there is a special portion of memory called the heap where objects live Instead 26 CHAPTER 2 OBJECT BASED PROGRAMMING of holding an object itself a variable holds the information necessary to find the object in memory This information is called a reference or pointer to the object In effect a refer ence to an object is the address of the memory location where the object is stored When you use a variable of class type the computer uses the reference in the variable to find the actual object Objects are act
192. edQ mouseExitedQ mousePressedQ mouseReleasedQ paintComponent e eeo faceUp boolea F height int YF width int amp Cardo colorg draw equalsd faceUpo flipo a maing rank suit toStringd interfaces CardNames F ace int F black int F club int F diamond int SF heart int F jack int F king int F queen int red int F spade int Figure 7 2 Class diagram for the Solitaire app we do is write all the common functionality in a base class called CardPile We then specialise this class to create the concrete classes for each pile A class diagram for this application is shown above The CardPile class the base class package solitaire import java awt Graphics import java util LinkedList import java util List public abstract class CardPil protected List pile protected int x protected int y Make an Empty P public CardPile int x pile this x x this y y e ile int y f new LinkedList 146 CHAPTER 7 A SOLITAIRE GAME KLONDIKE public boolean empty return pile isEmpty public Card topCard if empty return Card pile get pile size 1 else return null public Card pop if empty return Card pile remove pile size 1 else return null public boolean includes int tx int ty return x lt tx amp amp tx lt x Card width amp amp y lt ty amp amp
193. efficient for both types of lists You would probably find it easier to write an orderedInsert method using array like indexing with the methods get index and add index obj However that method would be horribly inefficient for LinkedLists because get index is so inefficient for such lists You can find a program that tests this method in the file ListInsert java 9 4 2 Sorting Sorting a list is a fairly common operation and there should really be a sorting method in the List interface For some reason there is not but methods for sorting Lists are available as static methods in the class java util Collections This class contains a variety of static utility methods for working with collections The command Collections sort list can be used to sort a list into ascending order The items in the list must implement the Comparable interface This method will work for example for lists of Strings If a Comparator is provided as a second argument Collections sort list comparator then the comparator will be used to compare the items in the list As mentioned in the previous section a Comparator is an object that defines a compare method that can be used to compare two objects We ll see an example of using a Comparator in Section4 The Collections class has at least two other useful methods for modifying lists Collections shuffle list will rearrange the elements of the list into a random or der Collections reverse list w
194. efined Instance members also have full names but remember that instance variables and methods are actually contained in objects not classes The full name of an instance member has to contain a reference to the object that contains the instance member To get at an instance variable or method from outside the class definition you need a variable that refers to the object Then the full name is of the form variable name simple name But suppose you are writing the definition of an instance method in some class How can you get a reference to the object that contains that instance method You might need such a reference for example if you want to use the full name of an instance variable because the simple name of the instance variable is hidden by a local variable or parameter Java provides a special predefined variable named this that you can use for such purposes The variable this is used in the source code of an instance method to refer to the object that contains the method This intent of the name this is to refer to this object the one right here that this very method is in If x is an instance variable in the same object then this x can be used as a full name for that variable If otherMethod 3 2 THIS AND SUPER 65 is an instance method in the same object then this otherMethod could be used to call that method Whenever the computer executes an instance method it automatically sets the variable this to refer
195. efinitions of instance methods The variable super is for use in a subclass Like this super refers to the object that contains the method But it s forgetful It forgets that the object belongs to the class you are writing and it remembers only that it belongs to the superclass of that class The point is that the class can contain additions and modifications to the superclass super doesn t know about any of those additions and modifications it can only be used to refer to methods and variables in the superclass Let s say that the class that you are writing contains an instance method doSomething Consider the subroutine call statement super doSomething Now super doesn t know anything about the doSomething method in the subclass It only knows about things in the superclass so it tries to execute a method named doSomething from the superclass If there is none if the doSomething method was an addition rather than a modification you ll get a syntax error The reason super exists is so you can get access to things in the superclass that are hid den by things in the subclass For example super x always refers to an instance variable named x in the superclass This can be useful for the following reason If a class contains an instance variable with the same name as an instance variable in its superclass then an object of that class will actually contain two variables with the same name one defined as part of the class
196. eflections It decides which reflec tions to draw based on the setting of the symmetry menu Where the mousePressed 182 CHAPTER 8 ADVANCED GUI PROGRAMMING mouseDragged and mouseReleased methods call drawFigure they should call putMultiFigure instead The source code also has a repaintRect method that calls repaint on a rectangle that contains two given points You can treat this in the same way as drawFigure adding a repaintMultiRect that calls repaintRect and replacing each call to repaintRect with a call to repaintMultiRect Al ternatively if you are willing to let your applet be a little less efficient about re painting you could simply replace each call to repaintRect with a simple call to repaint without parameters This just means that the applet will redraw a larger area than it really needs to If x y is a point in a component that is width pixels wide and height pixels high then the reflections of this point are obtained as follows The horizontal reflection is width x y The two vertical reflections are x height y and width x height y To get the four diagonal reflections first compute the diagonal reflection of x y as a int double y height width b int double x width height Then use the horizontal and vertical reflections of the point a b a b width a b a height b width a height b The diagonal reflections are harder t
197. ehave A Player object represents a player of one particular poker game If Player were a static nested class on the other hand it would represent the general idea of a poker player independent of a particular poker game In some cases you might find yourself writing a nested class and then using that class in just a single line of your program Is it worth creating such a class Indeed it can be but for cases like this you have the option of using an anonymous nested class An anonymous class is created with a variation of the new operator that has the form new superclass or interface methods and variables This constructor defines a new class without giving it a name and it simultaneously creates an object that belongs to that class This form of the new operator can be used in any statement where a regular new could be used The intention of this expression is to create a new object belonging to a class that is the same as superclass or interface but with these methods and variables added The effect is to create a uniquely customized object just at the point in the program where you need it Note that it is possible to base an anonymous class on an interface rather than a class In this case the anonymous class must implement the interface by defining all the methods that are declared in the interface Anonymous classes are most often used for handling events in graphical user inter faces and we will enco
198. el In then next few sections we will be studying Swing components in more detail Most Swing components are defined by subclasses of the javax swing JComponent class A JComponent cannot stand on its own It must be contained in some other component We have seen for example that JPanels can act as containers for other JComponents At the top level of this containment hierarchy are classes such as JApplet A JApplet is not a JComponent but it can serve as a container for JComponents A JApplet is a top level container that is meant to appear on a Web page We ll see two more top level container classes JFrame and JDialog which can be used to create independent windows on the computer screen The basic properties of components and containers are actually defined by the AWT classes java awt Component and java awt Container Occasionally you will see these classes used in Swing For example the getContentPane method in a JApplet has a return type of Container rather than JPanel or JComponent as you might expect A JPanel is a container that is itself a Jcomponent A JPanel can contain other com ponents and it can in turn be contained in another component The fact that panels can contain other panels means that you can have many levels of components containing other components as shown in the illustration on the right Several other classes such as Box and TabbedPane also define components that can be used as containers This leads to two q
199. el buttonBar new JPanel This panel will hold three buttons and will appear at the bottom of the applet buttonBar setBackground Color gray Change the background color of the button panel so that the buttons will stand out better JButton redButton new JButton Red Create a new button Red is the text displayed on the button redButton addActionListener this Set up the button to send an action event to this applet when the user clicks the button The parameter this is a name for the applet object that we are creating so action events from the button will be handled by calling the actionPerformed method in this class buttonBar add redButton Add the button to the buttonBar panel JButton greenButton new JButton Green the second button greenButton addActionListener this buttonBar add greenButton JButton blueButton new JButton Blue the third button blueButton addActionListener this buttonBar add blueButton getContentPane add buttonBar BorderLayout SOUTH Add button panel to the bottom of the content pane end initO Notice that the variables buttonBar redButton greenButton and blueButton are local to the init method This is because once the buttons and panel have been added to the applet the variables are no longer needed The objects continue to exist since they 110 CHAPTER 5 APPLETS HTML AND GUI S have bee
200. en more is true Suppose the statement is in a loop and gets executed many times If the value of oneShape changes as the loop is executed it is possible that the very same statement oneShape redraw will call different methods and draw different shapes as it is executed over and over We say that the redraw method is polymorphic A method is polymorphic if the action performed by the method depends on the actual type of the object to which the method is applied Polymorphism is one of the major distinguishing features of object oriented programming Perhaps this becomes more understandable if we change our terminology a bit In object oriented programming calling a method is often referred to as sending a message to an object The object responds to the message by executing the appropriate method The statement oneShape redraw is a message to the object referred to by oneShape Since that object knows what type of object it is it knows how it should respond to the message From this point of view the computer always executes oneShape redraw in the same way by sending a message The response to the message depends naturally on who receives it From this point of view objects are active entities that send and receive messages and polymorphism is a natural even necessary part of this view Polymorphism just means that different objects can respond to the same message in different ways 3 1 INHERITANCE POLYMORPHIS
201. ence see Applet HelloWorldSpectrum Figure 8 4 HelloWorldSpectrum Here is part of the source code for this applet omitting the definition of the nested class that defines the drawing surface 8 5 TIMERS ANIMATION AND THREADS 171 public class HelloWorldSpectrum extends JApplet Display display A JPanel belonging to a nested Display class used for displaying Hello World It defines a method setColor Color for setting the color of the displayed message JButton startStopButton The button that will be used to start and stop the animation Timer timer The timer that drives the animation A timer is started when the user starts the animation Each time an ActionEvent is received from the timer the color of the message will change The value of this variable is null when the animation is not in progress int colorIndex This is be a number between O and 100 that will be used to determine the color It will increase by 1 each time a timer event is processed public void init This is called by the system to initialize the applet It adds a button to the south position in the applet s content pane and it adds a display panel to the center position so that it will fill the rest of the content pane display new Display The component that displays Hello World getContentPane add display BorderLayout CENTER Adds the disp
202. er applet on the other hand loads the image with its own getImage method Image cards getImage getCodeBase smallcards gif highLow new HighLowFrame cards 5 6 2 Dialogs In addition to JFrame there is another type of window in Swing A dialog box is a type of window that is generally used for short single purpose interactions with the user For example a dialog box can be used to display a message to the user to ask the user a question or to let the user select a color In Swing a dialog box is represented by an object belonging to the class JDialog Like a frame a dialog box is a separate window Unlike a frame however a dialog box is not completely independent Every dialog box is associated with a frame or another dialog box which is called its parent The dialog box is dependent on its parent For example if the parent is closed the dialog box will also be closed It is possible to create a dialog box without specifying a parent but in that case a default frame is used or an invisible frame is created to serve as the parent Dialog boxes can be either modal or modeless When a modal dialog is created its parent frame is blocked That is the user will not be able to interact with the parent until the dialog box is closed Modeless dialog boxes do not block their parents in the same way so they seem a lot more like independent windows In practice modal dialog boxes are easier to use and are much more common th
203. er can enter some text The applet should have a button When the user clicks on the button the applet should count the number of lines in the user s input the number of words in the user s input and the number of characters in the user s input This information should be displayed on three labels in the applet Recall that if textInput is a JTextArea then you can get the contents of the JTextArea by calling the function textInput getText This function returns a String containing all the text from the JTextArea The number of characters is just the length of this String Lines in the String are separated by the new line character n so the number of lines is just the number of new line characters in the String plus one Words are a little harder to count Exercise 3 4 has some advice about finding the words in a String Essentially you want to count the number of characters that are first characters in words Don t forget to put your JTextArea in a JScrollPane Scrollbars should appear when the user types more text than will fit in the available area Here is my applet 180 3 CHAPTER 8 ADVANCED GUI PROGRAMMING The RGBColorChooser applet lets the user set the red green and blue levels in a color by manipulating sliders Something like this could make a useful custom component Such a component could be included in a program to allow the user to specify a drawing color for example Rewrite the RGBColorChooser as a compo
204. er it has been created 8 2 BASIC COMPONENTS AND THEIR EVENTS 157 8 2 1 The JButton Class An object of class JButton is a push button You ve already seen buttons used in the previous chapter but we can use a review of JButtons as a reminder of what s involved in using components events and listeners Some of the methods described here are new e Constructors The JButton class has a constructor that takes a string as a parame ter This string becomes the text displayed on the button For example stopGoButton new JButton Go Events When the user clicks on a button the button generates an event of type ActionEvent This event is sent to any listener that has been registered with the button Listeners An object that wants to handle events generated by buttons must imple ment the ActionListener interface This interface defines just one method pubic void actionPerformed ActionEvent evt which is called to notify the object of an action event Registration of Listeners In order to actually receive notification of an event from a button an ActionListener must be registered with the button This is done with the button s addActionListener method For example stopGoButton addActionListener buttonHandler e Event methods When actionPerformed evt is called by the button the parame ter evt contains information about the event This information can be retrieved by calling methods in the Ac
205. er with pointers Both of these classes implement an interface java util List which specifies operations that are available for all lists Both list classes support the basic list operations and an abstract data type is defined by its operations not by its representation So why two classes Why not a single List class with a single representation The problem is that there is no single representation of lists for which all list operations are efficient For some operations linked lists are more efficient than arrays For others arrays are more efficient In a particular application of lists it s likely that only a few operations will be used frequently You want to choose the representation for which the frequently used operations will be as efficient as possible Broadly speaking the LinkedList class is more efficient in applications where items will often be added or removed at the beginning of the list or in the middle of the list In an array these operations require moving a large number of items up or down one position in the array to make a space for a new item or to fill in the hole left by the removal of an item In a linked list nodes can be added or removed at any position by changing a few pointer values The ArrayList class is more efficient when random access to items is required Random access means accessing the n th item in the list for any integern This is trivial for an array but for a linked list it means starting at the
206. ers Similarly std can be used to call the getAverage instance method in the object by saying std getAverage To print out the student s average you could say System out println Your average is std getAverage More generally you could use std name any place where a variable of type String is legal You can use it in expressions You can assign a value to it You can even use it to call subroutines from the String class For example std name length is the number of characters in the student s name It is possible for a variable like std whose type is given by a class to refer to no object at all We say in this case that std holds a null reference The null reference is written in Java as null You can store a null reference in the variable std by saying std null and you could test whether the value of std is null by testing if std null If the value of a variable is null then it is of course illegal to refer to instance variables or instance methods through that variable since there is no object and hence no instance variables to refer to For example if the value of the variable std is nu11 then it would be illegal to refer to std test1 If your program attempts to use a null reference illegally like this the result is an error called a null pointer exception Let s look at a sequence of statements that work with objects 2 1 OBJECTS INSTANCE METHODS AND INSTANCE VARIABLES 27 Studen
207. ers or only non static However it is possible to mix static and non static members in a single class and we ll see a few examples later in this chapter where it is reasonable to do so By the way static member variables and static member subroutines in a class are sometimes called class variables and class methods since they belong to the class itself rather than to instances of that class This terminology is most useful when the class contains both static and non static members So far I ve been talking mostly in generalities and I haven t given you much idea what you have to put in a program if you want to work with objects Let s look at a specific example to see how it works Consider this extremely simplified version of a Student class which could be used to store information about students taking a course class Student String name Student s name double testi test2 test3 Grades on three tests double getAverage compute average test grade return testi test2 test3 3 end of class Student None of the members of this class are declared to be static so the class exists only for creating objects This class definition says that any object that is an instance of the Student class will include instance variables named name test1 test2 and test3 and it will include an instance method named getAverage The names and tests in different objects will generally have different values When call
208. es the mouse tell the system to call the drawingSurface s paintComponent method drawingSurface repaint end init end class ClickableRandomStrings2 5 5 Introduction to Layouts and Components IN PRECEDING SECTIONS YOU VE SEEN how to use a graphics context to draw on the screen and how to handle mouse events and keyboard events In one sense that s all there is to GUI programming If you re willing to program all the drawing and handle all the mouse and keyboard events you have nothing more to learn However you would either be doing a lot more work than you need to do or you would be limiting yourself to very simple user interfaces A typical user interface uses standard GUI components such as buttons scroll bars text input boxes and menus These components have already been written for you so you don t have to duplicate the work involved in developing them They know how to draw themselves and they can handle the details of processing the mouse and keyboard events that concern them Consider one of the simplest user interface components a push button The button has a border and it displays some text This text can be changed Sometimes the button is disabled so that clicking on it doesn t have any effect When it is disabled its appearance changes When the user clicks on the push button the button changes appearance while the mouse button is pressed and changes back when the mouse button is re
209. ess obvious aspect of the state Sometimes a game is in progress and the user is supposed to make a prediction about the next card Sometimes we are between games and the user is supposed to click the New Game button It s a good idea to keep track of this basic difference in state The canvas class uses a boolean variable named gameInProgress for this purpose The state of the applet can change whenever the user clicks on a button The HighLowCanvas class implements the ActionListener interface and defines an actionPerformed method to respond to the user s clicks This method simply calls one of three other methods doHigher doLower or newGame depending on which button was pressed It s in these three event handling methods that the action of the game takes place We don t want to let the user start a new game if a game is currently in progress That would be cheating So the response in the newGame method is different depending on whether the state variable gameInProgress is true or false If a game is in progress the message instance variable should be set to show an error message If a game is not in progress then all the state variables should be set to appropriate values for the beginning 114 CHAPTER 5 APPLETS HTML AND GUI S of a new game In any case the board must be repainted so that the user can see that the state has changed The complete newGame method is as follows void doNewGame
210. ess to one another Following this model I will tend to declare a class public if it seems like it might have some general applicability If it is written just to play some sort of auxiliary role in a larger project Iam more likely not to make it public A member variable or subroutine in a class can also be declared to be public which means that it is accessible from anywhere It can be declared to be private which means that it accessible only from inside the class where it is defined Making a variable private gives you complete control over that variable The only code that will ever manipulate it is the code you write in your class This is an important kind of protection If no access modifier is specified for a variable or subroutine then it is accessible from any class in the same package as the class As with classes in this textbook there is no practical difference between declaring a member public and using no access modifier at all However there might be stylistic reasons for preferring one over the other And a real difference does arise once you start writing your own packages There is a third access modifier that can be applied to a member variable or subroutine If it is declared to be protected then it can be used in the class where it is defined and in any subclass of that class This is obviously less restrictive than private and more restrictive than public Classes that are written specifically to be used as a basis for ma
211. ess to the instance variables of the main class such as message and font1 public void paintComponent Graphics g super paintComponent g Call the paintComponent method from the superclass JPanel This simply fills the entire component with the component s background color int width getSize width Get this component s width int height getSize height Get this component s height for int i 0 i lt 25 i Draw one string First set the font to be one of the five available fonts at random int fontNum int 5 Math random 1 switch fontNum case 1 g setFont font1 break case 2 g setFont font2 break case 3 g setFont font3 break case 4 g setFont font4 break case 5 g setFont font5 break end switch Set the color to a bright saturated color with random hue float hue float Math random g setColor Color getHSBColor hue 1 0F 1 0F Select the position of the string at random int x y x 50 int Math random width 40 y int Math random height 20 Draw the message g drawString message x y end for end paintComponent end nested class Display end class RandomStrings 5 4 MOUSE EVENTS 101 5 4 Mouse Events EVENTS ARE CENTRAL to programming for a graphical user interface A GUI program doesn t have a main routine that outlines what will happen w
212. et the contents of the input boxes the results are Strings not numbers We need a 166 CHAPTER 8 ADVANCED GUI PROGRAMMING method to convert a string such as 42 17 into the number that it represents The stan dard class Double contains a static method Double parseDouble String for doing just that So we can get the first number entered by the user with the commands String xStr xInput getText x Double parseDouble xStr where x is a variable of type double Similarly if we wanted to get an integer value from the string xStr we could use a static method in the standard Integer class x Integer parseInt xStr This makes it fairly easy to get numerical values from a JTextField but one problem remains We can t be sure that the user has entered a string that represents a legal real number We could ignore this problem and assume that a user who doesn t enter a valid input shouldn t expect to get an answer However a more friendly program would notice the error and display an error message to the user This requires using a try catch statement which is not covered until Chapter9 of this book My program does in fact use a try catch statement to handle errors so you can get a preview of how it works Here is the actionPerformed method that responds when the user clicks on one of the buttons in the applet public void actionPerformed ActionEvent evt When the user clicks a button get the numbers fro
213. extend RuntimeException or one of its subclasses if the programmer does not want to require mandatory exception handling To create a new exception class that does 222 CHAPTER 10 CORRECTNESS AND ROBUSTNESS require mandatory handling the programmer can extend one of the other subclasses of Exception or can extend Exception itself Here for example is a class that extends Exception and therefore requires mandatory exception handling when it is used public class ParseError extends Exception public ParseError String message Constructor Create a ParseError object containing the given message as its error message super message The class contains only a constructor that makes it possible to create a ParseError object containing a given error message The statement super message calls a con structor in the superclass Exception See Section 5 5 Of course the class inherits the getMessage and printStackTrace routines from its superclass If e refers to an ob ject of type ParseError then the function call e getMessage will retrieve the error message that was specified in the constructor But the main point of the ParseError class is simply to exist When an object of type ParseError is thrown it indicates that a certain type of error has occurred Parsing by the way refers to figuring out the meaning of a string A ParseError would indicate presumably that some string being processed by the program
214. f for example you wanted to give the user a chance to save some unsaved work before closing the window To get this behavior I had to turn off the system s default handling of the close box with the command setDefaultCloseOperation DO_NOTHING_ON_CLOSE and I had to program my own response instead I did this by registering a window listener for the frame When certain operations are performed on a window the window generates an event of type WindowEvent You can program a response to such events in the usual way by registering a listening object of type WindowListener with the frame The JFrame class has an addWindowListener method for this purpose The WindowListener must define seven event handling methods including the poorly named public void windowClosing WindowEvent evt and public void windowClosed WindowEvent evt The windowClosing event is generated when the user clicks the close box of the window The windowClosed event is generated when the window is actually being dis posed The other five methods in the WindowListener interface are more rarely used Fortunately you don t have to worry about them if you use the WindowAdapter class The WindowAdapter class implements the WindowListener interface but defines all the WindowListener methods to be empty You can define a WindowListener by creating a subclass of WindowAdapter and providing definitions for just those methods that you ac tually need In the HighLowFrame cl
215. f the program is a simple while loop As usual the I O operations can throw exceptions so this must be done in a try catch statement while true int data source read if data lt 0 break copy write data The file copy command in an operating system such as DOS or UNIX uses command line arguments to specify the names of the files For example the user might say copy original dat backup dat to copy an existing file original dat to a file named backup dat Command line arguments can also be used in Java programs The com mand line arguments are stored in the array of strings args which is a parameter to the main routine The program can retrieve the command line arguments from this array For example if the program is named CopyFile and if the user runs the program with the command java CopyFile work dat oldwork dat then in the program args 0 will be the string work dat and args 1 will be the string oldwork dat The value of args length tells the program how many command line arguments were specified by the user My CopyFile program gets the names of the files from the command line arguments It prints an error message and exits if the file names are not specified To add a little interest there are two ways to use the program The command line can simply specify the two file names In that case if the output file already exists the program will print an error message and en
216. fficiently construct programs that verifyably meet their specifications and that are easy to modify if necessary There is a wide range of method ologies that can be applied to help in the systematic design of programs Most of these methodologies seem to involve drawing little boxes to represent program components with labeled arrows to represent relationships among the boxes We have been discussing object orientation in programming languages which is rel evant to the coding stage of program development But there are also object oriented methodologies for analysis and design The question in this stage of the software life cycle is How can one discover or invent the overall structure of a program As an example of a rather simple object oriented approach to analysis and design consider this advice Write down a description of the problem Underline all the nouns in that description The nouns should be considered as candidates for becoming classes or objects in the program design Similarly underline all the verbs These are candidates for methods This is your starting point Further analysis might uncover the need for more classes and methods and it might reveal that subclassing can be used to take advantage of similarities among classes This is perhaps a bit simple minded but the idea is clear and the general approach can be effective Analyze the problem to discover the concepts that are involved and create classes to represent th
217. g JFrame parent String question Again the parent can be null and the question can include newline characters The dialog box will contain an input box and an OK button and a Cancel button If the user clicks Cancel or closes the dialog box in some other way then the return value of 5 7 QUIZ QUESTIONS 123 the method is null If the user clicks OK then the return value is the string that was entered by the user Note that the return value can be an empty string which is not the same as a null value if the user clicks OK without typing anything in the input box If you want to use an input dialog to get a numerical value from the user you will have to convert the return value into a number Finally a confirm dialog presents a question and three response buttons Yes No and Cancel A confirm dialog can be shown by calling int JOptionPane showConfirmDialog JFrame parent String question The return value tells you the user s response It is one of the following constants e JOptionPane YES_OPTION the user clicked the Yes button e JOptionPane NO_OPTION the user clicked the No button e JOptionPane CANCEL_OPTION the user clicked the Cancel button e JOptionPane CLOSE_OPTION the user closed the dialog in some other way I use a confirm dialog in the HighLowFrame example earlier on this page When the user clicks the close box of a HighLowFrame I display a confirm
218. g to be able to do we can provide instance methods for sorting the cards in the hand Here is a full specification for a reusable Hand class Constructor and instance methods in class Hand public Hand f Create a Hand object that is initially empty public void clear Discard all cards from the hand making the hand empty public void addCard Card c Add the card c to the hand c should be non null If c is null nothing is added to the hand public void removeCard Card c If the specified card is in the hand it is removed public void removeCard int position If the specified position is a valid position in the hand then the card in that position is removed public int getCardCount Return the number of cards in the hand public Card getCard int position Get the card from the hand in given position where positions are numbered starting from 0 If the specified position is not the position number of a card in the hand then null is returned public void sortBySuit Sorts the cards in the hand so that cards of the same suit are grouped together and within a suit the cards are sorted by value Note that aces are considered to have the lowest value 1 public void sortByValue Sorts the cards in the hand so that cards of the same value are grouped together Cards with the same value are sorted by suit Note that aces are considered to have the l
219. galArgumentException A can t be zero else double disc B B 4x AxC if disc lt 0 throw new IllegalArgumentException Discriminant lt zero return B Math sqrt disc 2 A As discussed in the previous section The computation in this subroutine has the pre conditions that A 0 and B B 4 A C gt 0 The subroutine throws an exception of type IllegalArgumentException when either of these preconditions is violated When an illegal condition is found in a subroutine throwing an exception is often a reasonable response If the program that called the subroutine knows some good way to handle the error it can catch the exception If not the program will crash and the programmer will know that the program needs to be fixed 10 3 1 Mandatory Exception Handling In the preceding example declaring that the subroutine root can throw an IllegalArgumentException is just a courtesy to potential readers of this routine This 10 4 PROGRAMMING WITH EXCEPTIONS 221 is because handling of IllegalArgumentExceptions is not mandatory A routine can throw an IllegalArgumentException without announcing the possibility And a program that calls that routine is free either to catch or to ignore the exception just as a program mer can choose either to catch or to ignore an exception of type Nul1lPointerException For those exception classes that require mandatory handling the situation is different If a subroutin
220. generate events and the program must respond to these events as they occur Event driven programming builds on all the skills you have learned in the first five chapters of this text You need to be able to write the subroutines that respond to events Inside these subroutines you are doing the kind of programming in the small that was covered in Chapters 2 and 3 And of course objects are everywhere Events are objects Applets and other GUI components are objects Events are handled by instance methods contained in objects In Java event oriented programming is object oriented program ming This chapter covers the basics of applets graphics components and events There is also a section that covers HyperText Markup Language HTML the language used for writing Web pages The discussion of applets and GUI s will continue in another next chapter with more details and with more advanced techniques 5 1 The Basic Java Applet and JApplet JAVA APPLETS ARE SMALL PROGRAMS that are meant to run on a page in a Web browser Very little of that statement is completely accurate however An applet is not a complete program It doesn t have to be small And while applets are generally meant to be used on Web pages there are other ways to use them An applet is inherently part of a graphical user interface It is a type of graphical component that can be displayed in a window whether belonging to a Web browser or to some other program When shown
221. getParameter String paramName The parameter paramName corresponds to the param name in a PARAM tag If the specified paramName actually occurs in one of the PARAM tags then getParameter returns 5 3 GRAPHICS AND PAINTING 91 the associated param value If the specified paramName does not occur in any PARAM tag then getParameter returns the value null Parameter names are case sensitive so you can t use size in the PARAM tag and ask for Size in getParameter By the way if you put anything besides PARAM tags between lt APPLET gt and lt APPLET gt it will be ignored by any browser that supports Java On the other hand a browser that does not support Java will ignore the APPLET and PARAM tags This means that if you put a message such as Your browser doesn t support Java between lt APPLET gt and lt APPLET gt then that message will only appear in browsers that don t support Java Here is an example of an APPLET tag with PARAMs and some extra text for display in browsers that don t support Java lt APPLET code ShowMessage class WIDTH 200 HEIGHT 50 gt lt PARAM NAME message VALUE Goodbye World gt lt PARAM NAME font VALUE Serif gt lt PARAM NAME size VALUE 36 gt lt p align center gt Sorry but your browser doesn t support Java lt p gt lt APPLET gt The applet ShowMessage would presumably read these parameters in its init method which might go something like this String
222. git number representing the card s rank 1 ACE 2 10 cards 2 to 10 11 JACK 12 QUEEN 13 KING e g c12 is the Queen of clubs d1 is the Ace of Diamonds h8 8 of hearts There are two images of the back of a card b1fv gif and b2fv gif The testing of the Card class can be done by setting up a test harness This could simply be a main method in the Card class like this one You will off course make changes to this to do various tests public static void main String args class Panel extends JPanel a method local inner class Card c Panel c new Card 1 13 public void PanelTest method to test Cards repaint c flipQ repaint public void paintComponent Graphics g super paintComponent g c draw g 20 10 end of class Panel 144 CHAPTER 7 A SOLITAIRE GAME KLONDIKE JFrame frame new JFrame frame setSize new Dimension 500 500 frame setDefaultCloseOperation JFrame EXIT_ON_CLOSE Panel p new Panel frame setContentPane p frame show p PanelTest end of main method The CardNames Interface The CardNames class is an interface defining names public interface CardNames public static final int heart 0 public static final int diamond 1 public static final int club 2 public static final int spade 3 public static final int ace 1 public static final int jack 11 public static final int queen 12 public static final int king 13 publi
223. gt Hobart and William Smith Colleges lt BR gt Geneva NY 14456 lt BR gt 5 2 HTML BASICS 89 If you want extra vertical space in your web page you can use several lt BR gt s ina row Similarly you need a tag to indicate how the text should be broken up into paragraphs This is done with the lt P gt tag which should be placed at the beginning of every para graph The lt P gt tag has a matching lt P gt which should be placed at the end of each paragraph The closing lt P gt is technically optional but it is considered good form to use it If you want all the lines of the paragraph to be shoved over to the right you can use lt PALIGN RIGHT gt instead of lt P gt This is mostly useful when used with one short line or when used with lt BR gt to make several short lines You can also use lt PALIGN CENTER gt for centered lines By the way if tags like lt P gt and lt HR gt have special meanings in HTML you might wonder how I can get them to appear here on this page To get certain special characters to appear on the page you have to use an entity name in the HTML source code The entity name for lt is amp 1t and the entity name for gt is amp gt Entity names begin with amp and end with a semicolon The character amp is itself a special character whose entity name is amp There are also entity names for nonstandard characters such as the accented e which has the entity name amp eacute The res
224. gure 3 2 Hierarchy Several classes can be declared as subclasses of the same superclass The subclasses which might be referred to as sibling classes share some structures and behaviors namely the ones they inherit from their common superclass The superclass expresses these shared structures and behaviors In the diagram to the left classes B C and D are sibling classes Inheritance can also extend over several generations of classes This is shown in the diagram where class E is a subclass of class D which is itself a subclass of class A In this case class E is considered to be a subclass of class A even though it is not a direct subclass Let s look at an example Suppose that a program has to deal with motor vehicles including cars trucks and motorcycles This might be a program used by a Depart ment of Motor Vehicles to keep track of registrations The program could use a class named Vehicle to represent all types of vehicles The Vehicle class could include in stance variables such as registrationNumber and owner and instance methods such as transferOwnership These are variables and methods common to all vehicles Three subclasses of Vehicle Car Truck and Motorcycle could then be used to hold vari ables and methods specific to particular types of vehicles The Car class might add an instance variable numberOfDoors the Truck class might have numberOfAxels and the Motorcycle class could have a boolea
225. h keys and values are of type Object just as for HashMaps Every Object has a hash code so at least you don t have to define your own hash functions Also you do not have to worry about increasing the size of the table when it becomes too full You should also write a short program to test your solution 4 A predicate is a boolean valued function with one parameter Some languages use predicates in generic programming Java doesn t but this exercise looks at how predicates might work in Java In Java we could use predicate objects by defining an interface public interface Predicate public boolean test OUbject obj The idea is that an object that implements this interface knows how to test objects in some way Define a class Predicates that contains the following generic methods for working with predicate objects public static void remove Collection coll Predicate pred Remove every object obj from coll for which pred test obj is true public static void retain Collection coll Predicate pred Remove every object obj from coll for which pred test obj is false That is retain the objects for which the predicate is true 9 6 PROGRAMMING EXERCISES 205 public static List collect Collection coll Predicate pred Return a List that contains all the objects obj from the collection coll such that pred test obj is true public static int find ArrayList list Predicate pred
226. han they would be if the canvas were square Then the height would equal the width and the reflection of x y would just be y x To reflect a figure determined by two points x1 y1 and x2 y2 compute the reflections of both points to get the reflected figure This is really not so hard The changes you have to make to the source code are not as long as the explanation I have given here Turn your applet from the previous exercise into a stand alone application that runs as a JFrame If you didn t do the previous exercise you can do this exercise with the original SimplePaint3 java To make the exercise more interesting remove the JButtons and JComboBoxes and replace them with a menubar at the top of the frame You can design the menus any way you like but you should have at least the same functionality as in the original program As an improvement you might add an Undo command When the user clicks on the Undo button the previous drawing operation will be undone This just means returning to the image as it was before the drawing operation took place This is easy to implement as long as we allow just one operation to be undone When the off screen canvas OSI is created make a second off screen canvas undoBuf fer of the same size Before starting any drawing operation copy the image from OSI to undoBuffer You can do this with the commands Graphics undoGr undoBuffer getGraphics undoGr drawImage OSI
227. haracters and will be used for the union intersection and difference operations The user of the program will type in lines of input containing two sets separated by an operator The program should perform the operation and print the resulting set Here are some examples Input Output 1 2 3 3 5 7 1 2 3 5 7 10 9 8 7 2 4 6 8 8 5 10 15 20 0 10 20 5 15 To represent sets of non negative integers use TreeSets containing objects belong ing to the wrapper class Integer Read the user s input create two TreeSets and use the appropriate TreeSet method to perform the requested operation on the two sets Your program should be able to read and process any number of lines of input If a line contains a syntax error your program should not crash It should report the error and move on to the next line of input Note To print out a Set A of Integers you can just say System out print1n A I ve chosen the syntax for sets to be the same as that used by the system for outputting a set 3 The fact that Java has a HashMap class means that no Java programmer has to write an implementation of hash tables from scratch unless of course you are a com puter science student Write an implementation of hash tables from scratch Define the following methods get key put key value remove key containsKey key and size Do not use any of Java s generic data structures Assume that bot
228. hat just fits inside the rectangle specified by x y width and height If width equals height the oval is a circle e drawRoundRect int x int y int width int height int xdiam int ydiam Draws the outline of a rectangle with rounded corners The basic rectangle is speci fied by x y width and height but the corners are rounded The degree of rounding is given by xdiam and ydiam The corners are arcs of an ellipse with horizontal di ameter xdiam and vertical diameter ydiam A typical value for xdiam and ydiam is 16 But the value used should really depend on how big the rectangle is e draw3DRect int x int y int width int height boolean raised Draws the outline of a rectangle that is supposed to have a three dimensional effect as if it is raised from the screen or pushed into the screen The basic rectangle is specified by x y width and height The raised parameter tells whether the rectangle seems to be raised from the screen or pushed into it The 3D effect is achieved by using brighter and darker versions of the drawing color for different edges of the rectangle The documentation recommends setting the drawing color equal to the background color before using this method The effect won t work well for some colors e drawArc int x int y int width int height int startAngle int arcAngle Draws part of the oval that just fits inside the rectangle specified by x y width and height The part drawn is an arc that extends ar
229. hat this means and why it is important Question 2 Java has two types of streams character streams and byte streams Why What is the difference between the two types of streams Question 3 What is a file Why are files necessary Question 4 What is the point of the following statement out new PrintWriter new FileWriter data dat Why would you need a statement that involves two different stream classes PrintWriter and FileWriter Question 5 The package java io includes a class named URL What does an object of type URL represent and how is it used Question 6 Explain what is meant by the client server model of network communi cation Question 7 What is a Socket Question 8 What is a ServerSocket and how is it used Question 9 Network server programs are often multithreaded Explain what this means and why it is true Question 10 Write a complete program that will display the first ten lines from a text file The lines should be written to standard output System out The file name is given as the command line argument args 0 You can assume that the file contains at least ten lines Don t bother to make the program robust 11 6 Programming Exercises 1 The WordList program from Section 10 3 reads a text file and makes an alphabetical list of all the words in that file The list of words is output to another file Improve the program so that it also keeps track of the number of times that each word occurs in
230. have a text input box where the user can enter an expression involving the variable x such as x 2 or sin x 3 x This ex pression is the definition of the function When the user presses return in the text input box the applet should use the contents of the text input box to construct an object of type Expr If an error is found in the definition then the applet should dis play an error message Otherwise it should display a graph of the function Note A JTextField generates an ActionEvent when the user presses return The applet will need a JPanel for displaying the graph To keep things simple this panel should represent a fixed region in the xy plane defined by 5 lt x lt 5 and 5 lt y lt 5 To draw the graph compute a large number of points and connect them with line segments This method does not handle discontinuous functions properly doing so is very hard so you shouldn t try to do it for this exercise My applet divides the interval 5 lt x lt 5 into 300 subintervals and uses the 301 endpoints of these subintervals for drawing the graph Note that the function might be undefined at one of these x values In that case you have to skip that point A point on the graph has the form x y where y is obtained by evaluating the user s expression at the given value of x You will have to convert these real numbers to the integer coordinates of the corresponding pixel on the canvas The formulas for the conversion are a
231. he determinant of M is determinant catch RuntimeException e System out println Sorry an error has occurred e printStackTrace Since objects of type Array IndexOutOfBoundsException or NullPointerException is also of type RuntimeException this will catch array index errors and null pointer errors as well as any other type of runtime exception This shows why exception classes are organized into a class hierarchy It allows you the option of casting your net narrowly to catch only a specific type of exception Or you can cast your net widely to catch a wide class of exceptions The example I ve given here is not particularly realistic You are not very likely to use exception handling to guard against null pointers and bad array indices This is a case where careful programming is better than exception handling Just be sure that your program assigns a reasonable non null value to the array M You would certainly resent it if the designers of Java forced you to set up a try catch statement every time you wanted to use an array This is why handling of potential RuntimeExceptions is not mandatory There are just too many things that might go wrong This also shows that 10 3 EXCEPTIONS AND THE TRY CATCH STATEMENT 219 exception handling does not solve the problem of program robustness It just gives you a tool that will in many cases let you approach the problem in a more organized way The syntax of a try statement
232. he exception the computer executes the finally clause of the try statement if there is one It then continues normally with the rest of the program which follows the try statement If the exception is not immediately caught and handled the processing of the exception will continue When an exception is thrown during the execution of a subroutine and the exception is not handled in the same subroutine then that subroutine is terminated after the ex ecution of any pending finally clauses Then the routine that called that subroutine gets a chance to handle the exception That is if the subroutine was called inside a try statement that has an appropriate catch clause then that catch clause will be executed and the program will continue on normally from there Again if that routine does not handle the exception then it also is terminated and the routine that called it gets the next shot at the exception The exception will crash the program only if it passes up through the entire chain of subroutine calls without being handled A subroutine that might generate an exception can announce this fact by adding the clause throws exception class name to the header of the routine For example static double root double A double B double C throws IllegalArgumentException Returns the larger of the two roots of the quadratic equation A x x B x C 0 Throws an exception if A 0 or B B 4 A C lt 0 if A 0 throw new Ille
233. he first time but you should understand that they solve a difficult problem in an elegant way The Collection interface defines a method that can be used to obtain an iterator for any collection If coll is a collection then coll iterator returns an iterator that can be used to traverse the collection You should think of the iterator as a kind of generalized pointer that starts at the beginning of the collection and can move along the collection from one item to the next Iterators are defined by an interface named Iterator This interface defines just three methods If iter refers to an Iterator then e iter next returns the next item and advances the iterator The return value is of type Object Note that there is no way to look at an item without advancing the iterator past that item If this method is called when no items remain it will throw a NoSuchElementException e iter hasNext returns a boolean value telling you whether there are more items to be processed You should test this before calling iter next e iter remove if you call this after calling iter next it will remove the item that you just saw from the collection This might produce an UnsupportedOperationException if the collection does not support removal of items Using iterators we can write code for printing all the items in any collection Suppose that coll is of type Collection Then we can say Iterator iter coll iterator while i
234. he name properly In the new version of the class there is no way to create a Student object except by calling the constructor and that constructor automatically sets the name The programmer s life is made easier and whole hordes of frustrating bugs are squashed before they even have a chance to be born Another type of guarantee is provided by the private modifier Since the instance variable name is private there is no way for any part of the program outside the Student class to get at the name directly The program sets the value of name indirectly when it calls the constructor I ve provided a function getName that can be used from outside the class to find out the name of the student But I haven t provided any way to change the name Once a student object is created it keeps the same name as long as it exists 2 2 1 Garbage Collection So far this section has been about creating objects What about destroying them In Java the destruction of objects takes place automatically An object exists in the heap and it can be accessed only through variables that hold references to the object What should be done with an object if there are no variables that refer to it Such things can happen Consider the following two statements though in reality you d never do anything like this Student std new Student John Smith std null 36 CHAPTER 2 OBJECT BASED PROGRAMMING In the first line a reference to a newly create
235. he object oriented concepts of encapsulation inheritance and polymorphism The game is inspired by Timothy Budd s version in his book AN INTRODUCTION TO OBJECT ORIENTED PROGRAMMING Klondike Solitaire The most popular solitare game is called klondike It can be described as follows The layout of the game is shown in the figure below A single standard pack of 52 cards is used i e 4 suits spades diamonds lt hearts Q clubs amp and 13 cards 13 ranks in each suit The tableau or playing table consists of 28 cards in 7 piles The first pile has 1 card the second 2 the third 3 and so on up to 7 The top card of each pile is initially face up all other cards are face down The suit piles sometimes called foundations are built up from aces to kings in suits They are constructed above the tableau as the cards become available The object of the game is to build all 52 cards into the suit piles The cards that are not part of the tableau are initially all in the deck Cards in the deck are face down and are drawn one by one from the deck and placed face up on the discard pile From there they can be moved onto either a tableau pile or a foundation Cards are drawn from the deck until the pile is empty at this point the game is over if no further moves can be made Cards can be placed on a tableau pile only on a card of next higher rank and opposite color They can be placed on a foundation only if they are the sam
236. he square with that color The point is that this is a long computation When you click the Start button of the applet the applet creates a separate thread to do this computation In Java a thread is an object belonging to the class java lang Thread The purpose of a thread is to execute a single subroutine from beginning to end In the Mandelbrot applet that subroutine implements the above algorithm The subroutine for a thread is usually an instance method public void run that is defined in an object that implements the interface named Runnable The Runnable interface defines run as its only method The Runnable object is provided as a parameter to the constructor of the thread object It is also possible to define a thread by declaring a subclass of class Thread and defining a run method in the sub class but it is more common to use a Runnable object to provide the run method for a thread If runnableObject is an object that implements the Runnable interface then a thread can be constructed with the command Thread runner new Thread runnableObject The job of this thread is to execute the run method in runnableObject Just as with a Timer it is not enough to construct a thread You must also start it running by 178 CHAPTER 8 ADVANCED GUI PROGRAMMING calling its start method runner start When this method is called the thread will begin executing the run method in the runnableObject and it will do this
237. he thread checks this value regularly and will terminate when running becomes false running false There are a few more details of threads that you need to understand before looking at the run method from the applet First on some platforms once a thread starts running it grabs control of the CPU and no other thread can run until it yields control In Java a thread can yield control and allow other threads to run by calling the static method Thread yield I do this regularly in the run method of the applet If I did not do this then on some platforms the computational thread would block the rest of the program from running Another way for a thread to yield control is to go to sleep for a specified period of time by calling Thread sleep time where time is the number of milliseconds for which the thread will be inactive The thread in a Timer for example sleeps between events since it has nothing else to do 8 6 QUIZ QUESTIONS 179 8 6 Quiz Questions THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 7 of this on line Java textbook You should be able to answer these questions after studying that chapter Question 1 Question 2 Question 3 One of the main classes in Swing is the JComponent class What is meant by a component What are some examples Question 4 What is the function of a LayoutManager in Java Question 5 What does it mean to use a null layout manager and why would you want to do so Que
238. hen the program is run in a step by step process from beginning to end Instead the program must be prepared to respond to various kinds of events that can happen at unpredictable times and in an order that the program doesn t control The most basic kinds of events are generated by the mouse and keyboard The user can press any key on the keyboard move the mouse or press a button on the mouse The user can do any of these things at any time and the computer has to respond appropriately In Java events are represented by objects When an event occurs the system collects all the information relevant to the event and constructs an object to contain that informa tion Different types of events are represented by objects belonging to different classes For example when the user presses one of the buttons on a mouse an object belonging to a class called MouseEvent is constructed The object contains information such as the GUI component on which the user clicked the x y coordinates of the point in the com ponent where the click occurred and which button on the mouse was pressed When the user presses a key on the keyboard a KeyEvent is created After the event object is constructed it is passed as a parameter to a designated subroutine By writing that subroutine the programmer says what should happen when the event occurs As a Java programmer you get a fairly high level view of events There is lot of pro cessing that goes on between the
239. her it is black or red Create a card specified by rank and suit Know if it is face down or up Display itself face up or down Flip itself change from face down to face up and vice versa Your tasks is to design the Card class and program it It is also necessary to test your class Using Images In order to program the class we need to use images of cards There are several ways to work with images Heres a quick how to describing one way 143 a Copy the images folder into the project folder It should be copied into the top level of the CardGames folder b Using an image is a three step process Declare a variable of type Image e g Image backImage Read an image into this variable This must be done within a try catch block and assumes the images are stored in the images folder in the project try backImage ImageI0 read new File images bifv gif catch IOException i System err println Image load error Draw the image Off course you draw method will be different since you have to worry about whether the card is face up and face down and the image you draw depends on the particular card public void draw Graphics g int x int y g drawlmage backImage x y null c The naming convention of the image files is straight forward xnn gif is the format were x is a letter of the suit s spades d diamonds h hearts Q c clubs amp and nn is a one or two di
240. her small applet Here s our sample JApplet which is named HelloSwing Applet Reference see Applet HelloSwing If you click this button a new window will open with a message and an OK button Click the OK button to dismiss the window 5 1 THE BASIC JAVA APPLET AND JAPPLET 85 Java Applet Window i Hello from the Swing User Interface Library Click Me Figure 5 2 Hello Swing The button in this applet is an object that belongs to the class JButton more properly javax swing JButton When the applet is created the button must be created and added to the applet This is part of the process of initializing the applet and is done in the applet s init method In this method the button is created with the statement JButton bttn new JButton Click Me The parameter to the constructor specifies the text that is displayed on the button The button does not automatically appear on the screen It has to be added to the applet s content pane This is done with the statement getContentPane add bttn Once it has been added to the applet a JButton object mostly takes care of itself In particular it draws itself so you don t have to worry about drawing it When the user clicks the button it generates an event The applet or in fact any object can be programmed to respond to this event Event handling is the major topic in GUI programming and I will cover it in detail later Bu
241. hod which has almost the same form as the init method in the previous example 5 5 INTRODUCTION TO LAYOUTS AND COMPONENTS 113 public void init The init method lays out the applet A HighLowCanvas occupies the CENTER position of the layout On the bottom is a panel that holds three buttons The HighLowCanvas object listens for ActionEvents from the buttons and does all the real work of the program setBackground new Color 130 50 40 HighLowCanvas board new HighLowCanvas getContentPane add board BorderLayout CENTER JPanel buttonPanel new JPanel buttonPanel setBackground new Color 220 200 180 getContentPane add buttonPanel BorderLayout SOUTH JButton higher new JButton Higher higher addActionListener board buttonPanel add higher JButton lower new JButton Lower lower addActionListener board buttonPanel add lower JButton newGame new JButton New Game newGame addActionListener board buttonPanel add newGame end init In programming the drawing surface class HighLowCanvas it is important to think in terms of the states that the game can be in how the state can change and how the response to events can depend on the state The state of the game includes the cards and the message The cards are stored in an object of type Hand The message is a String These values are stored in instance variables There is also another l
242. hod is called by the system not by your program Your job as a programmer is just to provide a definition of the init method The definition of the method must have the form public void init do initialization You might wonder by the way why initialization is done in the init method rather than in a constructor In fact it is possible to define a constructor for your applet class To create the applet object the system calls the constructor that has no parameters You can write such a constructor for an applet class and can do initializations in the constructor as well as in the init method The most significant difference is that when the constructor is called the size of the applet is not available By the time init is called the size is known and can be used to customize the initialization according to the size In general though it is customary to do applet initialization in the init method Suppose for example that we want to change the colors used by the HelloWorldAp plet An applet has a background color which is used to fill the entire area of the applet before any other drawing is done and it has a foreground color which is used as the 84 CHAPTER 5 APPLETS HTML AND GUI S default color for drawing in the applet It is convenient to set these colors in the init method Here is a version of the HelloWorldApplet that does this Applet Reference see Applet HelloWorldApplet2
243. iable of type StringBuffer and x is a value of any type then the command buffer append x will add x converted into a string representation onto the end of the data that was already in the buffer This command actually modifies the buffer rather than making a copy and that can be done efficiently A long string can be built up in a StringBuffer using a sequence of append commands When the string is complete the function buffer toString will return a copy of the string in the buffer as an ordinary value of type String A number of useful classes are collected in the package java util For example this package contains classes for working with collections of objects one of the contexts in which wrapper classes for primitive types are useful We will study these collection classes in Chapter12 The class java util Date is used to represent times When a Date object is constructed without parameters the result represents the current date and time so an easy way to display this information is System out println new Date Of course to use the Date class in this way you must make it available by importing it with one of the statements import java util Date or importjava util at the beginning of your program Finally I will mention the class java util Random An object belonging to this class is a source of random numbers The standard function Math random uses one of these objects behind the scenes to generate its r
244. ial class can be created just for the purpose of defining a listening object Many people consider it to be good form to use anonymous nested classes to define listening objects You will see all of these patterns in examples in this textbook 5 4 1 MouseEvent and MouseListener The MouseListener interface specifies five different instance methods public void mousePressed MouseEvent evt public void mouseReleased MouseEvent evt public void mouseClicked MouseEvent evt public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt 5 4 MOUSE EVENTS 103 The mousePressed method is called as soon as the user presses down on one of the mouse buttons and mouseReleased is called when the user releases a button These are the two methods that are most commonly used but any mouse listener object must define all five methods You can leave the body of a method empty if you don t want to define a response The mouseClicked method is called if the user presses a mouse button and then releases it quickly without moving the mouse When the user does this all three routines mousePressed mouseReleased and mouseClicked will be called in that order In most cases you should define mousePressed instead of mouseClicked The mouseEntered and mouseExited methods are called when the mouse cursor enters or leaves the component For example if you want the component to change appearance whenever the user moves the mouse over
245. ic input with a try catch statement of the form 8 7 PROGRAMMING EXERCISES 181 try betAmount Integer parseInt betInput getText catch NumberFormatException e The input is not a number Respond by showing an error message and exiting from the doNewGame method It would be a good idea to make the JTextField uneditable while the game is in progress If betInput is the JTextField you can make it editable and uneditable by the user with the commands betAmount setEditable true and betAmount setEditable false In the paintComponent method you should include commands to display the amount of money that the user has left There is one other thing to think about The applet should not start a new game when it is first created The user should have a chance to set a bet amount before the game starts So in the constructor for the canvas class you should not call doNewGame You might want to display a message such as Welcome to Blackjack before the first game starts 6 The StopWatch component displays the text Timing when the stop watch is running It would be nice if it displayed the elapsed time since the stop watch was started For that you need to create a Timer Add a Timer to the original source code StopWatch java to display the elapsed time in seconds Create the timer in the mousePressed routine when the stop watch is started Stop the timer in the mousePressed routine when
246. ic void roll Roll the dice and redraw them super roll Call the roll method from PairOfDice redraw Call a method to draw the dice More stuff including definition of redraw Note that this allows you to extend the behavior of the ro11 method even if you don t know how the method is implemented in the superclass Here is a more complete example The RandomBrighten applet shows a disturbance that moves around in a mosaic of little squares As it moves the squares it visits become a brighter red The result looks interesting but I think it would be prettier if the pattern were symmetric The symmetric applet can be programmed as an easy extension of the original applet Figure 3 7 Symmetric Brighten Applet 3 2 THIS AND SUPER 67 In the symmetric version each time a square is brightened the squares that can be obtained from that one by horizontal and vertical reflection through the center of the mosaic are also brightened The four red squares in the picture for example form a set of such symmetrically placed squares as do the purple squares and the green squares The blue square is at the center of the mosaic so reflecting it doesn t produce any other squares it s its own reflection The original applet is defined by the class RandomBrighten This class uses features of Java that you won t learn about for a while yet but the actual task of brightening a square is done by a single method c
247. ics This method is a function that returns a graphics context that can be used for drawing on the component outside its paintComponent method The official line is that you should not do this and I will avoid it for the most part But I have found it convenient to use getGraphics in some cases since it can mean better performance for certain types of drawing Anyway if the people who designed Java really didn t want us to use it they shouldn t have made the getGraphics method public Most components do in fact do all drawing operations in their paintComponent methods What happens if in the middle of some other method you realize that the content of the component needs to be changed You should not call paintComponent directly to make the change this method is meant to be called only by the system Instead 5 3 GRAPHICS AND PAINTING 93 you have to inform the system that the component needs to be redrawn and let the system do its job by calling paintComponent You do this by calling the repaint method The method public void repaint is defined in the Component class and so can be used with any component You should call repaint to inform the system that the component needs to be redrawn The repaint method returns immediately without doing any painting itself The sys tem will call the component s paintComponent method later as soon as it gets a chance to do so after processing other pending
248. ilarities can be expressed using inher itance and polymorphism The topics covered in this section are relatively advanced aspects of object oriented programming Any programmer should know what is meant by subclass inheritance and polymorphism 3 1 Inheritance Polymorphism and Abstract Classes The term inheritance refers to the fact that one class can inherit part or all of its structure and behavior from another class class superclass class B subclass Figure 3 1 Inheritance The class that does the inheriting is said to be a subclass of the class from which it 53 54 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I inherits If class B is a subclass of class A we also say that class A is a superclass of class B Sometimes the terms derived class and base class are used instead of subclass and superclass A subclass can add to the structure and behavior that it inherits It can also replace or modify inherited behavior though not inherited structure The relationship between subclass and superclass is sometimes shown by a diagram in which the subclass is shown below and connected to its superclass In Java when you create a new class you can declare that it is a subclass of an existing class If you are defining a class named B and you want it to be a subclass of a class named A you would write class B extends A additions to and modifications of stuff inherited from class A Fi
249. ill reverse the order of the elements so that the last 9 4 LIST CLASSES 201 element is moved to the beginning of the list the next to last element to the second posi tion and so on Since an efficient sorting method is provided for Lists there is no need to write one yourself You might be wondering whether there is an equally convenient method for standard arrays The answer is yes Array sorting methods are available as static methods in the class java util Arrays The command Arrays sort A will sort an array A provided either that the base type of A is one of the primitive types except boolean or that A is an array of Objects that implement the Comparable interface You can also sort part of an array This is important since arrays are often only partially filled The command Arrays sort A fromIndex toIndex sorts the elements A fromIndex A fromIndext 1i A toIndex 1 into ascend ing order You can use Arrays sort A 0 N to sort a partially filled array which has elements in the first N positions Java does not support generic programming for primitive types In order to implement the command Arrays sort A the Arrays class contains eight methods one method for arrays of Objects and one method for each of the primitive types byte short int long float double and char 9 4 3 The TreeSet and HashSet Classes A set is a collection of Objects in which no object occurs more than once Objects obj1
250. implest type of component An object of type JLabel is just a single line of text The text cannot be edited by the user although it can be changed by your program The constructor for a JLabel specifies the text to be displayed 158 CHAPTER 8 ADVANCED GUI PROGRAMMING JLabel message new JLabel Hello World There is another constructor that specifies where in the label the text is located if there is extra space The possible alignments are given by the constants JLabel LEFT JLabel CENTER and JLabel RIGHT For example JLabel message new JLabel Hello World JLabel CENTER creates a label whose text is centered in the available space You can change the text displayed in a label by calling the label s setText method message setText Goodby World Since the JLabel class is a subclass of Component you can use methods such as setForeground with labels If you want the background color to have any effect you should call setOpaque true on the label since otherwise the JLabel might not fill in its background depending on the look and feel For example JLabel message new JLabel Hello World message setForeground Color red Display red text message setBackground Color black on a black background message setFont new Font Serif Font BOLD 18 in a bold font message setOpaque true Make sure background is filled in 8 2 3 The JCheckBox Class A JChec
251. in multiple behaviors but at the same time we can have some general methods which treat all Sounds the same For example we could create an array of Sounds to represent some music Then an iterator through the array calling play at each step will produce the right sound at the right place We can also create sub classes that do not override the method then the play of Sound will be used Another example of polymorphism is our writing of a toString method for most of our classes 72 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I Chapter 4 Object Oriented Programming II Interfaces and Nested Classes THIS SECTION simply pulls together a few more miscellaneous features of object ori ented progrmming in Java Read it now or just look through it and refer back to it later when you need this material 4 1 Interfaces Some object oriented programming languages such as C allow a class to extend two or more superclasses This is called multiple inheritance In the illustration below for example class E is shown as having both class A and class B as direct superclasses while class F has three direct superclasses class A class B Multiple Inheritance NOT allowed in Java Such multiple inheritance is not allowed in Java The designers of Java wanted to keep the language reasonably simple and felt that the benefits of multiple inheritance were not worth the cost in increased complexity However Java does have a featu
252. ine and return to the main program To avoid having an over abundance of variables in your subroutine remember that a method call such as 52 CHAPTER 2 OBJECT BASED PROGRAMMING userHand getBlackjackValue can be used anywhere that a number could be used including in an output statement or in the condition of an if statement Write a main program that lets the user play several games of Blackjack To make things interesting give the user 100 dollars and let the user make bets on the game If the user loses subtract the bet from the user s money If the user wins add an amount equal to the bet to the user s money End the program when the user wants to quit or when she runs out of money Chapter 3 Object Oriented Programming I Inheritance Polymorphism and Abstract Classes A CLASS REPRESENTS A SET OF OBJECTS which share the same structure and be haviors The class determines the structure of objects by specifying variables that are contained in each instance of the class and it determines behavior by providing the in stance methods that express the behavior of the objects This is a powerful idea However something like this can be done in most programming languages The central new idea in object oriented programming the idea that really distinguishes it from traditional pro gramming is to allow classes to express the similarities among objects that share some but not all of their structure and behavior Such sim
253. ine of the string The baseline is a horizontal line on which the characters rest Some parts of the characters such as the tail on a y or g extend below the baseline e drawLine int x1 int y1 int x2 int y2 Draws a line from the point x1 y1 to the point x2 y2 The line is drawn as if with a pen that hangs one pixel to the right and one pixel down from the x y point where the pen is located For example if g refers to an object of type Graphics then the command g drawLine x y x y which corresponds to putting the pen down at a point draws the single pixel located at the point x y e drawRect int x int y int width int height Draws the outline of a rectangle The upper left corner is at x y and the width and height of the rectangle are as specified If width equals height then the rectangle is a square If the width or the height is negative then nothing is drawn The rectangle is drawn with the same pen that is used for drawLine This means that the actual width of the rectangle as drawn is width 1 and similarly for the height There is an extra pixel along the right edge and the bottom edge For example if you want to draw a rectangle around the edges of the component you can say e drawRect 0 0 getSize width 1 getSize height 1 where g is a graphics context for the component 5 3 GRAPHICS AND PAINTING 97 e drawOval int x int y int width int height Draws the outline of an oval The oval is one t
254. ingle command ArrayList list new ArrayList new TreeSet coll This makes a sorted list of the elements of coll with no duplicates A nice example of the power of generic programming It seems by the way there is no equally easy way to get the list with duplicates To do this we would need something like a TreeSet that allows duplicates The C programming language has such a thing and refers to it as a multiset The Smalltalk language has something similar and calls it a bag Java for the time being at least lacks this data type 9 5 Quiz Questions THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 12 of this on line Java textbook You should be able to answer these questions after studying that chapter Question 1 What is meant by generic programming and what is the alternative Question 2 Java does not support generic programming with the primitive types Why not What is it about generic programming in Java that prevents it from working with primitive types such as int and double Question 3 What is an iterator and why are iterators necessary for generic program ming Question 4 Suppose that integers is a variable of type Collection and that every object in the collection belongs to the wrapper class Integer Write a code segment that will compute the sum of all the integer values in the collection Question 5 Interfaces such as List Set and Map define abstract data types Explain what this means Question 6
255. int you are ready to begin writing the program In the simple calculator applet the user types in two numbers and clicks a button The computer responds by doing a computation with the user s numbers and display ing the result The program uses two JTextField components to get the user s input The JTextFields do a lot of work on their own They respond to mouse focus and keyboard events They show blinking cursors when they are active They collect and display the characters that the user types The program only has to do three things with each JTextField Create it add it to the applet and get the text that the user has input by calling its getText method The first two things are done in the applet s init method The third getting the user s input from the input boxes is done in an actionPerformed method which responds when the user clicks on one of the buttons When a component is created in one method and used in another as the input boxes are in this case we need an instance variable to refer to it In this case I use two instance variables xInput and yInput of type JTextField to refer to the input boxes The JLabel that is used to display the result is treated similarly A JLabel is created and added to the applet in the init method When an answer is computed in the actionPerformed method the JLabel s setText method is used to display the answer in the label I use an instance variable named answer of ty
256. ion is an exception class that requires mandatory exception handling this means that you can t use the read method except inside a try statement or in a subroutine that is itself declared with a throws IOException clause The InputStream class also defines methods for reading several bytes of data in one step into an array of bytes However InputStream provides no convenient methods for reading other types of data such as int or double from a stream This is not a problem because you ll never use an object of type InputStream itself Instead you ll use subclasses of InputStream that add more convenient input methods to InputStream s rather primitive capabilities Similarly the OutputStream class defines a primitive output method for writing one byte of data to an output stream the method public void write int b throws IOException but again in practice you will almost always use higher level output operations de fined in some subclass of OutputStream 11 2 STREAMS READERS AND WRITERS 237 The Reader and Writer classes provide very similar low level read and write oper ations But in these character oriented classes the I O operations read and write char values rather than bytes In practice you will use sub classes of Reader and Writer as discussed below One of the neat things about Java s I O package is that it lets you add capabilities to a stream by wrapping it in another stream object that provides those c
257. ional circumstances without crashing One approach to writing robust programs is to anticipate the problems that might arise and to include tests in the program for each possible prob lem For example a program will crash if it tries to use an array element A i when i is not within the declared range of indices for the array A A robust program must antic ipate the possibility of a bad index and guard against it This could be done with an if statement if i lt 0 i gt A length Do something to handle the out of range index i else Process the array element A i There are some problems with this approach It is difficult and sometimes impossible to anticipate all the possible things that might go wrong It s not always clear what to do when an error is detected Furthermore trying to anticipate all the possible problems can turn what would otherwise be a straightforward program into a messy tangle of if statements Java like its cousin C provides a neater more structured alternative method for dealing with errors that can occur while a program is running The method is referred to as exception handling The word exception is meant to be more general than error It includes any circumstance that arises as the program is executed which is meant to be treated as an exception to the normal flow of control of the program An exception might be an error or it might just be a special case that you w
258. is a function that returns the value of the expression when the number num is sub stituted for the variable x in the expression For example if Expr represents the 10 6 PROGRAMMING EXERCISES 229 expression 3 x 1 then func value 5 is 3 5 1 or 16 If the expression is unde fined for the specified value of x then the special value Double NaN is returned Finally func getDefinition returns the definition of the expression This is just the string that was used in the constructor that created the expression object For this exercise you should write a program that lets the user enter an expression If the expression contains an error print an error message Otherwise let the user enter some numerical values for the variable x Print the value of the expression for each number that the user enters However if the expression is undefined for the specified value of x print a message to that effect You can use the boolean valued function Double isNaN val to check whether a number val is Double NaN The user should be able to enter as many values of x as desired After that the user should be able to enter a new expression Here is an applet that simulates my solution to this exercise so that you can see how it works 5 This exercises uses the class Expr which was described in Exercise9 4 For this ex ercise you should write an applet that can graph a function f x whose definition is entered by the user The applet should
259. itself and one defined as part of the superclass The variable in the 66 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I subclass does not replace the variable of the same name in the superclass it merely hides it The variable from the superclass can still be accessed using super When you write a method in a subclass that has the same signature as a method in its superclass the method from the superclass is hidden in the same way We say that the method in the subclass overrides the method from the superclass Again however super can be used to access the method from the superclass The major use of super is to override a method with a new method that extends the behavior of the inherited method instead of replacing that behavior entirely The new method can use super to call the method from the superclass and then it can add addi tional code to provide additional behavior As an example suppose you have a PairOfDice class that includes a ro11 method Suppose that you want a subclass GraphicalDice to represent a pair of dice drawn on the computer screen The rol1 method in the GraphicalDice class should do everything that the rol1 method in the PairOfDice class does We can express this with a call to super rol1 But in addition to that the rol1 method for a GraphicalDice object has to redraw the dice to show the new values The GraphicalDice class might look something like this public class GraphicalDice extends PairOfDice publ
260. ject and a Deck object Pick a random number between 2 and 6 Deal that many cards from the deck and add them to the hand Print out all the cards in the hand and then print out the value computed for the hand by getBlackjackValue Repeat this as long as the user wants to continue Your program will depend on Card java Deck java Hand java and BlackjackHand java 5 Write a program that let s the user play Blackjack The game will be a simplified version of Blackjack as it is played in a casino The computer will act as the dealer As in the previous exercise your program will need the classes defined in Card java Deck java Hand java and BlackjackHand java This is the longest and most com plex program that has come up so far in the exercises You should first write a subroutine in which the user plays one game The subroutine should return a boolean value to indicate whether the user wins the game or not Return true if the user wins false if the dealer wins The program needs an object of class Deck and two objects of type BlackjackHand one for the dealer and one for the user The general object in Blackjack is to get a hand of cards whose value is as close to 21 as possible without going over The game goes like this First two cards are dealt into each player s hand If the dealer s hand has a value of 21 at this point then the dealer wins Otherwise if the user has 21 then the user wins This is called a Blackjack
261. jects not objects themselves has a couple of other consequences that you should be aware of They follow logically if you just keep in mind the basic fact that the object is not stored in the variable The object is somewhere else the variable points to it Suppose that a variable that refers to an object is declared to be final This means that the value stored in the variable can never be changed once the variable has been initialized The value stored in the variable is a reference to the object So the variable will continue to refer to the same object as long as the variable exists However this does not prevent the data in the object from changing The variable is final not the object It s perfectly legal to say 2 2 CONSTRUCTORS AND OBJECT INITIALIZATION 29 final Student stu new Student stu name John Doe Change data in the object The value stored in stu is not changed Next suppose that obj is a variable that refers to an object Let s consider what hap pens when obj is passed as an actual parameter to a subroutine The value of obj is assigned to a formal parameter in the subroutine and the subroutine is executed The subroutine has no power to change the value stored in the variable obj It only has a copy of that value However that value is a reference to an object Since the subroutine has a reference to the object it can change the data stored in the object After the sub routine ends obj still point
262. k with more than one colour system A colour system defines how a colur is represented We ll first look at the RGB color system An RGB color is specified by three numbers that give the level of red green and blue respectively in that color A color in Java is an object of the class java awt Color You can construct a new color by specifying its red blue and green components For example myColor new Color r g b There are two constructors that you can call in this way In the one that I almost al ways use r g and b are integers in the range 0 to 255 In the other they are numbers of type float in the range 0 0F to 1 0F You might recall that a literal of type float is written with an F to distinguish it from a double number Often you can avoid constructing new colors altogether since the Color class defines several named constants 5 3 GRAPHICS AND PAINTING 95 representing common colors Color white Color black Color red Color green Color blue Color cyan Color magenta Color yellow Color pink Color orange Color lightGray Color gray and Color darkGray One of the instance variables in a Graphics object is the current drawing color which is used for all drawing of shapes and text If g is a graphics context you can change the current drawing color for g using the method g setColor c where c is a Color For example if you want to draw in green you would just say g setColor Color green before doing the
263. kBox is a component that has two states selected or unselected The user can change the state of a check box by clicking on it The state of a checkbox is represented by a boolean value that is true if the box is selected and false if the box is unselected A checkbox has a label which is specified when the box is constructed JCheckBox showTime new JCheckBox Show Current Time Usually it s the user who sets the state of a JCheckBox but you can also set the state in your program The current state of a checkbox is set using its setSelected boolean method You can check or uncheck a checkbox showTime programmatically by saying showTime setSelected true or showTime setSelected false You can de termine the current state of a checkbox by calling its isSelected method which returns a boolean value In many cases you don t need to worry about events from checkboxes Your program can just check the state whenever it needs to know it by calling the isSelected method However a checkbox does generate an event when its state changes and you can detect this event and respond to it if you want something to happen at the moment the state changes When the state of a checkbox is changed by the user it generates an event of type ActionEvent If you want something to happen when the user changes the state of a checkbox you must register an ActionListener with the checkbox Note that if you change the state
264. king subclasses 3 2 this and super ALTHOUGH THE BASIC IDEAS of object oriented programming are reasonably simple and clear they are subtle and they take time to get used to And unfortunately beyond the basic ideas there are a lot of details This section and the next cover more of those annoying details You should not necessarily master everything in these two sections the first time through but you should read it to be aware of what is possible For the most part when I need to use this material later in the text I will explain it again briefly or I will refer you back to it In this section we ll look at two variables this and super that are automatically defined in any instance method 3 2 1 The Special Variables this and super A static member of a class has a simple name which can only be used inside the class definition For use outside the class it has a full name of the form class name simple name For example System out is a static member variable with simple name out in the class System It s always legal to use the full name of a static member even within the class where it s defined Sometimes it s even necessary as when the simple name of a static member variable is hidden by a local variable of the same name Instance variables and instance methods also have simple names The simple name of such an instance member can be used in instance methods in the class where the instance member is d
265. king subclasses often have protected members The protected members are there to provide a foundation for the subclasses to build on But they are still invisible to the public at large 2 6 Mixing Static and Non static Classes as I ve said have two very distinct purposes A class can be used to group together a set of static member variables and static member subroutines Or it can be used as a factory for making objects The non static variables and subroutine definintions in the class definition specify the instance variables and methods of the objects In most cases a class performs one or the other of these roles not both Sometimes however static and non static members are mixed in a single class In this case the class plays a dual role Sometimes these roles are completely separate It is also possible for the static and non static parts of a class to interact This happens when instance methods use static member variables or call static member subroutines An instance method belongs to an object not to the class itself and there can be many objects with their own versions of the instance method But there is only one copy of a static member variable So effectively we have many objects sharing that one variable Suppose for example that we want to write a PairOfDice class that uses the Random class mentioned previously for rolling the dice To do this a PairOfDice object needs access to an object of type Random But there is n
266. l from one template The most recent version of C comes with a large number of pre written templates called the Standard Template Library or STL The STL is quite complex Many people would say that its much too complex But it is also one of the most interesting features of C 9 1 3 Generic Programming in Java Like C Java is a strongly typed language However generic programming in Java is closer in spirit to Smalltalk than it is to C As I ve already noted generic programming in Java is based on the fact that class Object is a superclass of every other class To some extent this makes Java similar to Smalltalk A data structure designed to hold Objects can hold values belonging to any class There is no need for templates or any other new language feature to support generic programming Of course primitive type values such as integers are not objects in Java and therefor cannot be stored in generic data structures In fact there is no way to do generic program ming with the primitive data types in Java The Smalltalk approach doesn t work except for objects and the C approach is not available Furthermore generic subroutines are more problematic in Java than they are in either Smalltalk or C In Smalltalk a sub routine can be called with parameter values of any type and it will work fine as long as 188 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES all the operations used by the subroutine are supported by the
267. lay panel to the CENTER position of the JApplet s content pane JPanel buttonBar new JPanel This panel will hold the button and appears at the bottom of the applet buttonBar setBackground Color gray getContentPane add buttonBar BorderLayout SOUTH startStopButton new JButton Start buttonBar add startStopButton 172 CHAPTER 8 ADVANCED GUI PROGRAMMING startStopButton addActionListener new ActionListener The action listener that responds to the button starts or stops the animation It checks the value of timer to find out which to do Timer is non null when the animation is running so if timer is null the animation needs to be started public void actionPerformed ActionEvent evt if timer null startAnimation else stopAnimation H end init ActionListener timerListener new ActionListener Define an action listener to respond to events from the timer When an event is received the color of the display is changed public void actionPerformed ActionEvent evt colorIndext A number between O and 100 if colorIndex gt 100 colorIndex 0 float hue colorIndex 100 0F Between 0 OF and 1 0F display setColor Color getHSBColor hue 1 1 F3 void startAnimation Start the animation unless it is already running We can check if it is running since the value of timer is non null when the animation i
268. le HelloWorldJApplet java For a second example let s look at something a little more interesting Here s a simple card game in which you look at a playing card and try to predict whether the next card will be higher or lower in value Aces have the lowest value in this game A previous section has already defined Deck Hand and Card classes that are used in this applet In this GUI version of the game you click on a button to make your prediction If you predict wrong you lose If you make three correct predictions you win After completing one game you can click the New Game button to start a new game Try it See what happens if you click on one of the buttons at a time when it doesn t make sense to do so Applet Reference see Applet HighLowGUI 10 3 of of Diamonds Diamonds Lower New Game Figure 5 5 HighLowGUI Applet The overall form of this applet is the same as that of the previous example It has three buttons in a panel at the bottom of the applet and a large drawing surface that displays the cards and a message However I ve organized the code a little differently in this example In this case it s the drawing surface object rather than the applet that listens for events from the buttons and I ve put almost all the programming into the display surface class The applet object is only responsible for creating the components and adding them to the applet This is done in the following init met
269. le is a car As in the case of ints and shorts the solution here is to use type casting If for some reason you happen to know that myVehicle does in fact refer to a Car you can use the type cast Car myVehicle to tell the computer to treat myVehicle as if it were actually of type Car So you could say myCar Car myVehicle and you could even refer to Car myVehicle numberOfDoors As an example of how this could be used in a program suppose that you want to print out relevant data about a vehicle You could say System out println Vehicle Data System out println Registration number myVehicle registrationNumber if myVehicle instanceof Car System out println Type of vehicle Car Car c c Car myVehicle System out println Number of doors c numberO0fDoors else if myVehicle instanceof Truck System out println Type of vehicle Truck Truck t t Truck myVehicle System out println Number of axels t numberOfAxels else if myVehicle instanceof Motorcycle System out println Type of vehicle Motorcycle Motorcycle m m Motorcycle myVehicle System out println Has a sidecar m hasSidecar Note that for object types when the computer executes a program it checks whether 3 1 INHERITANCE POLYMORPHISM AND ABSTRACT CLASSES 57 type casts are valid So for example if myVehicle refers to an object of type Truck then the type cast Car myVehicle will produce
270. leased In fact it s more complicated than that If the user moves the mouse outside the push button before releasing the mouse button the button changes to its regular appearance To im plement this it is necessary to respond to mouse exit or mouse drag events Furthermore on many platforms a button can receive the input focus The button changes appearance when it has the focus If the button has the focus and the user presses the space bar the button is triggered This means that the button must respond to keyboard and focus events as well Fortunately you don t have to program any of this provided you use an object be longing to the standard class javax swing JButton A JButton object draws itself and 5 5 INTRODUCTION TO LAYOUTS AND COMPONENTS 107 processes mouse keyboard and focus events on its own You only hear from the Button when the user triggers it by clicking on it or pressing the space bar while the button has the input focus When this happens the JButton object creates an event object belonging to the class java awt event ActionEvent The event object is sent to any registered listen ers to tell them that the button has been pushed Your program gets only the information it needs the fact that a button was pushed Another aspect of GUI programming is laying out components on the screen that is deciding where they are drawn and how big they are You have probably noticed that computing coordinates can be a diffi
271. lection figure draw Note that this works not just for Collection but for other container classes such as ArrayList and Vector In fact it even works for applying the same operation to all the elements in an array 194 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES 9 3 2 Equality and Comparison The discussion of methods in the Collection interface had an unspoken assumption It was assumed that it s known what it means for two objects to be equal For example the methods coll contains object and coll remove object look for an item in the collection that is equal to object However equality is not such a simple matter The obvious technique for testing equality using the operator does not usually give a reasonable answer when applied to objects The operator tests whether two objects are identical in the sense that they share the same location in memory Usually however we want to consider two objects to be equal if they represent the same value which is a very different thing Two values of type String should be considered equal if they contain the same sequence of characters The question of whether those characters are stored in the same location in memory is irrelevant Two values of type Date should be considered equal if they represent the same time The Object class defines a boolean valued method equals Object for testing whether one object is equal to another For the purposes of collections
272. lete access to the members of the containing class object Looking at an example will help and will hopefully convince you that non static nested classes are really very natural Consider a class that represents poker games 78 CHAPTER 4 OBJECT ORIENTED PROGRAMMING II This class might include a nested class to represent the players of the game This structure of the PokerGame class could be class PokerGame Represents a game of poker class Player Represents one of the players in this game end class Player private Deck deck A deck of cards for playing the game private int pot The amount of money that has been bet end class PokerGame If game is a variable of type PokerGame then conceptually game contains its own copy of the Player class In an an instance method of a PokerGame object a new Player object would be created by saying newPlayer just as for any other class A Player object could be created outside the PokerGame class with the expression newgame Player Again however this is rather rare The Player object will have access to the deck and pot instance variables in the PokerGame object Each PokerGame object has its own deck and pot and Players Players of that poker game use the deck and pot for that game playes of another poker game use the other game s deck and pot That s the effect of making the Player class non static This is the most natural way for players to b
273. letely in Java buffer overflow errors are impossible The language simply does not provide any way to store data into memory that has not been properly allocated To do that you would need a pointer that points to unallocated mem ory or you would have to refer to an array location that lies outside the range allocated for the array As explained above neither of these is possible in Java However there could conceivably still be errors in Java s standard classes since some of the methods in these classes are actually written in the C programming language rather than in Java It s clear that language design can help prevent errors or detect them when they occur Doing so involves restricting what a programmer is allowed to do Or it requires tests such as checking whether a pointer is null that take some extra processing time Some programmers feel that the sacrifice of power and efficiency is too high a price to pay for the extra security In some applications this is true However there are many situations where safety and security are primary considerations Java is designed for such situations There is one area where the designers of Java chose not to detect errors automatically numerical computations In Java a value of type int is represented as a 32 bit binary number With 32 bits it s possible to represent a little over four billion different values The values of type int range from 2147483648 to 2147483647 What happens when
274. lic void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt public void mouseClicked MouseEvent evt public void mouseReleased MouseEvent evt Fourth and finally the applet must be registered to listen for mouse events Since the drawing surface fills the entire applet it is actually the drawing surface on which the user clicks We want the applet to listen for mouse events on the drawing surface This can be arranged by adding this line to the applet s init method drawingSurface addMouseListener this This calls the addMouseListener method in the drawing surface object It tells that 104 CHAPTER 5 APPLETS HTML AND GUI S object where to send the mouse events that it generates The parameter to this method is the object that will be listening for the events In this case the listening object is the applet itself The special variable this is used here to refer to the applet We could make all these changes in the source code of the original RandomStrings applet However since we are supposed to be doing object oriented programming it might be instructive to write a subclass that contains the changes This will let us build on previous work and concentrate just on the modifications Here s the actual source code for the above applet import java awt event public class ClickableRandomStrings extends RandomStrings implements MouseListener public void init When the ap
275. locations Changing that data could cause the whole system to crash a consequence more severe than a single failed program Another type of pointer error occurs when a pointer value is pointing to an object of the wrong type or to a segment of memory that does not even hold a valid object at all These types of errors are impossible in Java which does not allow programmers to manipulate pointers directly In other languages it is possible to set a pointer to point essentially to any location in memory If this is done incorrectly then using the pointer can have unpredictable results Another type of error that cannot occur in Java is a memory leak In Java once there are no longer any pointers that refer to an object that object is garbage collected so that the memory that it occupied can be reused In other languages it is the programmer s responsibility to return unused memory to the system If the programmer fails to do this unused memory can build up leaving less memory for programs and data There is a story that many common programs for Windows computers have so many memory leaks that 210 CHAPTER 10 CORRECTNESS AND ROBUSTNESS the computer will run out of memory after a few days of use and will have to be restarted Many programs have been found to suffer from buffer overflow errors Buffer overflow errors often make the news because they are responsible for many network security prob lems When one computer receives data f
276. lt 100 amp amp in peek 0 line in next text append line n lineCt if hasNext text append n n Text truncated to 100 lines n in close catch Exception e Some error has occurred while trying to read the file Show an error message JOptionPane showMessageDialog this Sorry some error occurred n e getMessage The doSave and doOpen methods are the only part of the text editor program that deal with files If you would like to see the entire program you will find the source code in the file TrivialEdit java For a final example of files used in a complete program you might want to look at ShapeDrawWithFiles java This file defines one last version of the ShapeDraw pro gram This version has a File menu for saving and loading the patterns of shapes that are created with the program The program also serves as an example of using ObjectInputStream and ObjectOutputStream If you check you ll see that the Shape class in this version has been declared to be Serializable so that objects of type Shape can be written to and read from object streams 11 5 QUIZ QUESTIONS 255 11 5 Quiz Questions THIS PAGE CONTAINS A SAMPLE quiz on material from Chapter 10 of this on line Java textbook You should be able to answer these questions after studying that chapter Question 1 In Java input output is done using streams Streams are an abstraction Explain w
277. m the input boxes and perform the operation indicated by the button Put the result in the answer label If an error occurs an error message is put in the label double x y The numbers from the input boxes Get a number from the xInput JTextField Use xInput getText to get its contents as a String Convert this String to a double The try catch statement will check for errors in the String If the string is not a legal number the error message Tllegal data for x is put into the answer and the actionPerformed method ends try String xStr xInput getText x Double parseDouble xStr catch NumberFormatException e xStr is not a legal number answer setText Illegal data for x return Get a number from yInput in the same way try String yStr yInput getText y Double parseDouble yStr catch NumberFormatException e answer setText Illegal data for y return 8 4 MENUS AND MENUBARS 167 Perform the operation based on the action command from the button Note that division by zero produces an error message String op evt getActionCommand if op equals answer setText x y x y else if op equals answer setText x y x y else if op equals answer setText x x y x y else if op equals if y 0 answer setText Can t divide by zero else answer setText x y x y
278. mal value 1 to 13 if cardVal gt 10 cardVal 10 For a Jack Queen or King if cardVal 1 ace true There is at least one ace val val cardVal Now val is the value of the hand counting any ace as 1 If there is an ace and if changing its value from 1 to 11 would leave the score less than or equal to 21 then do so by adding the extra 10 points to val if ace true amp amp val 10 lt 21 val val 10 return val end getBlackjackValue end class BlackjackHand 64 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I Since BlackjackHand is a subclass of Hand an object of type BlackjackHand contains all the instance variables and instance methods defined in Hand plus the new instance method getBlackjackValue For example if bHand is a variable of type BlackjackHand then the following are all legal method calls bHand getCardCount bHand removeCard 0 and bHand getBlackjackValue Inherited variables and methods from the Hand class can also be used in the definition of BlackjackHand except for any that are declared to be private The statement cards getCardCount in the above definition of getBlackjackValue calls the instance method getCardCount which was defined in the Hand class Extending existing classes is an easy way to build on previous work We ll see that many standard classes have been written specifically to be used as the basis for ma
279. mbered from right to left starting with 0 Testing if N testBit 0 is an easy way to check whether N is even or odd N testBit 0 is true if and only if N is an odd number For this exercise you should write a program that prints 3N 1 sequences with start ing values specified by the user In this version of the program you should use BigIntegers to represent the terms in the sequence If the user s input is legal print out the 3N 1 sequence Count the number of terms in the sequence and print the count at the end of the sequence Exit the program when the user inputs an empty line 3 A Roman numeral represents an integer using letters Examples are XVII to represent 17 MCMLIII for 1953 and MMMCCCIII for 3303 By contrast ordinary numbers such as 17 or 1953 are called Arabic numerals The following table shows the Arabic equivalent of all the single letter Roman numerals M 1000 X 10 D 500 V C 100 I 1 L 50 When letters are strung together the values of the letters are just added up with the following exception When a letter of smaller value is followed by a letter of larger value the smaller value is subtracted from the larger value For example IV represents 5 1 or 4 And MCMXCV is interpreted as M CM XC V or 1000 1000 100 100 10 5 which is 1995 In standard Roman numerals no more than thee consecutive copies of the same letter are used Following these rules every number between 1 and 3999
280. med when user clicks Higher Check the user s prediction Game ends if user guessed wrong or if the user has made three correct predictions if gameInProgress false If the game has ended it was an error to click Higher so set up an error message and abort processing message Click New Game to start a new game repaint return hand addCard deck dealCard Deal a card to the hand int cardCt hand getCardCount How many cards in the hand Card thisCard hand getCard cardCt 1 Card just dealt Card prevCard hand getCard cardCt 2 The previous card 5 6 FRAMES AND DIALOGS 115 if thisCard getValue lt prevCard getValue f gameInProgress false message Too bad You lose else if thisCard getValue prevCard getValue gameInProgress false message Too bad You lose on ties else if cardCt 4 gameInProgress false message You win You made three correct guesses else message Got it right Try for cardCt repaint The paintComponent method of the HighLowCanvas class uses the values in the state variables to decide what to show It displays the string stored in the message variable It draws each of the cards in the hand There is one little tricky bit If a game is in progress it draws an extra face down card which is not in the hand to represent the next card in the de
281. mer support by the telephone and takes orders from the customer into the system would be represented by an actor Support Staff and an actor Sales Representative Use Case Descriptions are textual narratives of the Use Case They usually take the form of a note or a document that is somehow linked to the Use Case and explains the processes or activities that take place in the Use Case Class Diagrams Class Diagrams show the different classes that make up a system and how they relate to each other Class Diagrams are said to be static diagrams because they show the classes along with their methods and attributes as well as the static relationships between them which classes know about which classes or which classes are part of another class but do not show the method calls between them A Class defines the attributes and the methods of a set of objects All objects of this class instances of this class share the same behavior and have the same set of attributes each object has its own set The term Type is sometimes used instead of Class but it is important to mention that these two are not the same and Type is a more general term In UML Classes are represented by rectangles with the name of the class and can also show the attributes and operations of the class in two other compartments inside 134 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN attri int attr2 string operation p bool
282. methods are defined e calc enter item where item is a number adds the item to the dataset e calc getCount is a function that returns the number of items that have been added to the dataset e calc getSum is a function that returns the sum of all the items that have been added to the dataset e calc getMean is a function that returns the average of all the items e calc getStandardDeviation is a function that returns the standard devia tion of the items Typically all the data are added one after the other calling the enter method over and over as the data become available After all the data have been entered any of the other methods can be called to get statistical information about the data The methods getMean and getStandardDeviation should only be called if the number of items is greater than zero Modify the current source code StatCalc java to add instance methods getMax and getMin The getMax method should return the largest of all the items that have been added to the dataset and getMin should return the smallest You will need to add two new instance variables to keep track of the largest and smallest items that have been seen so far Test your new class by using it in a program to compute statistics for a set of non zero numbers entered by the user Start by creating an object of type StatCalc StatCalc calc Object to be used to process the data calc new StatCalc Read number
283. metricBrighten This is the entire source code for the applet 3 2 2 Constructors in Subclasses Constructors are not inherited That is if you extend an existing class to make a subclass the constructors in the superclass do not become part of the subclass If you want con structors in the subclass you have to define new ones from scratch If you don t define any constructors in the subclass then the computer will make up a default constructor with no parameters for you This could be a problem if there is a constructor in the superclass that does a lot of necessary work It looks like you might have to repeat all that work in the subclass This could be a real problem if you don t have the source code to the superclass and don t know how it works or if the constructor in the superclass initializes private member variables that you don t even have access to in the subclass 68 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I Obviously there has to be some fix for this and there is It involves the special variable super As the very first statement in a constructor you can use super to call a constructor from the superclass The notation for this is a bit ugly and misleading and it can only be used in this one particular circumstance It looks like you are calling super as a subroutine even though super is not a subroutine and you can t call constructors the same way you call other subroutines anyway As an example assume that
284. model the problem at hand Software objects in the program can represent real or abstract entities in the problem domain This is supposed to make the design of the program more natural and hence easier to get right and easier to understand To some extent OOP is just a change in point of view We can think of an object in standard programming terms as nothing more than a set of variables together with some subroutines for manipulating those variables In fact it is possible to use object oriented techniques in any programming language However there is a big difference between a language that makes OOP possible and one that actively supports it An object oriented programming language such as Java includes a number of features that make it very different from a standard language In order to make effective use of those features you have to orient your thinking correctly Objects are closely related to classes We have already been working with classes for several chapters and we have seen that a class can contain variables and subroutines If an object is also a collection of variables and subroutines how do they differ from classes And why does it require a different type of thinking to understand and use them effectively I have said that classes describe objects or more exactly that the non static portions of classes describe objects But it s probably not very clear what this means The more usual terminology is to say that o
285. mpossible which means that when they hap pen you have the satisfaction of blaming the system rather than your own program When an exception occurs the thing that is actually thrown is an object This object can carry information in its instance variables from the point where the exception occurs to the point where it is caught and handled This information always includes the subrou tine call stack which is a list of the subroutines that were being executed when the excep tion was thrown Since one subroutine can call another several subroutines can be active at the same time Typically an exception object also includes an error message describing what happened to cause the exception and it can contain other data as well The object thrown by an exception must be an instance of the standard class java 1lang Throwable or of one of its subclasses In general each different type of exception is represented by its own subclass of Throwable Throwable has two direct subclasses Error and Exception These two subclasses in turn have many other predefined subclasses In addition a pro grammer can create new exception classes to represent new types of exceptions Most of the subclasses of the class Error represent serious errors within the Java virtual machine that should ordinarily cause program termination because there is no reasonable way to handle them You should not try to catch and handle such errors An example is the ClassForm
286. n created by an applet or a standalone application Note that an applet s getImage method is used to load an image from a Web server while a Toolkit s getImage loads an image from the same computer on which the program is running An applet cannot in general use a Toolkit to load an image since for security reasons an applet is not usually allowed to read files from the computer on which it is running 5 6 FRAMES AND DIALOGS 121 The HighLowFrame example uses an image file named smallcards gif for the cards that it displays I designed HighLowFrame with a main routine so that it can be run as a stand alone application When it is run as an application the smallcards gif file must be in the same directory with the class files But a HighLowFrame can also be opened by an applet as is done in the example on this page When it is run as an application the image file must be loaded by a Toolkit When it is opened by an applet the image file must be loaded by the getImage method of the applet How can this be handled I decided to do it by making the Image object a parameter to the HighLowFrame constructor The Image must be loaded before the frame is constructed so that it can be passed as a parameter to the constructor The main routine in HighLowFrame does this using a Toolkit Image cards Toolkit getDefaultToolkit getImage smallcards gif HighLowFrame game new HighLowFrame cards The HighLowLaunch
287. n added to the applet But they will take care of themselves and there is no need to manipulate them elsewhere in the applet The display variable on the other hand is an instance variable that can be used throughout the applet This is because we are not finished with the display object after adding it to the applet When the user clicks a button we have to change the color of the display We need a way to keep the variable around so that we can refer to it in the actionPerformed method In general you don t need an instance variable for every component in an applet just for the components that will be referred to outside the init method The drawing surface in our example is defined by a nested class named Display which is a subclass of JPanel The class contains a paintComponent method that is responsible for drawing the message Hello World on a black background The Display class also contains a variable that it uses to remember the current color of the message and a method that can be called to change the color This class is more self contained than than most of the drawing surface classes that we have looked at and in fact it could have been defined as an independent class instead of as a nested class Here is the definition of the nested class Display class Display extends JPanel This nested class defines a component that displays the string Hello World The color and font for the string are recorded in
288. n item that does not belong to class Shape When that item is encountered the type cast Shape iter next will cause an exception of type ClassCastException Although it s unfortunate that we can t have a Collection of Shapes in Java rather than a Collection of Objects it s not a big problem in practice You just have to be aware of what type of objects you are storing in your collections Java 1 5 Note Java 1 5 introduces a new variation on the for loop that makes Iterators unnecessary in many cases An iterator is often used to apply the same operation to all the elements in a Collection In Java 1 5 this can be done with a for loop something like this for Object x c applyOperation x where c is the Collection and x is the for loop variable The notation for Object x c has the meaning for every Object x in the Collection c do the following Using this notation the drawAllShapes example above could be written simply as void drawAllShapes Collection shapeCollection Precondition Every item in shapeCollection is non null and belongs to the class Shape for Object obj shapeCollection Shape figure Shape obj figure draw Using the template notation discussed in the previous Java 1 5 Note this becomes even nicer void drawAllShapes Collection lt Shape gt shapeCollection Precondition Every item in shapeCollection is non null for Shape figure shapeCol
289. n variable hasSidecar Well it could in theory at 3 1 INHERITANCE POLYMORPHISM AND ABSTRACT CLASSES 55 Motorcycle Figure 3 3 An Example Hierarchy least even if it might give a chuckle to the people at the Department of Motor Vehicles The declarations of these classes in Java program would look in outline like this class Vehicle int registrationNumber Person owner assuming that a Person class has been defined void transferOwnership Person newOwner class Car extends Vehicle int numberOfDoors class Truck extends Vehicle int numberOfAxels class Motorcycle extends Vehicle boolean hasSidecar Suppose that myCar is a variable of type Car that has been declared and initialized with the statement Car myCar new Car Note that as with any variable it is OK to declare a variable and initialize it in a single statement This is equivalent to the declaration Car myCar followed by the as signment statement myCar new Car Given this declaration a program could refer to myCar numberOfDoors since numberOfDoors is an instance variable in the class Car But since class Car extends class Vehicle a car also has all the structure and be havior of a vehicle This means that myCar registrationNumber myCar owner and myCar transferOwnership also exist Now in the real world cars trucks and motorcycles are in fact vehicles The same is true in a program That i
290. name If the name entered by the user is not a directory a message is printed and the program ends public static void main String args String directoryName Directory name entered by the user File directory File object referring to the directory String files Array of file names in the directory Scanner keyboard new Scanner System in System out print Enter a directory name directoryName keyboard next directory new File directoryName if directory isDirectory false if directory exists false System out println There is no such directory else System out println That file is not a directory else files directory list System out println Files in directory directory for int i 0 i lt files length i System out println files i end main end class DirectoryList All the classes that are used for reading data from files and writing data to files have constructors that take a File object as a parameter For example if file is a variable of type File and you want to read character data from that file you can create a FileReader to do so by saying new FileReader file If you want to use a Scanner to read from the file you might use Scanner data try data new Scanner new FileReader file catch FileNotFoundException e handle the exception 11 3 FILES 245 11 3 2 File Dialog
291. named data dat in the current directory Another constructor new File File String has two parameters The first is a File object that refers to the directory that contains the file The second is the name of the file Later in this section we ll look at a convenient way of letting the user specify a File in a GUI program File objects contain several useful instance methods Assuming that file is a variable of type File here are some of the methods that are available file exists This boolean valued function returns true if the file named by the File object already exists You could use this method if you wanted to avoid overwriting the contents of an existing file when you create a new FileWriter file isDirectory This boolean valued function returns true if the File object refers to a directory It returns false if it refers to a regular file or if no file with the given name exists file delete Deletes the file if it exists file list If the File object refers to a directory this function returns an array of type String containing the names of the files in that directory Otherwise it returns null 244 CHAPTER 11 BASIC INPUT OUTPUT Here for example is a program that will list the names of all the files in a directory specified by the user import java io File public class DirectoryList This program lists the files in a directory specified by the user The user is asked to type in a directory
292. nce variables and then returns a reference to the object 2 2 CONSTRUCTORS AND OBJECT INITIALIZATION 31 This reference is the value of the expression and that value is stored by the assignment statement in the variable dice Part of this expression PairOfDice looks like a subroutine call and that is no accident It is in fact a call to a special type of subroutine called a constructor This might puzzle you since there is no such subroutine in the class definition However every class has a constructor If the programmer doesn t provide one then the system will provide a default constructor This default constructor does nothing beyond the basics allocate memory and initialize instance variables If you want more than that to happen when an object is created you can include one or more constructors in the class definition The definition of a constructor looks much like the definition of any other subroutine with three exceptions A constructor does not have any return type not even void The name of the constructor must be the same as the name of the class in which it is defined The only modifiers that can be used on a constructor definition are the access modifiers public private and protected In particular a constructor can t be declared static However a constructor does have a subroutine body of the usual form a block of state ments There are no restrictions on what statements can be used And it can have a
293. nd Components Sod SBoOrder bayou s mope g al wi arg Rost Heh ew ae ee a 8 1 2 G dLayo t geeks gle a GG we Ag a Rg IR 8 1 3 An Example os 6 aos eee Rca eae dosh a oe eee 8 2 Basic Components and Their Events 8 2 1 The JButton Class cen pe hg Rink owl Sw oe 9 Se ey 8 2 2 The JLabel Class e waste ert iOS ihe Hoke ee 8 2 3 The JCheckBox Class s sceia e esa fla SE 8 2 4 The JRadioButton and ButtonGroup Classes 8 2 5 The JTextField and JIextArea Classes 8 2 6 Other Components woe 3 Se ae es ee 8 3 Programming with Components 8 3 1 An Example with Text Input Boxes 8 4 Menus and Me nubars 9 a 4 3 eratenen Soe ne eS Geode wo 8 4 1 Menu Bars and Menus CONTENTS CONTENTS 8 5 Timers Animation and Threads 0000 ee eens 8 5 1 Animation In Swing 0 220 202 e eee 8 5 2 JApplet s start and stopQ Methods 8 5 3 Other Useful Timer Methods 0 000004 8 5 4 Using Threads ny rome hee thea e Gy once 2a rae Ge An eae a A 8 6 QUIZ QUESHONS s i ged BB Ae ee Me Sete Bek RR ae Se et 8 7 Programming Exercises 2 2 ee 9 Generic Programming and Collection Classes 9 1 Generic Programming 0 0 0 eee eee eee eee 9 1 1 Generic Programming in Smalltalk 0 9 1 2 Generic Programming in C 2 2 000 9 1 3 Generic Programming in Ja
294. ne add bttn end init end class HelloSwing2 5 2 HTML Basics APPLETS GENERALLY APPEAR ON PAGES in a Web browser program Such pages are themselves written in a language called HTML HyperText Markup Language An HTML document describes the contents of a page A Web browser interprets the HTML code to determine what to display on the page The HTML code doesnt look much like the resulting page that appears in the browser The HTML document does contain all the text that appears on the page but that text is marked up with commands that determine the structure and appearance of the text and determine what will appear on the page in addition to the text HTML has developed rapidly in the last few years and it has become a rather com plicated language In this section I will cover just the basics of the language While that leaves out all the fancy stuff it does include just about everything I ve used to make the Web pages in this on line text It is possible to write an HTML page using an ordinary text editor typing in all the mark up commands by hand However there are many Web authoring programs that 88 CHAPTER 5 APPLETS HTML AND GUI S make it possible to create Web pages without ever looking at the underlying code Using these tools you can compose a Web page in much the same way that you would write a paper with a word processor For example Netscape Composer which is part of Netscape Communicator works
295. nement is inherently a top down process but the process does have a bottom that is a point at which you stop refining the pseudocode algorithm and trans late what you have directly into a programming language In the absence of subroutines the process would not bottom out until you get down to the level of assignment statements and very primitive input output operations But if you have subroutines lying around to perform certain useful tasks you can stop refining as soon as you ve managed to express your algorithm in terms of those tasks This allows you to add a bottom up element to the top down approach of stepwise refinement Given a problem you might start by writing some subroutines that perform tasks relevant to the problem domain The subroutines become a toolbox of ready made tools that you can integrate into your algorithm as you develop it Alternatively you might be able to buy or find a software toolbox written by someone else containing sub routines that you can use in your project as black boxes Subroutines can also be helpful even in a strict top down approach As you refine your algorithm you are free at any point to take any sub task in the algorithm and make it into a subroutine Developing that subroutine then becomes a separate problem which you can work on separately Your main algorithm will merely call the subroutine This of course is just a way of breaking your problem down into separate smaller problems
296. nent Make it a subclass of JPanel instead of JApplet Instead of doing the initialization in an init method you ll have to do it in a constructor The component should have a method getColor that returns the color currently displayed on the component It should also have a method setColor Color c to set the color to a specified value Both these methods would be useful to a program that uses your component In order to write the setColor Color c method you need to know that if c is a variable of type Color then c getRed is a function that returns an integer in the range 0 to 255 that gives the red level of the color Similarly the functions c getGreen and c getBlue return the blue and green components Test your component by using it in a simple applet that sets the component to a random color when the user clicks on a button like this one In the Blackjack game BlackjackGUI java from Exercise 6 8 the user can click on the Hit Stand and NewGame buttons even when it doesn t make sense to do so It would be better if the buttons were disabled at the appropriate times The New Game button should be disabled when there is a game in progress The Hit and Stand buttons should be disabled when there is not a game in progress The instance variable gameInProgress tells whether or not a game is in progress so you just have to make sure that the buttons are properly enabled and disabled wheneve
297. next long e nextBoolean returns the next boolean Each of these methods has a companion hasNext method hasNextInteger hasNextDouble hasNextLong etc that returns true if the next token is an integer double or long etc The next methods and their primitive type companion methods such as next Int and hasNextInt first skip any input that matches the delimiter pattern e g spaces and then attempt to return the next token Unfortunately there is no next method for reading in a character You can read the character as a string and convert it as shown below Scanner sc new Scanner System in char c sc next charAt 0 This code allows a user to read input from System in import java util Scanner Scanner keyboard new Scanner System in Create a Scanner object System out print Enter your name gt String name keyboard next Read the next input token as a string System out println System out print Enter grade gt int grade keyboard nextInt Read the next input token The steps are straight forward create a scanner object you may choose any name for it and read from it using the appropriate next method You can also use the Scanner class to read input from a file this is not much different from doing input from the keyboard instead of passing System in as a parameter to the Scanner constructor we pass a file object Once the Scanner object has been created with ei
298. nner class looks just like any other class except that it is nested inside another class It can even contain further levels of nested classes but you shouldn t carry these things too far Like any other item in a class a named inner class can be either static or non static A static nested class is part of the static structure of the containing class It can be used inside that class to create objects in the usual way If it has not been declared private then it can also be used outside the containing class but when it is used outside the class its name must indicate its membership in the containing class This is similar to other static components of a class A static nested class is part of the class itself in the same way that static member variables are parts of the class itself For example suppose a class named WireFrameModel represents a set of lines in three dimensional space Such models are used to represent three dimensional objects in graphics programs Suppose that the WireFrameModel class contains a static nested class Line that represents a single line Then outside of the class WireFrameModel the Line class would be referred to as WireFrameModel Line Of course this just fol lows the normal naming convention for static members of a class The definition of the WireFrameModel class with its nested Line class would look in outline like this 4 2 NESTED CLASSES 77 public class WireFrameModel other members of
299. nstructed there is no way to change it Similarly an object of type String is immutable There are wrapper classes for all the primitive types All objects belonging to these classes are immutable The wrapper class for values of type double is java lang Double The value stored in an object of type Double can be retrieved by calling the instance method doubleValue The wrapper classes define a number of useful methods Some of them exist to sup port generic programming For example the wrapper classes all define instance meth ods equals Object and compareTo Object in a reasonable way Other methods in the wrapper classes are utility functions for working with the primitive types For example we encountered the static methods Integer parseInt String and Double parseDouble String in Section7 4 These functions are used to convert strings such as 42 or 2 71828 into the numbers they represent Java 1 5 Note Java 1 5 makes it easier to use the wrapper classes by introducing automatic conversions between the primitive types and the wrapper types For example it is possible to assign a value of type int to a variable of type Integer and vice versa In Java 9 4 LIST CLASSES 197 1 5 the statement Integer val new Integer 17 could be replaced by Integer val 17 Similarly it is possible to pass a value of type double to a function that has a parameter of type Double All this is especially convenient when
300. nt age In this case there is no such variable as PlayerData name or PlayerData age since name and age are not static members of PlayerData So there is nothing much in the class at all except the potential to create objects But it s a lot of potential since it can be used to create any number of objects Each object will have its own variables called name and age There can be many players because we can make new objects to represent new players on demand A program might use this class to store information about multiple players in a game Each player has a name and an age When a player joins the game a new PlayerData object can be created to represent that player If a player leaves the game the PlayerData object that represents that player can be destroyed A system of objects in the program is being used to dynamically model what is happening in the game You can t do this with static variables An object that belongs to a class is said to be an instance of that class The variables that the object contains are called instance variables The subroutines that the object contains are called instance methods Recall that in the context of object oriented programming method is a synonym for subroutine From now on for subroutines in objects I will prefer the term method For example if the PlayerData class as defined above is used to create an object then that object is an instance of the Player
301. nt data source read if data lt 0 break copy write data byteCount source close copy close System out println Successfully copied byteCount bytes catch Exception e System out println Error occurred while copying byteCount bytes copied System out println e toString end main end class CopyFile Both of the previous programs use a command line interface but graphical user in terface programs can also manipulate files Programs typically have an Open command that reads the data from a file and displays it in a window and a Save command that writes the data from the window into a file We can illustrate this in Java with a simple text editor program The window for this program uses a JTextArea component to dis play some text that the user can edit It also has a menu bar with a File menu that includes Open and Save commands To fully understand the examples in the rest of this section you must be familiar with the material on menus and frames from Section7 7 and Section7 7 The examples also use file dialogs which were introduced in Section2 When the user selects the Save command from the File menu in the TrivialEdit program the program pops up a file dialog box where the user specifies the file The text from the JTextArea is written to the file All this is done in the following instance method where the variable text refers to the
302. nt red int 256 Math random int green int 256 Math random int blue int 256 Math random mosaic setColor rowNum colNum red green blue Finally consider the randomMove subroutine which is supposed to randomly move the disturbance up down left or right To make a random choice among four directions we can choose a random integer in the range 0 to 3 If the integer is 0 move in one direction if it is 1 move in another direction and so on The position of the disturbance is given by the variables currentRow and currentColumn To move up means to subtract 1 from currentRow This leaves open the question of what to do if currentRow becomes 1 which would put the disturbance above the window Rather than let this happen I decide to move the disturbance to the opposite edge of the applet by setting currentRow to 9 Remember that the 10 rows are numbered from 0 to 9 Moving the disturbance 16 down left or right is handled similarly See the code below for randomMove It uses a CHAPTER 1 DESIGN I CLASS LEVEL DESIGN switch statement to decide which direction to move the code becomes Putting this all together we get the following complete program The variables currentRow and currentColumn are defined as static members of the class rather than local variables because each of them is used in several different subroutines This program actually depends on two other classes Mosaic and another class
303. o greenRadio blackRadio Nariables to represent the radio buttons These should probably be instance variables so that they can be used throughout the program ButtonGroup colorGroup new ButtonGroup redRadio new JRadioButton Red Create a button colorGroup add redRadio Add it to the group blueRadio new JRadioButton Blue colorGroup add blueRadio 160 CHAPTER 8 ADVANCED GUI PROGRAMMING greenRadio new JRadioButton Green colorGroup add greenRadio blackRadio new JRadioButton Black colorGroup add blackRadio redRadio setSelected true Make an initial selection The individual buttons must still be added to a container if they are to appear on the screen If you want to respond immediately when the user clicks on one of the radio buttons you should register an ActionListener for each button Here is an applet that demonstrates this When you click one of the radio buttons the background color of the label is changed Applet Reference see Applet RadioButtonDemo w Red Blue Green C3 Black Red is selected Figure 8 1 RadioButtonDemo The source code for the applet is in the file RadioButtonDemo java Just as for check boxes it is not always necessary to register listeners for radio buttons In many cases you can simply check the state of each button when you need to know it using the isSelected method 8 2 5 The JTextField and JTextArea Clas
304. o a data type for which it is not defined and the program will crash 9 1 2 Generic Programming in C Unlike Smalltalk C is a very strongly typed language even more so than Java Every variable has a type and can only hold data values of that type This means that the type of generic programming used in Smalltalk is impossible Furthermore C does not have anything corresponding to Java s Object class That is there is no class that is a superclass of all other classes This means that C can t use Java s style of generic programming either Nevertheless C has a powerful and flexible system of generic programming It is made possible by a language feature known as templates In C instead of writing a different sorting subroutine for each type of data you can write a single subroutine template The template is not a subroutine it s more like a factory for making subroutines We can look at an example since the syntax of C is very similar to Java s 9 1 GENERIC PROGRAMMING 187 template lt class ItemType gt void sort ItemType A int count Sort count items in the array A into increasing order The algorithm that is used here is selection sort for int i count 1 i gt 0 i int position_of_max 0 for int j 1 j lt count j if ALj gt Alposition_of_max position_of_max j ItemType temp Alcount A count Alposition_of_max A position_of_max temp This piec
305. o need for each PairOfDice object to have a separate Random object In fact it would not even be a good idea Because of the way random number generators work a program should in general use only one source of random numbers A nice solution is to have a single Random variable as a static member of the PairOfDice class so that it can be shared by all PairOfDice objects For example 2 6 MIXING STATIC AND NON STATIC 45 class PairOfDice private static Random randGen new Random public int die1 Number showing on the first die public int die2 Number showing on the second die public PairOfDice Constructor Creates a pair of dice that initially shows random values roll public void roll Roll the dice by setting each of the dice to be a random number between 1 and 6 die1 randGen nextInt 6 1 die2 randGen nextInt 6 1 end class PairOfDice As another example let s rewrite the Student class ve added an ID for each student and a static member called nextUniqueID Although there is an ID variable in each student object there is only one nextUniquelID variable public class Student private String name Student s name private int ID Unique ID number for this student public double testi test2 test3 Grades on three tests private static int nextUniquelID 0 keep track of next available unique ID number Student String theName Constructor for S
306. of the parts For Aggregations the class acting as the whole always has a multiplicity of one In UML Aggregations are represented by an association that shows a rhomb on the side of the whole Composition Compositions are associations that represent very strong aggregations This means Compositions form whole part relationships as well but the relationship is so strong that the parts cannot exist on its own They exist only inside the whole and if the whole is destroyed the parts die too In UML Compositions are represented by a solid rhomb on the side of the whole Other Class Diagram Items Class diagrams can contain several other items besides classes e Interfaces are abstract classes which means instances can not be directly created of them They can contain operations but no attributes Classes can inherit from tar Wheel O O E Figure 6 6 Visual representation of an Aggregation relationship in UML 136 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN Figure 6 7 interfaces through a realisation association and instances can then be made of these diagrams e Datatypes are primitives which are typically built into a programming language Common examples include integers and booleans They can not have relationships to classes but classes can have relationships to them e Enums are a simple list of values A typical example is an enum for days of the week The options of an enum are called En
307. ol has been set to the color specified by r g amp b r gives the amount of red in the color with O representing no red and 255 representing the maximum possible amount of red The larger the value of r the more red in the color g and b work similarly for the green and blue color components int Mosaic getRed int row int col int Mosaic getBlue int row int col int Mosaic getGreen int row int col Precondition row and col are in the valid ranges of row numbers and column numbers Also the mosaic window should be open Postcondition Returns an int value that represents one of the three color components of the rectangle in row number row and column number col The return value is in the range 0 to 255 Mosaic getRed returns the red component of the rectangle Mosaic getGreen the green component and Mosaic getBlue the blue component void Mosaic delay int milliseconds Precondition milliseconds is a positive integer Postcondition The program has paused for at least the number of milliseconds given by the parameter where one second is equal to 1000 milliseconds Note This can be used to insert a time delay in the program to regulate the speed at which the colors are changed for example boolean Mosaic isOpen Precondition None Postcondition The return value is true if the mosaic window is open on the screen and is false otherwise Note The window will be closed if the user clicks its close box It can
308. olor for the page is white You can add modifiers to control the color of regular text TEXT hypertext links LINK and links to pages that have already been visited VLINK When the user clicks and holds the mouse button on a link the link is said to be active you can control the color of active links with the ALINK modifier For example how about a page with a black background white text blue links red active links and gray visited links lt BODY bgcolor black text white link blue alink red vlink gray gt There are several standard color names that you can use in this context but if you want complete control you ll have to learn how to specify colors using hexadecimal numbers It is also possible to use an image for the background of the page instead of a solid color Look up the details if you are interested 5 2 2 The Applet tag and Applet Parameters The lt APPLET gt tag is used to add a Java applet to a Web page This tag must have a matching lt APPLET gt A required modifier named CODE gives the name of the compiled class file that contains the applet HEIGHT and WIDTH modifiers are required to specify the size of the applet If you want the applet to be centered on the page you can put the applet in a paragraph with CENTER alignment So an applet tag to display an applet named HelloWorldApplet centered on a Web page would look like this lt P ALIGN CENTER gt lt APPLET CODE HelloWorldApplet class HEIGHT 50 WIDTH
309. om any other Scanner Working with output files is no more difficult than this You simply create an object belonging to the class FileWriter You will probably want to wrap this output stream in an object of type PrintWriter For example suppose you want to write data to a file named result dat Since the constructor for FileWriter can throw an exception of type IOException you should use a try statement PrintWriter result try result new PrintWriter new FileWriter result dat catch IOException e handle the exception If no file named result dat exists a new file will be created If the file already exists then the current contents of the file will be erased and replaced with the data that your program writes to the file An IOException might occur if for example you are trying to create a file on a disk that is write protected meaning that it cannot be modified After you are finished using a file it s a good idea to close the file to tell the operating system that you are finished using it If you forget to do this the file will ordinarily be closed automatically when the program terminates or when the file stream object is garbage collected but it s best to close a file as soon as you are done with it You can close a file by calling the close method of the associated stream Once a file has been closed it is no longer possible to read data from it or write data to it unless you open i
310. om std1 and std2 both point to the same object This illustrates a Very Important Point When one object variable is assigned to another only a reference is copied The object referred to is not copied When the assignment std2 std1 was executed no new object was created In stead std2 was set to refer to the very same object that std1 refers to This has some consequences that might be surprising For example std1 name and std2 name refer to exactly the same variable namely the instance variable in the object that both std1 and std2 refer to After the string Mary Jones is assigned to the variable std1 name it is also be true that the value of std2 name is Mary Jones There is a potential for a lot of confusion here but you can help protect yourself from it if you keep telling yourself The object is not in the variable The variable just holds a pointer to the object You can test objects for equality and inequality using the operators and but here again the semantics are different from what you are used to When you make a test if std1 std2 you are testing whether the values stored in std1 and std2 are the same But the values are references to objects not objects So you are testing whether std1 and std2 refer to the same object that is whether they point to the same location in memory This is fine if its what you want to do But sometimes what you want to check is whether
311. on presents some pro gramming examples that make use of those raw materials 8 3 PROGRAMMING WITH COMPONENTS 163 8 3 1 An Example with Text Input Boxes As a first example let s look at a simple calculator applet This example demonstrates typical uses of JTextFields JButtons and JLabels and it uses several layout managers In the applet you can enter two real numbers in the text input boxes and click on one of the buttons labeled and The corresponding operation is performed on the numbers and the result is displayed in a JLabel at the bottom of the applet If one of the input boxes contains an illegal entry a word instead of a number for example an error message is displayed in the JLabel Applet Reference see Applet SimpleCalculator Ss Eas x y 30 359999999999996 Figure 8 2 SimpleCalculator When designing an applet such as this one you should start by asking yourself questions like How will the user interact with the applet What components will I need in order to support that interaction What events can be generated by user actions and what will the applet do in response What data will I have to keep in instance variables to keep track of the state of the applet What information do I want to display to the user Once you have answered these questions you can decide how to lay out the components You might want to draw the layout on paper At that po
312. oncordance along with the current line number Your program will need to keep track of the line number The end of each line in the file is marked by the newline character n Every time you encounter this character add one to the line number Because it is so common don t include the word the in your concordance Also do not include words that have length less than3 206 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES Chapter 10 Correctness and Robustness COMPUTER PROGRAMS THAT FAIL are much too common Programs are fragile A tiny error can cause a program to misbehave or crash Most of us are familiar with this from our own experience with computers And we ve all heard stories about software glitches that cause spacecraft to crash telephone service to fail and in a few cases people to die Programs don t have to be as bad as they are It might well be impossible to guarantee that programs are problem free but careful programming and well designed program ming tools can help keep the problems to a minimum This chapter will look at issues of correctness and robustness of programs We ll also look at exceptions one of the tools that Java provides as an aid in writing robust programs 10 1 Introduction to Correctness and Robustness A PROGRAM IS correct if accomplishes the task that it was designed to perform It is robust if it can handle illegal inputs and other unexpected situations in a reasonable way
313. onsole Unlike other subroutines a constructor can only be called using the new operator in an expression that has the form new class name parameter list where the parameter list is possibly empty I call this an expression because it com putes and returns a value namely a reference to the object that is constructed Most often you will store the returned reference in a variable but it is also legal to use a constructor call in other ways for example as a parameter in a subroutine call or as part of a more complex expression Of course if you don t save the reference in a variable you won t have any way of referring to the object that was just created A constructor call is more complicated than an ordinary subroutine or function call It is helpful to understand the exact steps that the computer goes through to execute a constructor call 1 First the computer gets a block of unused memory in the heap large enough to hold an object of the specified type 2 It initializes the instance variables of the object If the declaration of an instance variable specifies an initial value then that value is computed and stored in the instance variable Otherwise the default initial value is used 3 The actual parameters in the constructor if any are evaluated and the values are assigned to the formal parameters of the constructor 4 The statements in the body of the constructor if any are executed 5 A reference to the
314. or nouns in this description there are several candidates for objects game player hand card deck value and suit Of these the value and the suit of a card are simple values and they will just be represented as instance variables in a Card object In a complete program the other five nouns might be represented by classes But let s work on the ones that are most obviously reusable card hand and deck If we look for verbs in the description of a card game we see that we can shuffle a deck and deal a card from a deck This gives use us two candidates for instance methods in a Deck class Cards can be added to and removed from hands This gives two candidates for instance methods in a Hand class Cards are relatively passive things but we need to be 2 4 PROGRAMMING EXAMPLES 39 able to determine their suits and values We will discover more instance methods as we go along First we ll design the deck class in detail When a deck of cards is first created it contains 52 cards in some standard order The Deck class will need a constructor to create a new deck The constructor needs no parameters because any new deck is the same as any other There will be an instance method called shuffle that will rearrange the 52 cards into a random order The dealCard instance method will get the next card from the deck This will be a function with a return type of Card since the caller needs to know what card is being dealt It has no parameter
315. ormation can be used by the listener object to determine its response The methods that are required in a mouse event listener are specified in an interface named MouseListener To be used as a listener for mouse events an object must imple ment this MouseListener interface An interface in Java is just a list of instance meth ods A class can implement an interface by doing two things First the class must be de clared to implement the interface asin class MyListener implements MouseListener or class RandomStrings extends JApplet implements MouseListener Second 102 CHAPTER 5 APPLETS HTML AND GUI S the class must include a definition for each instance method specified in the interface An interface can be used as the type for a variable or formal parameter We say that an object implements the MouseListener interface if it belongs to a class that implements the MouseListener interface Note that it is not enough for the object to include the spec ified methods It must also belong to a class that is specifically declared to implement the interface Every event in Java is associated with a GUI component For example when the user presses a button on the mouse the associated component is the one that the user clicked on Before a listener object can hear events associated with a given component the listener object must be registered with the component If a MouseListener object mListener needs to hear mouse e
316. orth South East and West as shown in the diagram at the right Each of the four bor dering components is optional The layout manager first allocates space to the bordering components Any space that is left over goes to the center component If a container uses a BorderLayout then components should be added to the container using a version of the add method that has two parameters The first parameter is the component that is being added to the container The second parameter specifies where the component is to be placed It must be one of the constants BorderLayout CENTER 8 1 MORE ABOUT LAYOUTS AND COMPONENTS 153 BorderLayout NORTH BorderLayout SOUTH BorderLayout EAST or BorderLayout WEST If the second parameter is omitted then BorderLayout CENTER is used by default For ex ample the following code creates a panel with drawArea as its center component and with scroll bars to the right and below JPanel panel new JPanel To use BorderLayout with a JPanel you have to change the panel s layout manager otherwise a FlowLayout is used Alternatively you can provide the layout manager as a parameter to the constructor panel new JPanel new BorderLayout panel setLayout new BorderLayout Assume drawArea already exists panel add drawArea BorderLayout CENTER Assume hScroll is a horizontal scroll bar component that already exists panel add h
317. ose concepts The design should arise from the problem itself and you should end up with a program whose structure reflects the structure of the problem in a natural way 6 1 1 Software Engineering Life Cycles A decades long goal has been to find repeatable predictable processes or methodologies that improve productivity and quality of software Some try to systematize or formalize the seemingly unruly task of writing software Others apply project management tech niques to writing software Without project management software projects can easily be delivered late or over budget With large numbers of software projects not meeting their expectations in terms of functionality cost or delivery schedule effective project management is proving difficult Software engineering requires performing many tasks notably the following some of which may not seem to directly produce software e Requirements Analysis Extracting the requirements of a desired software product is the first task in creating it While customers probably believe they know what the software is to do it may require skill and experience in software engineering to recognize incomplete ambiguous or contradictory requirements e Specification Specification is the task of precisely describing the software to be written usually in a mathematically rigorous way In reality most successful spec ifications are written to understand and fine tune applications that were already well
318. ou should modify the SubKiller game from Section 6 5 You can start with the existing source code from the file SubKillerGame java Modify the game so it keeps track of the number of hits and misses and displays these quantities That is every time the depth charge blows up the sub the number of hits goes up by one Every time the depth charge falls off the bottom of the screen without hitting the sub the number of misses goes up by one There is room at the top of the applet to display these numbers To do this exercise you only have to add a half dozen lines to the source code But you have to figure out what they are and where to add them To do this you ll have to read the source code closely enough to understand how it works 7 Consider the SimpleAnimationApplet2 a framework for writing simple animations You can define an animation by writing a subclass and defining a drawFrame method It is possible to have the subclass implement the MouseListener interface Then you can have an animation that responds to mouse clicks Write a game in which the user tries to click on a little square that jumps erratically around the applet To implement this use instance variables to keep track of the position of the square In the drawFrame method there should be a certain prob ability that the square will jump to a new location You can experiment to find a probability that makes the game play well In your mousePressed method check whether
319. ould rather not have clutter up your elegant algorithm When an exception occurs during the execution of a program we say that the excep tion is thrown When this happens the normal flow of the program is thrown off track and the program is in danger of crashing However the crash can be avoided if the ex ception is caught and handled in some way An exception can be thrown in one part of 216 CHAPTER 10 CORRECTNESS AND ROBUSTNESS a program and caught in a different part An exception that is not caught will gener ally cause the program to crash More exactly the thread that throws the exception will crash In a multithreaded program it is possible for other threads to continue even after one crashes By the way since Java programs are executed by a Java interpreter having a program crash simply means that it terminates abnormally and prematurely It doesn t mean that the Java interpreter will crash In effect the interpreter catches any exceptions that are not caught by the program The interpreter responds by terminating the program In the case of an applet only the current operation such as the response to a button will be terminated Parts of the applet might continue to function even when other parts are non functional because of exceptions In many other programming languages a crashed program will often crash the entire system and freeze the computer until it is restarted With Java such system crashes should be i
320. ove is 190 CHAPTER 9 GENERIC PROGRAMMING AND COLLECTION CLASSES for int i 0 i lt players size i Player plr Player players get i plr makeMove The two lines inside the for loop can be combined to a single line Player players get i makeMove This gets an item from the list type casts it and then calls the makeMove method on the resulting Player The parentheses around Player players get i are required because of Java s precedence rules The parentheses force the type cast to be performed before the makeMove method is called 9 3 Collections Java s generic data structures can be divided into two categories collections and maps A collection is more or less what it sound like a collection of objects A map associates objects in one set with objects in another set in the way that a dictionary associates defini tions with words or a phone book associates phone numbers with names A map is similar to what I called an association list in Section8 4 There are two types of collections lists and sets A list is a collection in which the objects are arranged in a linear sequence A list has a first item a second item and so on For any item in the list except the last there is an item that directly follows it A set is a collection in which no object can appear more than once Note that the terms collection list set and map tell you nothing about how the
321. owest value 1 Again you don t yet know enough to implement this class But given the source code Hand java you can use the class in your own programming projects We have covered enough material to write a Card class The class will have a construc tor that specifies the value and suit of the card that is being created There are four suits which can be represented by the integers 0 1 2 and 3 It would be tough to remember which number represents which suit so I ve defined named constants in the Card class to represent the four possibilities For example Card SPADES is a constant that represents the suit spades These constants are declared to be public final static ints This 2 4 PROGRAMMING EXAMPLES 41 is one case in which it makes sense to have static members in a class that otherwise has only instance variables and instance methods The possible values of a card are the numbers 1 2 13 with 1 standing for an ace 11 for a jack 12 for a queen and 13 for a king Again I ve defined some named constants to represent the values of aces and face cards So cards can be constructed by statements such as cardi new Card Card ACE Card SPADES Construct ace of spades card2 new Card 10 Card DIAMONDS Construct 10 of diamonds card3 new Card v s This is OK as long as v and s are integer expressions A Card object needs instance variables to represent its value and suit ve made these p
322. pe JLabel to refer to the label The applet also has four JButtons and two more JLabels The two extra labels display the strings x and y I use local variables rather than instance variables for these components because I don t need to refer to them outside the init method 164 CHAPTER 8 ADVANCED GUI PROGRAMMING The applet as a whole uses a GridLayout with four rows and one column The bot tom row is occupied by the JLabel answer The other three rows each contain several components Each of the first three rows is filled by a JPanel which has its own layout manager and contains several components The row that contains the four buttons is a JPanel which uses a GridLayout with one row and four columns The JPanels that con tain the input boxes use BorderLayouts The input box occupies the Center position of the BorderLayout with a JLabel on the West This example shows that BorderLayouts are more versatile than it might appear at first All the work of setting up the applet is done in its init method public void init Since I will be using the content pane several times declare a variable to represent it Note that the return type of getContentPane is Container Container content getContentPane Assign a background color to the applet and its content panel This color will show through between components and around the edges of the applet setBackground Color gray content setBackgro
323. per left corner of the applet The color of the shape is given by the pop up menu in the lower right Once a shape is on the screen you can drag it around with the mouse A shape will maintain the same front to back order with respect to other 62 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I Rect Oval RoundRect Magenta nA Figure 3 6 ShapeDraw Applet shapes on the screen even while you are dragging it However you can move a shape out in front of all the other shapes if you hold down the shift key as you click on it In this applet the only time when the actual class of a shape is used is when that shape is added to the screen Once the shape has been created it is manipulated entirely as an abstract shape The routine that implements dragging for example works only with variables of type Shape As the Shape is being dragged the dragging routine just calls the Shape s draw method each time the shape has to be drawn so it doesn t have to know how to draw the shape or even what type of shape it is The object is responsible for drawing itself If I wanted to add a new type of shape to the program I would define a new subclass of Shape add another button to the applet and program the button to add the correct type of shape to the screen No other changes in the programming would be necessary You might want to look at the source code for this applet ShapeDraw java even though you won t be able to understand all of
324. pes of connections between objects Associations are the mechanism that allows objects to communicate to each other It describes the connection between different classes the connection between the actual objects is called object connection or link Associations can have a role that specifies the purpose of the association and can be uni or bidirectional indicates if the two objects participating in the relationship can send messages to the other of if only one of them knows about the other Each end of the association also has a multiplicity value which dictates how many objects on this side of the association can relate to one object on the other side 6 2 THE UNIFIED MODELLING LANGUAGE 135 Figure 6 4 Visual representation of a generalization in UML Figure 6 5 Visual representation of an Association in UML Aggregation In UML associations are represented as lines connecting the classes participating in the relationship and can also show the role and the multiplicity of each of the participants Multiplicity is displayed as a range min max of non negative values with a star on the maximum side representing infinite Aggregations Aggregations are a special type of associations in which the two par ticipating classes don t have an equal status but make a whole part relationship An Aggregation describes how the class that takes the role of the whole is composed has of other classes which take the role
325. plet Reference see Applet ShapeDrawLauncher The window in this example belongs to a class named ShapeDrawFrame which is de fined as a subclass of JFrame The structure of a JFrame is almost identical to a JApplet and the programming is almost the same In fact only a few changes had to be made to 116 CHAPTER 5 APPLETS HTML AND GUI S Java Applet Window Add Color Options Close ShapeDraw Figure 5 6 Screen Coordinates the applet class ShapeDrawWithMenus to convert it from a JApplet to a JFrame First a frame does not have an init method so the initialization for ShapeDrawFrame is done in a constructor instead of in init In fact the only real change necessary to convert a typical applet class into a frame class is to convert the applet s init method into a constructor and to add a bit of frame specific initialization to the constructor Everything that you learned about creating and programming GUI components and adding them to a content pane applies to frames as well Menus are also handled in exactly the same way So we really only need to look at the additional programming that is necessary for frames One significant difference is that the size and location of an applet are determined externally to the applet by the HTML code for a Web page and by the browser that displays the page The size of a frame on the other hand has to be set by the frame itself or by the program that creates the frame
326. plet is created do the initialization of the superclass RandomStrings Then set this applet to listen for mouse events on the drawingSurface The drawingSurface variable is defined in the RandomStrings class and represents a component that fills the entire applet super init drawingSurface addMouseListener this public void mousePressed MouseEvent evt When user presses the mouse tell the system to call the drawingSurface s paintComponent method drawingSurface repaint The next four empty routines are required by the MouseListener interface public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt public void mouseClicked MouseEvent evt public void mouseReleased MouseEvent evt end class ClickableRandomStrings Often when a mouse event occurs you want to know the location of the mouse cur sor This information is available from the parameter to the event handling method evt This parameter is an object of type MouseEvent and it contains instance methods that return information about the event To find out the coordinates of the mouse cursor call evt getX and evt getY These methods return integers which give the x and y co ordinates where the mouse cursor was positioned The coordinates are expressed in the coordinate system of the component that generated the event where the top left corner of the component is 0 0 5 4
327. ply a fact that can be proven to be true after the program has finished executing A program can be proven to be correct by showing that the postconditions of the program meet the program s specification Consider the following program segment where all the variables are of type double disc B B 4 A C x B Math sqrt disc 2 A The quadratic formula from high school mathematics assures us that the value as signed to x is a solution of the equation A x2 B x C 0 provided that the value of disc is greater than or equal to zero and the value of A is not zero If we can assume or guarantee that B B 4 A C gt 0 and that A 0 then the fact that x is a solution of the equation becomes a postcondition of the program segment We say that the condition B B 4 A C gt 0 is a precondition of the program segment The condition that A 0 is another precondition A precondition is defined to be condition that must be true at a given point in the execution of a program in order for the program to continue correctly A precondition is something that you want to be true It s something that you have to check or force to be true if you want your program to be correct We ve encountered preconditions and postconditions once before That section intro duced preconditions and postconditions as a way of specifying the contract of a subroutine As the terms are being used here a precondition of a subroutine is just a precondition of the
328. ponent Diagrams Component Diagrams show the software components either component technologies such as KParts CORBA components or Java Beans or just sections of the system which are clearly distinguishable and the artifacts they are made out of such as source code files programming libraries or relational database tables Deployment Diagrams Deployment diagrams show the runtime component instances and their associations They include Nodes which are physical resources typically a single computer They also show interfaces and objects class instances 6 2 THE UNIFIED MODELLING LANGUAGE 139 PECES Server States MOCE PaF Documentation Figure 6 10 Umbrello UML Modeller showing a State Diagram gt screen shots xmi Umbrello UML Modeller File Edit Diagram Code Settings Help AJE XAH UML Diagrams f je Business Process Make Reservation a Views Logical View activity diagram sequence diagram authentification failed state diagram 5 Use Case View Joego roa cancel Reservation canceled Documentation Process sucessfully finished Figure 6 11 Umbrello UML Modeller showing an Activity Diagram 140 CHAPTER 6 DESIGN II OBJECT ORIENTED DESIGN Chapter 7 A Solitaire Game Klondike In this chapter will build a version of the Solitaire game We ll use the case study investi gate t
329. r this variable changes value Make this change in the Blackjack program This applet uses a nested class BlackjackCanvas to represent the board You ll have to do most of your work in that class In order to manipulate the buttons you will have to use instance variables to refer to the buttons I strongly advise writing a subroutine that can be called whenever it is necessary to set the value of the gameInProgress variable Then the subroutine can take responsibility for enabling and disabling the buttons Recall that if bttn is a variable of type JButton then bttn setEnabled false disables the button and bttn setEnabled true enables the button Building on your solution to the preceding exercise make it possible for the user to place bets on the Blackjack game When the applet starts give the user 100 Add a JTextField to the strip of controls along the bottom of the applet The user can enter the bet in this JTextField When the game begins check the amount of the bet You should do this when the game begins not when it ends because several errors can occur The contents of the JTextField might not be a legal number The bet that the user places might be more money than the user has or it might be lt 0 You should detect these errors and show an error message instead of starting the game The user s bet should be an integral number of dollars You can convert the user s input into an integer and check for illegal non numer
330. r errors by calling the checkError method if out checkError true System out println Some error occurred while writing output System out println Output might be incomplete or invalid else System out println wordCount words from inputFileName output to outputFileName end main Making a copy of a file is a pretty common operation and most operating systems already have a command for doing so However it is still instructive to look at a Java program that does the same thing Many file operations are similar to copying a file except that the data from the input file is processed in some way before it is written to the output file All such operations can be done by programs with the same general form Since the program should be able to copy any file we can t assume that the data in the file is in human readable form So we have to use InputStream and OutputStream to operate on the file rather than Reader and Writer The program simply copies all the 250 CHAPTER 11 BASIC INPUT OUTPUT data from the InputStream to the OutputStream one byte at a time If source is the variable that refers to the InputStream then the function source read can be used to read one byte This function returns the value 1 when all the bytes in the input file have been read Similarly if copy refers to the OutputStream then copy write b writes one byte to the output file So the heart o
331. r message and terminating the program is almost as bad since it gives the program no chance to handle the error The program that calls the subroutine or uses the class needs to know that the error has occurred In languages that do not support exceptions the only alternative is to return some special value or to set the value of some variable to indicate that an error has occurred Exceptions are a cleaner way for a subroutine to react when it encounters an error 10 4 3 Assertions Recall that a precondition is a condition that must be true at a certain point in a program for the execution of the program to continue correctly from that point In the case where there is a chance that the precondition might not be satisfied it s a good idea to insert 224 CHAPTER 10 CORRECTNESS AND ROBUSTNESS an if statement to test it But then the question arises What should be done if the precondition does not hold One option is to throw an exception This will terminate the program unless the exception is caught and handled elsewhere in the program The programming languages C and C have always had a facility for adding what are called assertions to a program These assertions take the form assert condition where condition is a boolean valued expression This condition expresses a precondition that must hold at that point in the program When the computer encounters an assertion during the execution of the program it evaluates the condi
332. ractions occur in time while the Collaboration Diagrams put the relationships between the objects and their topology in the foreground In Collaboration Diagrams messages sent from one object to another are represented by arrows showing the message name parameters and the sequence of the message Collaboration Diagrams are specially well suited to showing a specific program flow or situation and are one of the best diagram types to quickly demonstrate or explain one process in the program logic 6 2 THE UNIFIED MODELLING LANGUAGE 137 lL OE Eile Edit Diagram cae Settinge Help f8ae EC UML Diagrams ygoow amp connection cl amp jexecution Dispatcher Te a Evan receive Type H S ER oA loadWorker job Transaction Worker Ss a work f f E Use Case View Documentation Ready Figure 6 8 Umbrello UML Modeller showing a Sequence Diagram i o PEED E e P E E EEEE EEEE ASTEKA SARERA SALAA SA AAAA AANI fj E SEAT EE LTTE OO Elle Edit Diagram Code Settings Help PECERA UML Diagrams g Qgenent commu E common a connection a amp connection l i execution EX Dispatcner E worker E worker ioc execution cla fi G sequence dia E amp toois S E List t g Map ci g Queue Use Case View
333. random values as if a new pair of dice were being thrown onto the gaming table Since the initialization is executed for each new object a set of random initial values will be computed for each new pair of dice Different pairs of dice can have different initial values For initialization of static member variables of course the situation is quite different There is only one copy of a static variable and initialization of that variable is executed just once when the class is first loaded If you don t provide any initial value for an instance variable a default initial value is provided automatically Instance variables of numerical type int double etc are automatically initialized to zero if you provide no other values boolean variables are initialized to false and char variables to the Unicode character with code number zero An instance variable can also be a variable of object type For such variables the default initial value is null In particular since Strings are objects the default initial value for String variables is nu11 Objects are created with the operator new For example a program that wants to use a PairOfDice object could say PairOfDice dice Declare a variable of type PairOfDice dice new PairOfDice Construct a new object and store a reference to it in the variable In this example new PairOfDice is an expression that allocates memory for the object initializes the object s insta
334. rash In many cases such problems are just annoyances but even on a personal computer there can be more serious consequences such as lost work or lost money When computers are given more important tasks the consequences of failure can be proportionately more serious 207 208 CHAPTER 10 CORRECTNESS AND ROBUSTNESS Just a few years ago the failure of two multi million space missions to Mars was prominent in the news Both failures were probably due to software problems but in both cases the problem was not with an incorrect program as such In September 1999 the Mars Climate Orbiter burned up in the Martian atmosphere because data that was expressed in English units of measurement such as feet and pounds was entered into a computer program that was designed to use metric units such as centimeters and grams A few months later the Mars Polar Lander probably crashed because its software turned off its landing engines too soon The program was supposed to detect the bump when the spacecraft landed and turn off the engines then It has been determined that deployment of the landing gear might have jarred the spacecraft enough to activate the program causing it to turn off the engines when the spacecraft was still in the air The unpowered spacecraft would then have fallen to the Martian surface A more robust system would have checked the altitude before turning off the engines There are many equally dramatic stories of problems caused by in
335. re FileInputStream and FileOutputStream In this section I will only discuss character oriented file I O using the FileReader and FileWriter classes However FileInputStream and FileOutputStream are used in an exactly parallel fashion All these classes are defined in the java io package It s worth noting right at the start that applets which are downloaded over a network connection are generally not allowed to access files This is a security consideration You can download and run an applet just by visiting a Web page with your browser If downloaded applets had access to the files on your computer it would be easy to write an applet that would destroy all the data on a computer that downloads it To prevent such possibilities there are a number of things that downloaded applets are not allowed to do Accessing files is one of those forbidden things Standalone programs written in Java however have the same access to your files as any other program When you write a standalone Java application you can use all the file operations described in this section The FileReader class has a constructor which takes the name of a file as a parameter and creates an input stream that can be used for reading from that file This constructor will throw an exception of type FileNotFoundException if the file doesn t exist This exception type requires mandatory exception handling so you have to call the constructor in a try statement or inside a routine that
336. re than one In doing so it must implement every method in the interface it can have more An interface cannot spec ify the behavior of the constructor Note that an interface is a valid type for a variable Example the interface interface Number void increment void add Number other ETC the implementation class MyInteger implements Number int X void increment x ETC the calling program but then one can only execute Number s methods on ticket Number ticket new MyInteger ticket increment There are also abstract classes where some of the methods are implemented and so are not 76 CHAPTER 4 OBJECT ORIENTED PROGRAMMING II 4 1 2 Interface Example Comparable and Comparator The Comparable interface is an in built interface to enable Java to provide facilities for sorting and searching problems The idea is that to sort a collection be it Strings ints or database records one needs a way to compare two items Implementing the Comparable interface guarantees that there is a method e int compareTo Object other which returns one of three values 1 if this comes before other 1 if this comes after other and 0 if the two objects are to be considered the same This method would usually be compatible with the equals method compareTo 0 if and only if equals true In built classes such as String and Integer implement this interface The pack age java util Array has
337. re that can be used to accomplish many of the same goals as multiple inheritance interfaces We ve encountered the term interface before in connection with black boxes in gen eral and subroutines in particular The interface of a subroutine consists of the name of 73 74 CHAPTER 4 OBJECT ORIENTED PROGRAMMING II the subroutine its return type and the number and types of its parameters This is the information you need to know if you want to call the subroutine A subroutine also has an implementation the block of code which defines it and which is executed when the subroutine is called In Java interface is a reserved word with an additional technical meaning An interface in this sense consists of a set of subroutine interfaces without any associated implementations A class can implement an interface by providing an implementation for each of the subroutines specified by the interface Here is an example of a very simple Java interface public interface Drawable public void draw Graphics g This looks much like a class definition except that the implementation of the method draw is omitted A class that implements the interface Drawable must provide an implementation for this method Of course the class can also include other methods and variables For example class Line implements Drawable public void draw Graphics g do something presumably draw a line other methods and variables
338. recondi tion namely that A is not null but let s leave that aside for the moment Consider the following code which searches for the number x in the array A i 0 while A i x i As this program segment stands it has a precondition namely that x is actually in the array If this precondition is satisfied then the loop will end when A i x That is the value of i when the loop ends will be the position of x in the array However if x is not in the array then the value of i will just keep increasing until it is equal to A length At that time the reference to A i is illegal and the program will be terminated To avoid this we can add a test to make sure that the precondition for referring to A i is satisfied i 0 while i lt A length amp amp Ali x 10 3 EXCEPTIONS AND THE TRY CATCH STATEMENT 215 i Now the loop will definitely end After it ends i will satisfy either i A length or A i x An if statement can be used after the loop to test which of these conditions caused the loop to end is 0 while i lt A length amp amp Ali x i if i A length System out println x is not in the array else System out println x is in position i 10 3 Exceptions and the try catch Statement GETTING A PROGRAM TO WORK UNDER IDEAL circumstances is usually a lot easier than making the program robust A robust program can survive unusual or except
339. rface will be a JComponent belonging to a subclass of the JPanel class We will define this class as a nested class inside the main applet class All the drawing is done in the 98 CHAPTER 5 APPLETS HTML AND GUI S paintComponent method of the drawing surface class I will use nested classes consis tently to define drawing surfaces although it is perfectly legal to use an independent class instead of a nested class to define the drawing surface A nested class can be either static or non static In general a non static class must be used if it needs access to instance variables or instance methods that are defined in the main class This will be the case in most of my examples The applet will draw multiple copies of a message on a black background Each copy of the message is in a random color Five different fonts are used with different sizes and styles The displayed message is the string Java but a different message can be specified in an applet param The applet works OK no matter what size is specified for the applet in the lt applet gt tag Here s the applet Applet Reference see Applet RandomStrings The source for the applet is shown below I use an instance variable called message to hold the message that the applet will display There are five instance variables of type Font that represent different sizes and styles of text These variables are initialized in the applet s init method and are used in
340. rivate so that they cannot be changed from outside the class and I ve provided instance methods getSuit and getValue so that it will be possible to discover the suit and value from outside the class The instance variables are initialized in the constructor and are never changed after that In fact I ve declared the instance variables suit and value to be final since they are never changed after they are initialized An instance variable can be declared final provided it is either given an initial value in its declaration or is initialized in every constructor in the class Finally ve added a few convenience methods to the class to make it easier to print out cards in a human readable form For example I want to be able to print out the suit of a card as the word Diamonds rather than as the meaningless code number 2 which is used in the class to represent diamonds Since this is something that I ll probably have to do in many programs it makes sense to include support for it in the class So I ve provided instance methods getSuitAsString and getValueAsString to return string representations of the suit and value of a card Finally there is an instance method toString that returns a string with both the value and suit such as Queen of Hearts There is a good reason for calling this method toString When any object is output with System out print the objects toString method is called to produce the string
341. rmine the source of an event by calling evt getSource However Java programmers tend to be fond of anonymous classes The applet defines methods startAnimation and stopAnimation which are called when the user clicks the button Each time the user clicks Start a new timer is created and started timer new Timer 50 timerListener timer start Remember that without timer start the timer won t do anything at all The con structor specifies a delay time of 50 milliseconds so there should be about 20 action events from the timer every second The second parameter is an ActionListener object The timer events will be processed by calling the actionPerformed method of this object The stopAnimation method calls timer stop which ends the flow of events from the timer The startAnimation and stopAnimation methods also change the text on the button so that it reads Stop when the animation is running and Start when it is not running The actionPerformed method in timerListener responds to a timer event by set ting the color of the display The color is computed from a number colorIndex that is changed for each frame The color is specified as an HSB color 8 5 2 JApplet s start and stop Methods There is one other method in the HelloWorldSpectrum class that needs some explanation the stop method Every applet has several methods that are meant to be called by the 174 CHAPTER 8 ADVA
342. rom another computer over a network that data is stored in a buffer The buffer is just a segment of memory that has been allocated by a program to hold data that it expects to receive A buffer overflow occurs when more data is received than will fit in the buffer The question is what happens then If the error is detected by the program or by the networking software then the only thing that has hap pened is a failed network data transmission The real problem occurs when the software does not properly detect buffer overflows In that case the software continues to store data in memory even after the buffer is filled and the extra data goes into some part of memory that was not allocated by the program as part of the buffer That memory might be in use for some other purpose It might contain important data It might even contain part of the program itself This is where the real security issues come in Suppose that a buffer overflow causes part of a program to be replaced with extra data received over a network When the computer goes to execute the part of the program that was replaced it s actually executing data that was received from another computer That data could be anything It could be a program that crashes the computer or takes it over A malicious programmer who finds a convenient buffer overflow error in networking software can try to exploit that error to trick other computers into executing his programs For software written comp
343. rom last year s project that looks like it might be made to fit On the level of a corporation that wants to save money by not reinventing the wheel for each new project just keeping track of all the old wheels becomes a major task Well designed classes are software components that can be reused without editing A well designed class is not carefully crafted to do a particular job in a particular program Instead it is crafted to model some particular type of object or a single coherent concept Since objects and concepts can recur in many problems a well designed class is likely to be reusable without modification in a variety of projects Furthermore in an object oriented programming language it is possible to make sub classes of an existing class This makes classes even more reusable If a class needs to be customized a subclass can be created and additions or modifications can be made in the subclass without making any changes to the original class This can be done even if the programmer doesn t have access to the source code of the class and doesn t know any details of its internal hidden implementation We will discuss subclasses later 2 4 Programming Examples The PairOfDice class is already an example of a generalized software component al though one that could certainly be improved The class represents a single coherent concept a pair of dice The instance variables hold the data relevant to the state of the dice
344. rs changeToRandomColor int int and randomMove Writing each of these subroutines is a separate small task The fillWithRandomColors routine is defined by the postcondition that each of the rectangles in the mosaic has been changed to a random color Pseudocode for an algorithm to accomplish this task can be given as For each row For each column set the square in that row and column to a random color For each row and for each column can be implemented as for loops We ve already planned to write a subroutine changeToRandomColor that can be used to set the color The possibility of reusing subroutines in several places is one of the big payoffs of using them So il1WithRandomColors can be written in Java as static void fillWithRandomColors for int row 0 row lt 10 rowt for int column 0 column lt 20 column changeToRandomColor row column Turning to the changeToRandomColor subroutine we already have a method Mosaic setColor row col r g b that can be used to change the color of a square If we want a random color we just have to choose random values for r g and b According to the precondition of the Mosaic setColor subroutine these random values must be integers in the range from 0 to 255 A formula for randomly selecting such an integer is Cint 256 Math random So the random color subroutine becomes static void changeToRandomColor int rowNum int colNum i
345. ry where that file is located A simple file name like data dat or result dat is taken to refer to a file in a directory that is called the current directory or default directory or working directory The current directory is not a permanent thing It can be changed by the user or by a program Files not in the current directory must be referred to by a path name which includes both the name of the file and information about the directory where it can be found To complicate matters even further there are two types of path names absolute path names and relative path names An absolute path name uniquely identifies one file among all the files available to the computer It contains full information about which directory the file is in and what its name is A relative path name tells the computer how to locate the file starting from the current directory Unfortunately the syntax for file names and path names varies quite a bit from one type of computer to another Here are some examples e data dat on any computer this would be a file named data dat in the current directory e home eck java examples data dat This is an absolute path name in the UNIX 11 3 FILES 243 operating system It refers to a file named data dat in a directory named examples which is in turn in a directory named java e C eck java examples data dat An absolute path name on a DOS or Windows computer e Hard Drive java e
346. s What will happen if there are no more cards in the deck when its dealCard method is called It should probably be considered an error to try to deal a card from an empty deck But this raises another question How will the rest of the program know whether the deck is empty Of course the program could keep track of how many cards it has used But the deck itself should know how many cards it has left so the program should just be able to ask the deck object We can make this possible by specifying another instance method cardsLeft that returns the number of cards remaining in the deck This leads to a full specification of all the subrou tines in the Deck class Constructor and instance methods in class Deck public Deck Constructor Create an unshuffled deck of cards public void shuffle Put all the used cards back into the deck and shuffle it into a random order public int cardsLeft As cards are dealt from the deck the number of cards left decreases This function returns the number of cards that are still left in the deck public Card dealCard Deals one card from the deck and returns it This is everything you need to know in order to use the Deck class Of course it doesn t tell us how to write the class This has been an exercise in design not in programming In fact writing the class involves a programming technique arrays which will not be covered until Chapter 8 Nevertheless you
347. s an object of type Car or Truck or Motorcycle is automatically an object of type Vehicle 56 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I This brings us to the following Important Fact A variable that can hold a reference to an object of class A can also hold a reference to an object belonging to any subclass of A The practical effect of this in our example is that an object of type Car can be assigned to a variable of type Vehicle That is it would be legal to say Vehicle myVehicle myCar or even Vehicle myVehicle new Car After either of these statements the variable myVehicle holds a reference to a Vehicle object that happens to be an instance of the subclass Car The object remembers that it is in fact a Car and not just a Vehicle Information about the actual class of an object is stored as part of that object It is even possible to test whether a given object belongs to a given class using the instanceof operator The test if myVehicle instanceof Car determines whether the object referred to by myVehicle is in fact a car On the other hand the assignment statement myCar myVehicle would be illegal because myVehicle could potentially refer to other types of vehicles that are not cars The computer will not allow you to assign an int value to a variable of type short because not every int isa short Similarly it will not allow you to assign a value of type Vehicle toa variable of type Car because not every vehic
348. s constructor can be either a noarg call or can include arguments passed to the super constructor 3 3 SUMMARY 69 pA You can provide your own no arg constructor You cannot call an instance method or access an instance variable until after the super constructor runs You can access static variables and methods only as part of the call to super or this Example super Animal DoThings PA The only way a constructor can be invoked is from within another constructor you can t write code that actually calls a constructor 3 3 2 Overloaded and Overridden Methods Methods can be overloaded or overridden but constructors can be only overloaded Over loaded methods and constructors let you use the same method name or constructor but with different argument lists Overriding lets you redefine a method in a subclass when you need new subclassspecific behavior Overloaded methods let you reuse the same method name in a class but with different arguments and optionally a different return type Overloading a method often means you re being a little nicer to those who call your methods because your code takes on the burden of coping with different argument types rather than forcing the caller to do conversions prior to invoking your method The rules for overloaded methods are simple e Overloaded methods must change the argument list e Overloaded methods can change the return type e Overloaded methods can change th
349. s design is undertaken by a team of developers CRC cards are used as a brainstorming technique The team attempts to determine all the classes and their responsibilities that will be needed for the application The team runs through various usage scenarios of the application For e g one such scenario for a game of cards may be the player picks a card from the deck and hand adds it to his hand The team uses the CRC cards to check if this scenario can be handled by the responsibilites assigned to the classes In this way the design is refined until the team agrees on a set of classes and has agreed on their responsibilities Using a small card keeps the complexity of the design at a minimum It focuses the designer on the essentials of the class and prevents him from getting into its details and inner workings at a time when such detail is probably counter productive It also forces the designer to refrain from giving the class too many responsibilities 6 2 The Unified Modelling Language This section will give you a quick overview of the basics of UML It is taken from the user documentation of the UML tool Umbrello and wikipedia Keep in mind that this is not a comprehensive tutorial on UML but rather a brief introduction to UML which can be read as a UML tutorial If you would like to learn more about the Unified Modelling Language or in general about software analysis and design refer to one of the many books available on the topic There
350. s from the user and add them to the dataset Use 0 as a sentinel value that is stop reading numbers when the user enters 0 After all the user s non zero numbers have been entered print out each of the six statistics that available from calc This problem uses the PairOfDice class from Exercise 5 1 and the StatCalc class from Exercise 5 2 The program in Exercise 4 4 performs the experiment of counting how many times a pair of dice is rolled before a given total comes up It repeats this experiment 10000 times and then reports the average number of rolls It does this whole process for each possible total 2 3 12 Redo that exercise But instead of just reporting the average number of rolls you should also report the standard deviation and the maximum number of rolls Use a PairOfDice object to represent the dice Use a StatCalc object to compute the statistics You ll need a new StatCalc object for each possible total 2 3 12 You can use a new pair of dice if you want but it s not necessary 2 9 PROGRAMMING EXERCISES 51 4 The BlackjackHand class is an extension of the Hand class BlackjackHand includes an instance method getBlackjackValue that returns the value of the hand for the game of Blackjack For this exercise you will also need the Deck and Card classes A Blackjack hand typically contains from two to six cards Write a program to test the BlackjackHand class You should create a BlackjackHand ob
351. s running It should be impossible for this to be called when an animation is already running but it doesn t hurt to check if timer null Start the animation by creating a Timer that will fire an event every 50 milliseconds and will send those events to timerListener timer new Timer 50 timerListener timer start Make the time start running startStopButton setText Stop 8 5 TIMERS ANIMATION AND THREADS 173 void stopAnimation Stop the animation by stopping the timer unless the animation is not running if timer null timer stop Stop the timer timer null Set timer variable to null so that we can tell that the animation isn t running startStopButton setText Start public void stop The stop method of an applet is called by the system when the applet is about to be stopped either temporarily or permanently We don t want a timer running while the applet is stopped so stop the animation It s harmless to call stopAnimation if the animation is not running stopAnimation This applet responds to ActionEvents from two sources the button and the timer that drives the animation I decided to use a different ActionListener object for each source Each listener object is defined by an anonymous nested class It would of course be possible to use a single object such as the applet itself as a listener and to dete
352. s similar to the one in DiscardPile We must check if any SuitPile 4 of them or any TablePile 7 of them all in the Solitaire class can take the card If any of these piles can take the card we add the Card to that pile otherwise we leave it in this tabePile 150 CHAPTER 7 A SOLITAIRE GAME KLONDIKE Chapter 8 Advanced GUI Programming IT S POSSIBLE TO PROGRAM A WIDE VARIETY of GUI applications using only the tech niques covered in the previous chapter In many cases the basic events components layouts and graphics routines covered in that chapter suffice But the Swing graphical user interface library is far richer than what we have seen so far and it can be used to build highly sophisticated applications This chapter is a further introduction to Swing Although the title of the chapter is Advanced GUI Programming it is still just an intro duction Full coverage of Swing would require at least another complete book In this chapter we ll take a more detailed look at Swing starting with a few more features of the Graphics class We ll cover a number of new layout managers component classes and event types and we ll see how to open independent windows and dialog boxes on the screen We ll also look at two other programming techniques timers and threads 8 1 More about Layouts and Components SWING INCLUDES A VARIETY of GUI components We have already encountered a few of these such as JApplet JButton and JPan
353. s that after this statement is executed the value of the variable x will be 7 We can be absolutely sure of this fact so it is something upon which we can build part of a mathematical proof In fact it is often possible to look at a program and deduce that some fact must be true at a given point during the execution of a program For example consider the do loop Scanner keyboard new Scanner System in do System out println Enter a positive integer N keyboard nextInt while N lt 0 After this loop ends we can be absolutely sure that the value of the variable N is greater than zero The loop cannot end until this condition is satisfied This fact is part of the meaning of the while loop More generally if a while loop uses the test while condition then after the loop ends we can be sure that the condition is false We can then use this fact to draw further deductions about what happens as the execution of the program continues With a loop by the way we also have to worry about the question of whether the loop will ever end This is something that has to be verified separately 10 2 WRITING CORRECT PROGRAMS 213 A fact that can be proven to be true after a given program segment has been executed is called a postcondition of that program segment Postconditions are known facts upon which we can build further deductions about the behavior of the program A postcon dition of a program as a whole is sim
354. s this special non a number value by calling the boolean valued function Double isNaN x For real numbers there is the added complication that most real numbers can only be represented approximately on a computer A real number can have an infinite number of digits after the decimal point A value of type double is only accurate to about 15 digits The real number 1 3 for example is the repeating decimal 0 333333333333 and there is no way to represent it exactly using a finite number of digits Computations with real numbers generally involve a loss of accuracy In fact if care is not exercised the result of a 212 CHAPTER 10 CORRECTNESS AND ROBUSTNESS large number of such computations might be completely wrong There is a whole field of computer science known as numerical analysis which is devoted to studying algorithms that manipulate real numbers Not all possible errors are detected automatically in Java Furthermore even when an error is detected automatically the system s default response is to report the error and terminate the program This is hardly robust behavior So a programmer still needs to learn techniques for avoiding and dealing with errors These are the topics of the rest of this chapter 10 2 Writing Correct Programs CORRECT PROGRAMS DON T just happen It takes planning and attention to detail to avoid errors in programs There are some techniques that programmers can use to increase the likelihood that
355. s to the same object but the data stored in the object might have changed Suppose x is a variable of type int and stu is a variable of type Student Compare void dontChange int z z 42 The lines x 17 dontChange x System out println x output the value 17 The value of x is not changed by the subroutine void change Student s s name Fred The lines stu name Jane change stu System out println stu name output the value Fred The value of stu is not changed but stu name is which is equivalent to This is equivalent to Zz X s stu z 42 s name Fred 2 2 Constructors and Object Initialization OBJECT TYPES IN JAVA ARE VERY DIFFERENT from the primitive types Simply declaring a variable whose type is given as a class does not automatically create an object of that class Objects must be explicitly constructed For the computer the process of constructing an object means first finding some unused memory in the heap that can be used to hold the object and second filling in the object s instance variables As a programmer you don t care where in memory the object is stored but you will usually want to exercise some control over what initial values are stored in a new object s instance variables In many cases you will also want to do more complicated initialization or bookkeeping every time an object is created An instance variable can be assigned an initial
356. se a throw statement The syntax of the throw statement is throw exception object The exception object must be an object belonging to one of the subclasses of Throwable Usually it will in fact belong to one of the subclasses of Exception In most cases it will be a newly constructed object created with the new operator For example throw new ArithmeticException Division by zero The parameter in the constructor becomes the error message in the exception object You might find this example a bit odd because you might expect the system itself to throw an ArithmeticException when an attempt is made to divide by zero So why should a programmer bother to throw the exception The answer is a little surprising If the numbers that are being divided are of type int then division by zero will indeed 220 CHAPTER 10 CORRECTNESS AND ROBUSTNESS throw an ArithmeticException However no arithmetic operations with floating point numbers will ever produce an exception Instead the special value Double NaN is used to represent the result of an illegal operation An exception can be thrown either by the system or by a throw statement The ex ception is processed in exactly the same way in either case Suppose that the exception is thrown inside a try statement If that try statement has a catch clause that handles that type of exception then the computer jumps to the catch clause and executes it The exception has been handled After handling t
357. ses JTextFields and JTextAreas are boxes where the user can type in and edit text The dif ference between them is that a JTextField contains a single line of editable text while a JTextArea can display multiple lines It is also possible to set a JTextField or JTextArea to be read only so that the user can read the text that it contains but cannot edit the text JTextField and JTextArea are subclasses of javax swing text JTextComponent which defines their common behavior The JTextComponent class supports the idea of a selection A selection is a subset of the characters in the JTextComponent including all the characters from some starting position to some ending position The selection is high lighted on the screen The user selects text by dragging the mouse over it Some useful methods in class JTextComponent include the following They can of course be used for both JTextFields and JTextAreas 8 2 BASIC COMPONENTS AND THEIR EVENTS 161 void setText String newText substitute newText for current contents String getText return a copy of the current contents String getSelectedText return the selected text void select int start int end change the selection characters in the range start lt pos lt end are selected characters are numbered starting from zero void selectAll select the entire text int getSelectionStart get starting point of selection int getSelectionEnd get end point
358. shapes like rectangles and ovals can be drawn So why should there even be a redraw method in the Shape class Well it has to be there or it would be illegal to call it in the setColor method of the Shape class and it would be illegal to write oneShape redraw where oneShape is a variable of type Shape The compiler would complain that oneShape is a variable of type Shape and there s no redraw method in the Shape class Nevertheless the version of redraw in the Shape class will never be called In fact if you think about it there can never be any reason to construct an actual object of type Shape You can have variables of type Shape but the objects they refer to will always belong to one of the subclasses of Shape We say that Shape is an abstract class An abstract class is one that is not used to construct objects but only as a basis for making subclasses An abstract class exists only to express the common properties of all its subclasses Similarly we say that the redraw method in class Shape is an abstract method since it is never meant to be called In fact there is nothing for it to do any actual redrawing is done by redraw methods in the subclasses of Shape The redraw method in Shape has to be there But it is there only to tell the computer that all Shapes understand the redraw message As an abstract method it exists merely to specify the common interface of all the actual concrete versions of redraw
359. sition index move up one position to make room for the new item The value of index can be in the range 0 to list size inclusive list remove index removes the object at position number index Items after this position move up one space in the list to fill the hole list indexOf obj returns an int that gives the position of obj in the list if it occurs If it does not occur the return value is 1 If obj occurs more than once in the list the index of the first occurrence is returned These methods are defined in both the ArrayList class and in the LinkedList class although they are only efficient for ArrayLists The LinkedList class adds a few addi tional methods which are not defined for an ArrayList If linkedlist is an object of type LinkedList then linkedlist getFirst returns the Object that is the first item in the list The list is not modified linkedlist getLast returns the Object that is the last item in the list The list is not modified linkedlist removeFirst removes the first item from the list and returns that Object as its return value linkedlist removeLast removes the last item from the list and returns that Object as its return value linkedlist addFirst obj adds the Object obj to the beginning of the list linkedlist addLast obj adds the Object obj to the end of the list This is exactly the same as linkedlist add obj and is apparently defined just
360. sses The method iter remove removes an item from the list the item that is removed is the item that the iterator passed most recently in a call to either iter next or iter previous There is also a method iter add Object that adds the specified object to the list at the current position of the iterator This can be between two existing items or at the beginning of the list or at the end of the list By the way the lists that are used in the LinkedList class are doubly linked lists That is each node in the list contains two pointers one to the next node in the list and one to the previous node This makes it possible to implement effeciently both the next and previous methods of a ListIterator Also to make the addLast and getLast methods of a LinkedList efficient the LinkedList class includes an instance variable that points to the last node in the list As an example of using a ListIterator suppose that we want to maintain a list of items that is always sorted into increasing order When adding an item to the list we can use a ListIterator to find the position in the list where the item should be added The idea is to start at the beginning of the list and to move the iterator forward past all the items that are bigger than the item that is being inserted At that point the iterator s add method can be used to insert the item at its correct position in the list In order to say what it means for one item to be bigger
361. stance variables 94 CHAPTER 5 APPLETS HTML AND GUI S w O Width Y height Figure 5 3 Screen Coordinates width and height The width of the component is getSize width pixels and its height is getSize height pixels When you are writing an applet you don t necessarily know the applet s size The size of an applet is usually specified in an lt APPLET gt tag in the source code of a Web page and it s easy for the Web page author to change the specified size In some cases when the applet is displayed in some other kind of window instead of on a Web page the applet can even be resized while it is running So it s not good form to depend on the size of the applet being set to some particular value For other components you have even less chance of knowing the component s size in advance This means that it s good form to check the size of a component before doing any drawing on that component For example you can use a paintComponent method that looks like public void paintComponent Graphics g int width getSize width Find out the width of component int height getSize height Find out its height Draw the contents of the component Of course your drawing commands will have to take the size into account That is they will have to use x y coordinates that are calculated based on the actual height and width of the applet 5 3 2 Colors Java is designed to wor
362. stion 6 What is a JCheckBox and how is it used Question 7 What is a thread Question 8 Explain how Timers are used to do animation Question 9 Menus can contain sub menus What does this mean and how are sub menus handled in Java Question 10 What is the purpose of the JFrame class 8 7 Programming Exercises 1 StatCalc java computes some statistics of a set of numbers Write an applet that uses the StatCalc class to compute and display statistics of numbers entered by the user The applet will have an instance variable of type StatCalc that does the computations The applet should include a JTextField where the user enters a number It should have four labels that display four statistics for the numbers that have been entered the number of numbers the sum the mean and the standard deviation Every time the user enters a new number the statistics displayed on the labels should change The user enters a number by typing it into the JTextField and pressing return There should be a Clear button that clears out all the data This means creating a new StatCalc object and resetting the displays on the labels My applet also has an Enter button that does the same thing as pressing the return key in the JTextField Recall that a JTextField generates an ActionEvent when the user presses return so your applet should register itself to listen for ActionEvents from the JTextField 2 Write an applet with a JTextArea where the us
363. t again as a new stream Note that for most stream classes the close method can throw an IOException which must be handled however PrintWriter overrides this method so that it cannot throw such exceptions As a complete example here is a program that will read numbers from a file named data dat and will then write out the same numbers in reverse order to another file named result dat It is assumed that data dat contains only one number on each line and that 11 3 FILES 241 there are no more than 1000 numbers altogether Exception handling is used to check for problems along the way Although the application is not a particularly useful one this program demonstrates the basics of working with files By the way at the end of this program you ll find our first example of a finally clause in a try statement When the computer executes a try statement the commands in its finally clause are guaranteed to be executed no matter what import java io public class ReverseFile public static void main String args Scanner data Character input stream for reading data PrintWriter result Character output stream for writing data double number new double 1000 An array to hold all the numbers that are read from the file int numberCt Number of items actually stored in the array try Create the input stream data new Scanner new FileReader data dat catch FileNotFoundException e
364. t where a solution is computed and printed we know that the preconditions are fulfilled In the other parts we know that one of the preconditions fails to hold In any case the program is correct Scanner sc new Scanner System in System out println Enter your values for A B and C System out print A A sc nextDouble System out print B B sc nextDouble System out print C C sc nextDouble if A 0 amp amp B B 4 AxC gt 0 disc B B 4 Ax C x B Math sqrt disc 2 A System out printin A solution of A X X B X C 0 is x else if A 0 System out println The value of A cannot be zero else System out println Since B B 4 A C is less than zero the System out println equation A X X B X C 0 has no solution Whenever you write a program its a good idea to watch out for preconditions and think about how your program handles them Often a precondition can offer a clue about how to write the program For example every array reference such as A i has a precondition The index must be within the range of legal indices for the array For A i the precondition is that 0 lt i lt A length The computer will check this condition when it evaluates A i and if the condition is not satisfied the program will be terminated In order to avoid this you need to make sure that the index has a legal value There is actually another p
365. t Kit includes javadoc as a program that can be used on the command line This program takes one or more Java source code files extracts the javadoc comments from them and prepares Web pages containing the documentation Integrated development environments for Java such as the Eclipse IDE typically include a menu command for generating javadoc documentation Chapter 2 Object Based Programming WHEREAS A SUBROUTINE represents a single task an object can encapsulate both data in the form of instance variables and a number of different tasks or behaviors related to that data in the form of instance methods Therefore objects provide another more sophisticated type of structure that can be used to help manage the complexity of large programs 2 1 Objects Instance Methods and Instance Variables OBJECT ORIENTED PROGRAMMING OOP represents an attempt to make programs more closely model the way people think about and deal with the world In the older styles of programming a programmer who is faced with some problem must identify a computing task that needs to be performed in order to solve the problem Programming then consists of finding a sequence of instructions that will accomplish that task But at the heart of object oriented programming instead of tasks we find objects entities that have behaviors that hold information and that can interact with one another Programming consists of designing a set of objects that somehow
366. t a repeating timer the time until the first event is the same as the time between events Sometimes it would be convenient to have a longer or shorter delay before the first event If you start a timer in the init method of an applet for example you might want to give the applet some time to appear on the screen before receiving any events from the timer You can set a separate delay for the first event by calling timer setInitialDelay delay where timer is the Timer and delay is specified in milliseconds as usual 8 5 4 Using Threads Although Timers can replace threads in some cases there are times when direct use of threads is necessary When a Timer is used the processing is done in an event handler This means that the processing must be something that can be done quickly An event 176 CHAPTER 8 ADVANCED GUI PROGRAMMING handler should always finish its work quickly so that the system can move on to handle the next event If an event handler runs for a long time it will block other events from being processed and the program will become unresponsive So in a GUI application any computation or process that will take a long time to complete should be run in a separate thread Then the event handling thread can continue to run at the same time responding to user actions as they occur As a short and incomplete introduction to threads we ll look at one example The example requires some explanation but the main point for our dis
367. t can enforce this condition at compile time Java 1 5 templates still only work with object types and not with primitive types but see the note later on this page about automatic conversion between primitive types and wrapper types 9 2 An Example ArrayLists In Java every class is a subclass of the class named Object This means that every object can be assigned to a variable of type Object Any object can be put into an array of type Object If a subroutine has a formal parameter of type Object than any object can be passed to the subroutine as an actual parameter The ArrayList class is in the package java util so if you want to use the ArrayList class in a program you should put the directive import java util ArrayList or import java util at the beginning of your source code file The ArrayList class always has a definite size and it is illegal to refer to a position in the ArrayList that lies outside its size In this an ArrayList is more like a regular array However the size of an ArrayList can be increased at will The ArrayList class defines many instance methods Pll describe some of the most useful Suppose that list is a variable of type ArrayList e list size This function returns the current size of the ArrayList The only valid positions in the list are numbers in the range 0 to list size 1 Note that the size can be zero A call to the default constructor newArrayList creates an ArrayLis
368. t in outline it works like this The type of event generated by a button is called an ActionEvent For the applet to respond to an event of this type it must define a method public void actionPerformed ActionEvent evt Furthermore the button must be told that the applet will be listening for action events from the button This is done by calling one of the button object s instance methods addActionListener in the applet s init method What should the applet do in its actionPerformed method When the user clicks the button we want a message window to appear on the screen Fortunately Swing makes this easy The class swing javax JOptionPane has a static method showMessageDialog that can be used for this purpose so all we have to do in actionPerformed is call that method Given all this you can understand a lot of what goes on in the source code for the HelloSwing applet This example shows several aspects of applet programming An init method sets up the applet and adds components the components generate events and event handling methods say what happens in response to those events Here is the source code 86 CHAPTER 5 APPLETS HTML AND GUI S An applet that appears on the page as a button that says Click Me When the button is clicked an informational dialog box appears to say Hello from Swing import javax swing Swing GUI classes are defined here import java awt event
369. t it is the only class that has this property Thus class myClass is exactly equivalent to class myClass extends Object Every other class is directly or indirectly a gubcliss of Object This means that any ob ject belonging to any class whatsoever can be assigned to a variable of type Object The class Object represents very general properties that are shared by all objects belonging to any class Object is the most abstract class of all The Object class actually finds a use in some cases where objects of a very general sort are being manipulated For example java has a standard class java util ArrayList that represents a list of Objects The ArrayList class is in the package java util If you want to use this class in a program you write you would ordinarily use an import statement to make it possible to use the short name ArrayList instead of the full name java util ArrayList ArrayList is discussed more fully later The ArrayList class is very convenient because an ArrayList can hold any number of objects and it will grow when necessary as objects are added to it Since the items in the list are of type Object the list can actually hold objects of any type A program that wants to keep track of various Shapes that have been drawn on the screen can store those shapes in an ArrayList Suppose that the ArrayList is named listOfShapes A shape oneShape can be added to the end of the list by calling the instance metho
370. t meant for displaying large amounts of text the number of lines read from the file is limited to one hundred at most Before the file is read any text currently in the JTextArea is removed Then lines are read from the file and appended to the JTextArea one by one with a line feed character at the end of each line This process continues until one hundred lines have been read or until the end of the input file is reached If any error occurs during this process an error message is displayed to the user in a dialog box Here is the complete method 254 CHAPTER 11 BASIC INPUT OUTPUT private void doOpen Carry out the Open command by letting the user specify the file to be opened and reading up to 100 lines from that file The text from the file replaces the text in the JTextArea File file The file that the user wants to open JFileChooser fd File dialog that lets the user specify a file fd new JFileChooser new File fd setDialogTitle Open File int action fd show0penDialog this if action JFileChooser APPROVE_OPTION User canceled the dialog or an error occurred return file fd getSelectedFile try Read lines from the file until end of file is detected or until 100 lines have been read The lines are added to the JTextArea with a line feed after each line Scanner in new Scanner new FileReader file String line text setText int lineCt 0 while lineCt
371. t of size zero list add obj Adds an object onto the end of the ArrayList increasing the size by 1 The parameter obj can refer to an object of any type or it can be null list get N This function returns the value stored at position N in the ArrayList N must be an integer in the range 0 to list size 1 If N is outside this range an error occurs Calling this function is similar to referring to A N for an array A except that you can t use list get N on the left side of an assignment statement 9 2 AN EXAMPLE ARRAYLISTS 189 e list set N obj Assigns the object obj to position N in the ArrayList replac ing the item previously stored at position N The integer N must be in the range from 0 to list size 1 A call to this function is equivalent to the command A N 0bj for an arrayA e list remove obj If the specified object occurs somewhere in the ArrayList it is removed from the list Any items in the list that come after the removed item are moved down one position The size of the ArrayList decreases by 1 If obj occurs more than once in the list only the first copy is removed e list remove N For an integer N this removes the N th item in the ArrayList N must be in the range 0 to list size 1 Any items in the list that come after the removed item are moved down one position The size of the ArrayList decreases by 1 e list indexOf obj A function that searches for the object obj
372. t of this page discusses several other basic HTML tags This is not meant to be a complete discussion But it is enough to produce interesting pages 5 2 1 Overall Document Structure HTML documents have a standard structure They begin with lt HTML gt and end with lt HTML gt Between these tags there are two sections the head which is marked off by lt HEAD gt and lt HEAD gt and the body which as I m sure you have guessed is sur rounded by lt BODY gt and lt BODY gt Often the head contains only one item a title for the document This title might be shown for example in the title bar of a Web browser window The title should not contain any HTML tags The body contains the actual page contents that are displayed by the browser So an HTML document takes this form lt HTML gt lt HEAD gt lt TITLE gt page title lt TITLE gt lt HEAD gt lt BODY gt page contents lt BODY gt lt HTML gt Web browsers are not very picky about enforcing this structure you can probably get away with leaving out everything but the actual page contents But it is good form to follow this structure for your pages The lt BODY gt tag can take a number of modifiers that affect the appearance of the page when it is displayed The modifier named BGCOLOR can be used to set the background color of the page For example 90 CHAPTER 5 APPLETS HTML AND GUI S lt BODY bgcolor white gt will ensure that the background c
373. t std stdl Declare four variables of std2 std3 type Student std new Student Create a new object belonging to the class Student and store a reference to that object in the variable std std1 new Student Create a second Student object and store a reference to it in the variable stdi std2 stdl Copy the reference value in stdl into the variable std2 std3 null Store a null reference in the variable std3 std name John Smith Set values of some instance variables stdi name Mary Jones Other instance variables have default initial values of zero After the computer executes these statements the situation in the computer s memory looks like this std std std2 std3 Object class Student name testi o Object class Student name testi C Lol test2 o a test2 o test3 o getAverage test3 getAverage Object class String Object class String John Smith Mary Jones Figure 2 1 Objects in Memory This picture shows variables as little boxes labeled with the names of the variables Objects are shown as boxes with round corners When a variable contains a reference 28 CHAPTER 2 OBJECT BASED PROGRAMMING to an object the value of that variable is shown as an arrow pointing to the object The variable std3 with a value of null doesn t point anywhere The arrows fr
374. t the end of the frame s constructor but it can also be reasonable to leave it out In that case the constructor will produce an invisible window and the program that creates the frame is responsible for calling its show method A frame has a title a string that appears in the title bar at the top of the window This title can be provided as an argument to the constructor or it can be set by calling the method void setTitle String title In the ShapeDrawFrame class I set the title of the frame to be Shape Draw I do this by calling a constructor in the superclass with the command super Shape Draw at the beginning of the ShapeDrawFrame constructor Now you know how to get a frame onto the screen and how to give it a title There is still the matter of getting rid of the frame You can hide a frame by calling its hide method If you do this you can make it visible again by calling show If you are completely finished with the window you can call its dispose method to close the window and free the system resources that it uses After calling dispose you should not use the frame object again You also have to be prepared for the fact that the user can click on the window s close box to indicate that it should be closed By default the system will hide the window when the user clicks its close box However you can tell the system to handle this event differently Sometimes it makes more sense to dispose of the
375. ter hasNext Object item iter next System out println item The same general form will work for other types of processing For example here is a subroutine that will remove all null values from any collection as long as that collection supports removal of values void removeNullValues Collection coll Iterator iter coll iterator while iter hasNext Object item iter next if item null iter remove Collections can hold objects of any type so the return value of iter next is Object Now there s not very much you can do with a general Object In practical situations a 9 3 COLLECTIONS 193 collection is used to hold objects belonging to some more specific class and objects from the collection are type cast to that class before they are used Suppose for example that we are working with Shapes where Shape is a class that represents geometric figures Suppose that the Shape class includes a draw method for drawing the figure Then we can write a generic method for drawing all the figures in a collection of Shapes void drawAllShapes Collection shapeCollection Precondition Every item in shapeCollection is non null and belongs to the class Shape Iterator iter shapeCollection iterator while iter hasNext Shape figure Shape iter next figure draw The precondition of this method points out that the method will fail if the method contains a
376. terface does not declare any methods It exists only as a marker for the compiler to tell it that the object is meant to be writable and readable You only need to add the words implements Serializable to your class definitions Many of Java s standard classes are already declared to be serializable including all the compo 11 3 FILES 239 nent classes in Swing and in the AWT This means in particular that GUI components can be written to ObjectOutputStreams and read from ObjectInputStreams 11 3 Files THE DATA AND PROGRAMS IN A COMPUTER S MAIN MEMORY survive only as long as the power is on For more permanent storage computers use files which are collections of data stored on a hard disk on a floppy disk on a CD ROM or on some other type of storage device Files are organized into directories sometimes called folders A directory can hold other directories as well as files Both directories and files have names that are used to identify them Programs can read data from existing files They can create new files and can write data to files In Java such input and output is done using streams Human readable char acter data is read from a file using an object belonging to the class FileReader which is a subclass of Reader Similarly data is written to a file in human readable format through an object of type FileWriter a subclass of Writer For files that store data in ma chine format the appropriate I O classes a
377. terminate the program System out print Input file name inputFileName keyboard next try in new Scanner new FileReader inputFileName catch FileNotFoundException e System out println Can t find file inputFileName return Returning from main ends the program Get the output file name from the user and try to create the output stream If there is an IOException print a message and terminate the program System out print Output file name outputFileName keyboard next 11 4 PROGRAMMING WITH FILES 249 try out new PrintWriter new FileWriter outputFileName catch IOException e System out println Can t open file outputFileName for output System out println e toString return Read all the words from the input stream and insert them into the array of words try while in hasNext insertWord in next catch IOException e System out println An error occurred while reading from input file System out println e toString Write all the words from the list to the output stream for int i 0 i lt wordCount i out println words i Finish up by checking for an error on the output stream and printing either a warning message or a message that the words have been output to the output file The PrintWriter class does not throw an exception when an error occurs so we have to check fo
378. than character streams However as of Java 1 1 you should use Readers and Writers rather than InputStreams and OutputStreams when working with character data The standard stream classes discussed in this section are defined in the package java io along with several supporting classes You must import the classes from this package if you want to use them in your program That means putting the directive import java io at the beginning of your source file Streams are not used in Java s graphi cal user interface which has its own form of I O But they are necessary for working with files and for doing communication over a network They can be also used for communica tion between two concurrently running threads and there are stream classes for reading and writing data stored in the computer s memory The beauty of the stream abstraction is that it is as easy to write data to a file or to send data over a network as it is to print information on the screen The basic I O classes Reader Writer InputStream and OutputStream provide only very primitive I O operations For example the InputStream class declares the instance method public int read throws IOException for reading one byte of data a number in the range 0 to 255 from an input stream If the end of the input stream is encountered the read method will return the value 1 instead If some error occurs during the input attempt an IOException is thrown Since IOExcept
379. the PairOfDice class has a constructor that takes two integers as parameters Consider a subclass public class GraphicalDice extends PairOfDice public GraphicalDice Constructor for this class super 3 4 Call the constructor from the PairOfDice class with parameters 3 4 initializeGraphics Do some initialization specific to the GraphicalDice class More constructors methods variables This might seem rather technical but unfortunately it is sometimes necessary By the way you can use the special variable this in exactly the same way to call another constructor in the same class This can be useful since it can save you from repeating the same code in several constructors 3 3 Summary 3 3 1 Rules for Constructors Constructors can use any access modifier including private The constructor name must match the name of the class Constructors must not have a return type If you don t explicitly provide a constructor a default noargument constructor is automatically generated by the compiler The compiler won t generate any constructor if you ve typed in any other construc tor s Every constructor must have as its first statement either a call to an overloaded constructor this or a call to the superclass constructor super If you ve type in a constructor without a call to super the compiler will insert a noarg call to super automatically A call to super clas
380. the WireFrameModel class static public class Line Represents a line from the point xl y1 z1 to the point x2 y2 z2 in 3 dimensional space double x1 y1 21 double x2 y2 22 end class Line other members of the WireFrameModel class end WireFrameModel Inside the WireframeModel class a Line object would be created with the constructor new Line Outside the class new WireFrameModel Line would be used A static nested class has full access to the members of the containing class even to the private members This can be another motivation for declaring a nested class since it lets you give one class access to the private members of another class without making those members generally available to other classes When you compile the above class definition two class files will be created Even though the definition of Line is nested inside WireFrameModel the compiled Line class is stored in a separate file WireFrameModel Line class will be the name of the class file for Line Non static nested classes are not in practice very different from static nested classes but a non static nested class is actually associated to an object rather than to the class in which it is nested This can get some getting used to Any non static member of a class is not really part of the class itself although its source code is contained in the class definition This is true for non static nested classes just as
381. the component you could define these two methods As an example let s look at an applet that does something when the user clicks on it Here s an improved version of the RandomStrings applet In this version the applet will redraw itself when you click on it Applet Reference see Applet ClickableRandomStrings For this version of the applet we need to make four changes in the source code First add the line import java awt event before the class definition Second declare that some class implements the MouseListener interface If we want to use the applet itself as the listener we would do this by saying class RandomStrings extends JApplet implements MouseListener Third define the five methods of the MouseListener interface Only mousePressed will do anything We want to repaint the drawing surface of the applet when the user clicks the mouse The drawing surface is represented in this applet by an instance variable named drawingSurface so the mousePressed needs to call drawingSurface repaint to force the drawing surface to be redrawn The other mouse listener methods are empty The following methods are added to the applet class definition public void mousePressed MouseEvent evt When user presses the mouse tell the system to call the drawing surface s paintComponent method drawingSurface repaint The following empty routines are required by the MouseListener interface pub
382. the drawing surface s paintComponent method I also use the init method to create the drawing surface add it to the applet and set its background color to black The paintComponent method for the drawing surface simply draws 25 copies of the message For each copy it chooses one of the five fonts at random and it calls g setFont to select that font for drawing the text It creates a random HSB color and uses g setColor to select that color for drawing It then chooses random x y coordinates for the location of the message The x coordinate gives the horizontal position of the left end of the string The formula used for the x coordinate 50 int Math random width 40 gives a random integer in the range from 50 to width 10 This makes it possible for the string to extend beyond the left edge or the right edge of the applet Similarly the formula for y allows the string to extend beyond the top and bottom of the applet The drawing surface class which is named Display defines the paintComponent method that draws all the strings that appear in the applet The drawing surface is created in the applet s init method as an object of type Display This object is set to be the content pane of the applet A JApplet s content pane fills the entire applet except for an optional menu bar An applet comes with a default content pane and you can add components to that content pane However any JComponent can be a
383. the instance variables in the objects have the same values To do that you would need to ask whether std1 test1 std2 test1 amp amp stdi test2 std2 test2 amp amp std1l test3 std2 test3 amp amp stdi name equals std2 name I ve remarked previously that Strings are objects and I ve shown the strings Mary Jones and John Smith as objects in the above illustration A variable of type String can only hold a reference to a string not the string itself It could also hold the value null meaning that it does not refer to any string at all This explains why using the operator to test strings for equality is not a good idea Suppose that greeting is a variable of type String and that the string it refers to is Hello Then would the test greeting Hello be true Well maybe maybe not The variable greeting and the String literal Hello each refer to a string that contains the characters H e l l o But the strings could still be different objects that just happen to contain the same charac ters The function greeting equals Hello tests whether greeting and Hello contain the same characters which is almost certainly the question you want to ask The expression greeting Hello tests whether greeting and Hello contain the same characters stored in the same memory location The fact that variables hold references to ob
384. the quadratic equation A x x B x C 0 Throws an exception if A 0 or B B 4 A C lt 0 if A 0 throw new IllegalArgumentException A can t be zero else double disc B B 4 AxC if disc lt 0 throw new IllegalArgumentException Discriminant lt zero return B Math sqrt disc 2 A Your program should allow the user to specify values for A B and C It should call the subroutine to compute a solution of the equation If no error occurs it should print the root However if an error occurs your program should catch that error and print an error message After processing one equation the program should ask whether the user wants to enter another equation The program should continue until the user answers no 2 As discussed in Section 1 values of type int are limited to 32 bits Integers that are too large to be represented in 32 bits cannot be stored in an int variable Java has a standard class java math BigInteger that addresses this problem An object of type BigInteger is an integer that can be arbitrarily large The maximum size is limited only by the amount of memory on your computer Since BigIntegers are objects they must be manipulated using instance methods from the BigInteger class For example you can t add two BigIntegers with the operator Instead if N and M are variables that refer to BigIntegers you can compute the sum of N and M with the function call N add M The v
385. the redraw statement in the setColor method does not necessarily call the redraw method in the Shape class The redraw method that is executed could be in any subclass of Shape Again this is not a real surprise if you think about it in the right way Remember that an instance method is always contained in an object The class only contains the source code for the method When a Rectangle object is created it contains a redraw method The source code for that method is in the Rectangle class The object also contains a setColor method Since the Rectangle class does not define a setColor method the source code for the rectangle s setColor method comes from the superclass Shape But even though the source codes for the two methods are in different classes the meth ods themselves are part of the same object When the rectangle s setColor method is executed and calls redraw the redraw method that is executed is the one in the same object Whenever a Rectangle Oval or RoundRect object has to draw itself it is the redraw method in the appropriate class that is executed This leaves open the question What does the redraw method in the Shape class do How should it be defined 60 CHAPTER 3 OBJECT ORIENTED PROGRAMMING I The answer may be surprising We should leave it blank The fact is that the class Shape represents the abstract idea of a shape and there is no way to draw such a thing Only particular concrete
386. the result of a computation lies outside this range For example what is 2147483647 1 And what is 2000000000 2 The mathematically correct result in each case cannot be represented as a value of type int These are examples of integer overflow In most cases integer overflow should be considered an error However Java does not automatically detect such errors For example it will compute the value of 2147483647 1 to be the negative number 2147483648 What happens is that any extra bits beyond the 32 nd bit in the correct answer are discarded Values greater than 2147483647 will wrap around to negative values Mathematically speaking the result is always correct modulo 232 For example consider the 3N 1 program Starting from a positive integer N the program computes a certain sequence of integers 10 1 INTRODUCTION TO CORRECTNESS AND ROBUSTNESS 211 while N 1 f if N 2 0 If N is even N N 2 else N 3 N 1 System out println N But there is a problem here If N is too large then the value of 3 N 1 will not be mathematically correct because of integer overflow The problem arises whenever 3 N 1 gt 2147483647 that is when N gt 2147483646 3 For a completely correct program we should check for this possibility before computing 3 N 1 while N 1 if N 2 0 If N is even N N 2 else if N gt 2147483646 3 System out println Sorry but the value of N has become System o
387. the stop watch is stopped The elapsed time won t be very accurate anyway so just show the integral number of seconds You only need to set the text a few times per second For my Timer method I use a delay of 100 milliseconds for the timer Here is an applet that tests my solution to this exercise 7 The applet KaleidaAnimate shows animations of moving symmetric patterns that look something like the image in a kaleidescope Symmetric patterns are pretty Make the SimplePaint3 applet do symmetric kaleidoscopic patterns As the user draws a figure the applet should be able to draw reflected versions of that figure to make symmetric pictures The applet will have several options for the type of symmetry that is displayed The user should be able to choose one of four options from a JComboBox menu Using the No symmetry option only the figure that the user draws is shown Using 2 way symmetry the user s figure and its horizontal reflection are shown Using 4 way symmetry the two vertical reflections are added Finally using 8 way symmetry the four diagonal reflections are also added Formulas for computing the reflections are given below The source code SimplePaint3 java already has a drawFigure subroutine that draws all the figures You can add a putMultiFigure routine to draw a fig ure and some or all of its reflections putMultiFigure should call the existing drawFigure to draw the figure and any necessary r
388. their programs are correct In some cases it is possible to prove that a program is correct That is it is possi ble to demonstrate mathematically that the sequence of computations represented by the program will always produce the correct result Rigorous proof is difficult enough that in practice it can only be applied to fairly small programs Furthermore it depends on the fact that the correct result has been specified correctly and completely As I ve already pointed out a program that correctly meets its specification is not useful if its specification was wrong Nevertheless even in everyday programming we can apply some of the ideas and techniques that are used in proving that programs are correct The fundamental ideas are process and state A state consists of all the information relevant to the execution of a program at a given moment during the execution of the program The state includes for example the values of all the variables in the program the output that has been produced any input that is waiting to be read and a record of the position in the program where the computer is working A process is the sequence of states that the computer goes through as it executes the program From this point of view the meaning of a statement in a program can be expressed in terms of the effect that the execution of that statement has on the computer s state As a simple example the meaning of the assignment statement x 7 i
389. ther System in or a file you use the Scanner object in the same way As an example this code allows long types to be assigned from entries in a file myNumbers Scanner myFile new Scanner new File myNumbers while myFile hasNextLong f long aLong myFile nextLong 11 1 2 Formatted Output Java now includes the ability to format output The formatting system is similar to that in C language 11 1 INPUT AND OUPUT IN JAVA 233 To produce formatted output you require a format string and an argument list The format string is a String which may contain fixed text and one or more embedded format specifiers The format specifiers specify how the argument s should be formatted The format specifiers for general character and numeric types have the following syntax argument_index flags width precision conversion Consider 41 3 1f as an example e The optional argument_index is a decimal integer indicating the position of the ar gument in the argument list The first argument is referenced by 1 the second by 2 etc In the example above 1 refers to the first argument e The required conversion is a character indicating how the argument should be for matted The set of valid conversions for a given argument depends on the argument s data type The following conversions apply to integer types byte short int long and BigInte ger d The result is formatted as a decimal integer
390. tion If the condition is false the program is terminated Otherwise the program continues normally Assertions are not available in Java 1 3 but an assertion facility similar to the C C version has been added to the language as of Java 1 4 Even in versions of Java before 1 4 you can do something similar to assertions You can test the condition using an if statement and throw an exception if the condition does not hold if condition false throw new IllegalArgumentException Assertion Failed Of course you could use a better error message And it would be better style to define a new exception class instead of using the standard class IllegalArgumentException This sort of test is most useful during testing and debugging of the program Once you are sure that the program is correct the test in the if statement might be seen as a waste of the computer s time One advantage of assertions in C and C is that they can be turned off at compile time That is if the program is compiled in one way then the assertions are included in the compiled code If the program is compiled in another way the assertions are not included During debugging the first type of compilation is used The release version of the program is compiled with assertions turned off The release version will be more efficient because the computer won t have to evaluate all the assertions The nice part is that the source code doesn t have to be modified to
391. tion and handle it For example try double determinant M 0 0 M 1 1 M 0O 1 M 1 O System out println The determinant of M is determinant catch ArrayIndexOutOfBoundsException e System out println M is the wrong size to have a determinant The computer tries to execute the block of statements following the word try If no exception occurs during the execution of this block then the catch part of the statement is simply ignored However if an ArrayIndexOutOfBoundsException oc curs then the computer jumps immediately to the block of statements labeled catch ArrayIndexOutOfBoundsException e This block of statements is said to be an ex ception handler for ArrayIndexOutOfBoundsException By handling the exception in this way you prevent it from crashing the program You might notice that there is another possible source of error in this try state ment If the value of the variable M is null then a NullPointerException will be thrown when the attempt is made to reference the array In the above try statement NullPointerExceptions are not caught so they will be processed in the ordinary way by terminating the program unless the exception is handled elsewhere You could catch Nul1lPointerExceptions by adding another catch clause to the try statement 218 CHAPTER 10 CORRECTNESS AND ROBUSTNESS try double determinant M 0 0 M 1 1 M 0 1 M 1 O System out println
392. tionEvent class In particular evt getActionCommand returns a String giving the command associated with the button By default this command is the text that is displayed on the button The method evt getSource returns a reference to the Object that produced the event that is to the JButton that was pressed The return value is of type Object not JButton because other types of components can also produce ActionEvents e Component methods There are several useful methods in the JButton class For example stopGoButton setText Stop changes the text displayed on the but ton to Stop And stopGoButton setActionCommand sgb changes the action command associated to this button for action events JButtons also have all the general Component methods such as setEnabled and setFont The setEnabled and setText methods of a button are particularly useful for giving the user information about what is going on in the program A disabled button is better than a button that gives an obnoxious error message such as Sorry you can t click on me now By the way it s possible for a JButton to display an icon instead of or in addition to the text that it displays An icon is simply a small image Several other components can also display icons However I will not cover this aspect of Swing in this book Consult a Java reference if you are interested 8 2 2 The JLabel Class JLabels are certainly the s
393. to keep the naming consistent These methods are apparently defined to make it easy to use a LinkedList as if it were a stack or a queue See Section11 3 For example we can use a LinkedList as a queue by adding items onto one end of the list using the addLast method and removing them from the other end using the removeFirst method If list is an object of type List then the method list iterator defined in the Collection interface returns an Iterator that can be used to traverse the list from beginning to end However for Lists there is a special type of Iterator called a ListIterator which offers additional capabilities The method list listIterator returns a ListIterator for list 9 4 LIST CLASSES 199 A ListIterator has the usual Iterator methods hasNext and next but it also has methods hasPrevious and previous that make it possible to move backwards in the list To understand how these work its best to think of an iterator as pointing to a position between two list elements or at the beginning or end of the list In this diagram the items in a list are represented by squares and arrows indicate the possible positions of an iterator Pee If iter is a ListIterator iter next moves the iterator one space to the right along the list and returns the item that the iterator passes as it moves The method iter previous moves the iterator one space to the left along the list and returns the item that it pa
394. to key board input When the applet has the input focus it should be hilited with a colored border and the dice should be rolled whenever the user presses a key on the key board This is in addition to rolling them when the user clicks the mouse on the applet 3 In a previous exercise you wrote a pair of dice applet where the dice are rolled when the clicks on the applet Now make a pair of dice applet that uses the methods discussed Draw the dice on a JPanel and place a Roll button at the bottom of the applet The dice should be rolled when the user clicks the Roll button 4 In Exercise 3 5 you drew a checkerboard For this exercise write a checkerboard applet where the user can select a square by clicking on it Hilite the selected square by drawing a colored border around it When the applet is first created no square is selected When the user clicks on a square that is not currently selected it be comes selected If the user clicks the square that is selected it becomes unselected Assume that the size of the applet is 160 by 160 pixels so that each square on the checkerboard is 20 by 20 pixels 5 8 PROGRAMMING EXERCISES 125 5 Write an applet that shows two squares The user should be able to drag either square with the mouse You ll need an instance variable to remember which square the user is dragging The user can drag the square off the applet if she wants if she does this it s gone 6 For this exercise y
395. to the object that contains the method One common use of this is in constructors For example public class Student private String name Name of the student public Student String name Constructor Create a student with specified name this name name More variables and methods In the constructor the instance variable called name is hidden by a formal parameter However the instance variable can still be referred to by its full name this name In the assignment statement the value of the formal parameter name is assigned to the instance variable this name This is considered to be acceptable style There is no need to dream up cute new names for formal parameters that are just used to initialize instance variables You can use the same name for the parameter as for the instance variable There are other uses for this Sometimes when you are writing an instance method you need to pass the object that contains the method to a subroutine as an actual param eter In that case you can use this as the actual parameter For example if you wanted to print out a string representation of the object you could say System out println this Or you could assign the value of this to another variable in an assignment statement In fact you can do anything with this that you could do with any other variable except change its value Java also defines another special variable named super for use in the d
396. tory This is true since is a special path name that refers to the current directory at least on Windows and UNIX systems Constructing a JFileChooser object does not make the dialog box appear on the screen You have to call a methods in the object to do that There are two different methods that can be used because there are two types of file dialog An open file dialog allows the user to specify an existing file to be opened for reading data into the program a save file dialog lets the user specify a file which might or might not already exist to be opened for writing data from the program File dialogs of these two types are opened using the showOpenDialog and showSaveDialog methods A file dialog box always has a parent another component which is associated with the dialog box The parent is specified as a parameter to the showOpenDialog or showSaveDialog methods The parent is a GUI component and can usually be specified as this The parameter can be nu11 in which case an invisible component is used as the parent Both showOpenDialog and showSaveDialog have a return value which will be one of the con stants JFileChooser CANCEL_OPTION JFileChooser ERROR_OPTION or JFileChooser APPROVE_OPTION If the return value is JFileChooser APPROVE_OPTION then the user has selected a file If the return value is something else then the user did not select a file The user might have clicked a Cancel button for example You shoul
397. ts that are added to it in a row That s exactly the behavior we want here so there is nothing more to do If we wanted a different kind of layout it s possible to change the panel s layout manager One more step is required to make the button useful an object must be registered with the button to listen for ActionEvents The button will generate an ActionEvent when the user clicks on it ActionEvents are similar to MouseEvents or KeyEvents To use them a class should import java awt event The object that is to do the listening must implement an interface named ActionListener This interface requires a definition for the method public void actionPerformed ActionEvent evt Finally the listener must be registered with the button by calling the button s addActionListener method In this case the applet itself will act as listener and the registration is done with the command redButton addActionListener this After doing the same three commands for each of the other two buttons and setting the background color for the sake of aesthetics the buttonBar panel is ready to use It just has to be added to the applet As we have seen components are not added directly to an applet Instead they are added to the applet s content pane which is itself a container The content pane comes with a default layout manager that is capable of displaying up to five components Four of these components are placed along the edges of
398. tually another menu This is called a sub menu When you select this item the sub menu will appear Select an item from the sub menu to set the background color of the drawing area This applet also demonstrates a pop up menu The pop up menu appears when you click on one of the shapes in just the right way The exact action you have to take depends on the look and feel and is called the pop up trigger The pop up trigger is probably either clicking with the right mouse button clicking with the middle mouse button or clicking while holding down the Control key The pop up menu in this example contains commands for editing the shape on which you clicked In the rest of this section we ll look at how all this can be programmed If you would like to see the complete source code of the applet you will find it in the file Shape DrawWithMenus java The source code is just over 600 lines long The menus are created and configured in a very long init method 8 4 1 Menu Bars and Menus A menu bar is just an object that belongs to the class JMenuBar Since JMenuBar is a subclass of JComponent a menu bar could actually be used anywhere in any container In practice though a JMenuBar is generally added to a top level container such as a JApplet This can be done in the init method of a JApplet using commands of the form JMenuBar menubar new JMenuBar setMenuBar menubar 8 5 TIMERS ANIMATION AND THREADS 169 The applet s setMenuBar
399. tudent objects provides a name for the Student and assigns the student a unique ID number name theName nextUniquelID ID nextUniqueID public String getName Accessor method for reading value of private instance variable name return name public int getID Accessor method for reading value of ID return ID 46 CHAPTER 2 OBJECT BASED PROGRAMMING public double getAverage Compute average test grade return testi test2 test3 3 end of class Student The initialization nextUniqueID 0 is done only once when the class is first loaded Whenever a Student object is constructed and nextUniqueID is ex ecuted it s always the same static member variable that is being incremented When the very first Student object is created nextUniqueID becomes 1 When the second object is created nextUniqueID becomes 2 After the third object it becomes 3 And so on The constructor stores the new value of nextUniquelID in the ID variable of the object that is being created Of course ID is an instance variable so every object has its own individual ID variable The class is constructed so that each student will automatically get a different value for its ID variable Furthermore the ID variable is private so there is no way for this variable to be tampered with after the object has been created You are guaranteed just by the way the class is designed that every st
400. ty lt y Card height public void addCard Card aCard pile add aCard public void draw Graphics g if empty g drawRect x y Card width Card height i else topCard draw g x y public abstract boolean canTake Card aCard public abstract void select Notice that this class is abstract It has three protected attributes What does protected mean The x and y are coordinates of this pile on some drawing surface and the pile attribute is Collection of Cards Most of the methods are self explanatory The includes method is given a point a coordinate and returns true if this point is contained within the space occupied by the cards in the pile We intend to use this method to tell us if the user has clicked on this particular pile of cards The idea is to get the coordinates of the point the user has clicked on and then ask each pile if this coordinate falls within the space it occupies 147 The canTake abstract method should tell us whether a particular pile of cards can accept a card Different piles will have different criteria for accepting a Card For example suit piles will accept a card if it is the same suit as all others in the pile and if its rank is one more that its topCard The table piles will accept a card if its suit is opposite in color and its rank is one less than the pile s topCard The select abstract method is the action this pile takes if it can accept a Card Usually
401. ually created by an operator called new which creates an object and returns a reference to that object For example assuming that std is a variable of type Student declared as above the assignment statement std new Student would create a new object which is an instance of the class Student and it would store a reference to that object in the variable std The value of the variable is a reference to the object not the object itself It is not quite true then to say that the object is the value of the variable std though sometimes it is hard to avoid using this terminology It is certainly not at all true to say that the object is stored in the variable std The proper terminology is that the variable std refers to the object and I will try to stick to that terminology as much as possible So suppose that the variable std refers to an object belonging to the class Student That object has instance variables name test1 test2 and test3 These instance vari ables can be referred to as std name std testi std test2 and std test3 This follows the usual naming convention that when B is part of A then the full name of B is A B For example a program might include the lines System out println Hello std name Your test grades are System out println std test1 System out println std test2 System out println std test3 This would output the name and test grades from the object to which std ref
402. udent object will have its own permanent unique identification number Which is kind of cool if you think about it 2 7 Summary Classes define types Objects are concrete instances of class and there may be multiple instances of a class Objects store data in fields or instance variables initialize this data with constructors and manipulate this data with methods The current value of the fields is the state of the object A variable of class type stores either a reference to an object or the value null Constructors and Methods A constructor sets up an object at creation For example Prof wayne new Prof A constructor can have arguments that are used to initialize the instance variables Prof wayne new Prof goddard A constructor must NOT have a type public class Prof String name Prof String n name n When a class is created instance variables are initialized to default values 0 for int null for objects etc Nevertheless you should explicitly set primitive variables A method has a header with formal parameters and a body Its signature is the types of its parameters and its own type The return type void means that the method returns nothing A method is called with the notation For example 2 7 SUMMARY 47 object method Most objects have accessor methods these allow the user to get data from the object Mutator methods allow the user to set data in the object A method should normall
403. ues to streams in internal binary number format DataOutputStream bears the same relationship to OutputStream that PrintWriter bears to Writer That is whereas OutputStream only has methods for outputting bytes DataQutputStream has methods writeDouble double x for outputting values of type double writeInt int x for out putting values of type int and so on Furthermore you can wrap any OutputStream in a DataOutputStream so that you can use the higher level output methods on it For example if byteSink is of type OutputStream you could say DataQutputStream dataSink new DataQutputStream byteSink to wrap byteSink in a DataOutputStream dataSink For machine readable data such as that created by writing to a DataQutputStrean java io provides the class DataInputStream You can wrap any InputStream opbject in a DataInputStream object to provide it with the ability to read data of various types from the byte stream The methods in the DataInputStream for reading binary data are called readDouble readInt and so on Data written by a DataOutputStream is guaranteed to be in a format that can be read by a DataInputStream This is true even if the data stream is created on one type of computer and read on another type of com puter The cross platform compatibility of binary data is a major aspect of Java s platform independence The classes PrintWriter DataInputStream and DataOutputStream allow you to eas ily input and output all of J
404. uestions How are components added to a container How are their sizes and positions controlled 151 152 CHAPTER 8 ADVANCED GUI PROGRAMMING Three Panels shown in color containing six additional Components shown in gray The sizes and positions of the components in a container are usually controlled by a layout manager Different layout managers implement different ways of arranging com ponents There are several predefined layout manager classes including FlowLayout GridLayout BorderLayout BoxLayout CardLayout and GridBagLayout All these classes are defined in the package java awt It is also possible to define new layout managers if none of these suit your purpose Every container is assigned a default layout manager when it is first created For JPanels the default layout manager belongs to the class FlowLayout The content pane of a JApplet uses a BorderLayout by default You can change the layout manager of a container using its setLayout method As for adding components to a container that s easy You just use one of the con tainer s add methods There are several add methods Which one you should use depends on what type of LayoutManager is being used by the container so I will discuss the appropriate add methods as I go along 8 1 1 BorderLayout A BorderLayout places one component in the center of a container This central component is surrounded by up to four other components that border it to the N
405. uld say java awt Color rectColor to declare a variable named rectColor whose type is java awt Color Of course this can get tiresome so Java makes it possible to avoid using the full names of classes If you put import java awt Color at the beginning of a Java source code file then in the rest of the file you can abbrevi 20 CHAPTER 1 DESIGN I CLASS LEVEL DESIGN Graphics drawRectt setColort Subroutines nested in classes nested in two layers of packages The full name of sqrt Q is java lang Math sgrt ate the full name java awt Color to just the name of the class Color This would allow you to say just Color rectColor to declare the variable rectColor The only effect of the import statement is to allow you to use simple class names instead of full package class names you aren t really importing anything substantial If you leave out the import statement you can still access the class you just have to use its full name There is a shortcut for importing all the classes from a given package You can import all the classes from java awt by saying import java awt and you can import all the classes from javax swing with the line import javax swing In fact any Java program that uses a graphical user interface is likely to begin with one or both of these lines A program might also include lines such as import java net or import java io to get easy access to networking and inp
406. um Literals Like datatypes they can not have relationships to classes but classes can have relationships to them e Packages represent a namespace in a programming language In a diagram they are used to represent parts of a system which contain more than one class maybe hundereds of classes Sequence Diagrams Sequence Diagrams show the message exchange i e method call between several Ob jects in a specific time delimited situation Objects are instances of classes Sequence Diagrams put special emphasis in the order and the times in which the messages to the objects are sent In Sequence Diagrams objects are represented through vertical dashed lines with the name of the Object on the top The time axis is also vertical increasing downwards so that messages are sent from one Object to another in the form of arrows with the operation and parameters name Messages can be either synchronous the normal type of message call where control is passed to the called object until that method has finished running or asynchronous where control is passed back directly to the calling object Synchronous messages have a vertical box on the side of the called object to show the flow of program control Collaboration Diagrams Collaboration Diagrams show the interactions occurring between the objects participating in a specific situation This is more or less the same information shown by Sequence Diagrams but there the emphasis is put on how the inte
407. um are in the valid range of row and column numbers Postcondition The rectangle in the specified row and column has been changed to a random color int red int 256 Math random choose random levels in range int green int 256 Math random 0 to 255 for red green int blue int 256 Math random and blue color components Mosaic setColor rowNum colNum red green blue end of changeToRandomColor static void randomMove Precondition The global variables currentRow and currentColumn specify a valid position in the grid Postcondition currentRow or currentColumn is changed to one of the neighboring positions in the grid up down left or right from the previous position If this moves the position outside the grid then it is moved to the opposite edge of the window int directionNum Randomly set to 0 1 2 or 3 to choose direction directionNum int 4 Math random switch directionNum case 0 move up currentRow if currentRow lt 0 currentRow 9 break case 1 move right currentColumn t if currentColumn gt 20 currentColumn 0 break case 2 move down currentRow if currentRow gt 10 currentRow 0 break case 3 currentColumn if currentColumn lt 0 currentColumn 19 break end of randomMove end of class RandomMosaicWalk 18 CHAPTER 1 DESIGN I CLASS LEVEL DESIGN 1 2
408. und Color gray Create the input boxes and make sure that their background color is white They are likely to be white by default xInput new JTextField 0 xInput setBackground Color white yInput new JTextField 0 yInput setBackground Color white Create panels to hold the input boxes and labels x and y By using a BorderLayout with the JTextField in the Center position the JTextField will take up all the space left after the label is given its preferred size JPanel xPanel new JPanel xPanel setLayout new BorderLayout xPanel add new Label x BorderLayout WEST xPanel add xInput BorderLayout CENTER JPanel yPanel new JPanel yPanel setLayout new BorderLayout yPanel add new Label y BorderLayout WEST yPanel add yInput BorderLayout CENTER Create a panel to hold the four buttons for the four operations A GridLayout is used so that the buttons will all have the same size and will fill the panel The applet serves as ActionListener for the buttons JPanel buttonPanel new JPanel buttonPanel setLayout new GridLayout 1 4 8 3 PROGRAMMING WITH COMPONENTS 165 JButton plus new JButton plus addActionListener this buttonPanel add plus JButton minus new JButton minus addActionListener this buttonPanel add minus JButton times new JButton times addActionListener this buttonPanel add times
409. unter them several times in the next two chapters For now we 4 2 NESTED CLASSES 79 will look at one not very plausible example Consider the Drawable interface which is defined earlier Suppose that we want a Drawable object that draws a filled red 100 pixel square Rather than defining a separate class and then using that class to create the object we can use an anonymous class to create the object in one statement Drawable redSquare new Drawable void draw Graphics g g setColor Color red g fillRect 10 10 100 100 3 The semicolon at the end of this statement is not part of the class definition It s the semicolon that is required at the end of every declaration statement When a Java class is compiled each anonymous nested class will produce a separate class file If the name of the main class is MainClass for example then the names of the class files for the anonymous nested classes will be MainClass 1 class MainClass 2 class MainClass 3 class and so on 80 CHAPTER 4 OBJECT ORIENTED PROGRAMMING II Chapter 5 Applets HTML and GUI s JAVA IS A PROGRAMMING LANGUAGE DESIGNED for networked computers and the World Wide Web Java applets are downloaded over a network to appear on a Web page Part of learning Java is learning to program applets and other Graphical User Interface programs GUI programs are event driven That is user actions such as clicking on a button or pressing a key on the keyboard
410. ups At most one radio button in a group can be selected at any given time In Java a radio button is represented by an object of type JRadioButton When used in isolation a JRadioButton acts just like a JCheckBox and it has the same methods and events Ordinarily however a JRadioButton is used in a group A group of radio buttons is represented by an object belonging to the class ButtonGroup A ButtonGroup is not a component and does not itself have a visible representation on the screen A ButtonGroup works behind the scenes to organize a group of radio buttons so that at most one button in the group can be selected at any given time To use a group of radio buttons you must create a JRadioButton object for each button in the group and you must create one object of type ButtonGroup to organize the individual buttons into a group Each JRadioButton must be added individually to some container so that it will appear on the screen A ButtonGroup plays no role in the placement of the buttons on the screen Each JRadioButton must also be added to the ButtonGroup which has an add method for this purpose If you want one of the buttons to be selected at start up you can call setSelected true for that button If you don t do this then none of the buttons will be selected until the user clicks on one of them As an example here is how you could set up a set of radio buttons that can be used to select a color JRadioButton redRadio blueRadi
411. ust be between 1 and 13 Suit must be between 0 and 3 If the parameters are outside these ranges the constructed card object will be invalid value theValue suit theSuit public int getSuit Return the int that codes for this card s suit return suit public int getValue Return the int that codes for this card s value return value public String getSuitAsString Return a String representing the card s suit If the card s suit is invalid is returned switch suit case SPADES return Spades case HEARTS return Hearts 2 5 MORE ABOUT ACCESS MODIFIERS 43 case DIAMONDS return Diamonds case CLUBS default return Clubs return public String getValueAsString Return a String representing the card s value If the card s value is invalid is returned switch value case 1 return Ace case 2 return 2 case 3 return 3 case 4 return 4 case 5 return 5 case 6 return 6 case 7 return 7 case 8 return 8 case 9 return 9 case 10 return 10 case 11 return Jack case 12 return Queen case 13 return King default return public String toString Return a String representation of this card such as 10 of Hearts or Queen of Spades return getValueAsString of getSuitAsString end class Card 2 5 More about Access Modifiers A class can be declared to be pu
412. ut output classes When you start importing lots of packages in this way you have to be careful about one thing It s possible for two classes that are in different packages to have the same name For example both the java awt package and the java util package contain classes named List If you import both java awt and java util the simple name List will be ambiguous If you try to declare a variable of type List you will get a compiler error message about an ambiguous class name The solution is simple use the full name of the class either java awt List or java util List Another solution is to use import to import the individual classes you need instead of importing entire packages Because the package java lang is so fundamental all the classes in java lang are automatically imported into every program It s as if every program began with the state ment import java lang This is why we have been able to use the class name String instead of java lang String and Math sqrt instead of java lang Math sqrt 1 3 JAVADOC 21 It would still however be perfectly legal to use the longer forms of the names Programmers can create new packages Suppose that you want some classes that you are writing to be in a package named utilities Then the source code file that defines those classes must begin with the line package utilities Any program that uses the classes should include the directive import utilities
413. ut println too large for your computer break N 3 N 4 System out printl1n N The problem here is not that the original algorithm for computing 3N 1 sequences was wrong The problem is that it just cant be correctly implemented using 32 bit integers Many programs ignore this type of problem But integer overflow errors have been re sponsible for their share of serious computer failures and a completely robust program should take the possibility of integer overflow into account The infamous Y2K bug was in fact just this sort of error For numbers of type double there are even more problems There are still overflow errors which occur when the result of a computation is outside the range of values that can be represented as a value of type double This range extends up to about 1 7 times 10 to the power 308 Numbers beyond this range do not wrap around to negative values Instead they are represented by special values that have no numerical equiva lent The values Double POSITIVE_INFINITY and Double NEGATIVE_INFINITY represent numbers outside the range of legal values For example 20 1e308 is computed to be Double POSITIVE_INFINITY Another special value of type double Double NaN repre sents an illegal or undefined result NaN stands for Not a Number For example the result of dividing by zero or taking the square root of a negative number is Double NaN You can test whether a number x i
414. va 2 000 9 2 An Examples Atray Lists s secta eek ea ee ee eS Hi Ee eS 9 3 GOMCCHONS sii 6 se Gee a Se aca a Be ee el eS ay a a ee T 9 3 1 Generic Algorithms and Iterators ace ee SS eS 9 3 2 Equality and Comparison aaa e 933 97 Wrapper Classes s Glas a a i hae once wick AADA Way a hows 9 4 Eist GCIASSOGY acina gece a EMR p a a a Renae era ae Ree OE ls 9 4 1 The ArrayList and LinkedList Classes o oo IA 2 SOAS ie oe eke Methane ae Se NA et Buds Soe A dee Mis Beg 9 4 3 The TreeSet and HashSet Classes 2204 9 5 QUZ Q estions iee fi eon he Se PG we Bark 2 A RAR wie BG Bk 9 6 Programming Exercises 2 2 eee ee 10 Correctness and Robustness 10 1 Introduction to Correctness and Robustness 10 2 Writing Correct Programs 0 00 eee ee eee 10 3 Exceptions and the try catch Statement yho do 4 iH RO RS 10 3 1 Mandatory Exception Handling 10 4 Programming with Exceptions 0 0 000 10 4 1 Writing New Exception Classes 0 10 4 2 Exceptions in Subroutines and Classes 1O4 3 ASSErONS e dhe ard Aeeldegp nas a Coop a Glth we etree bd Soe 10 5 Quiz Questions sri i hatnia ad BUS ee Ge er ee Go ace aoa doe Pas 10 6 Programming Exercises 2 a 11 Basic Input Output 11 1 Input and Ouput in Java 20 ois ee ayete Gee Ba ee gl SB Be ae ee Re 11 1 1 The Scanner Class
415. va 11 1 1 The Scanner Class System is the name of the built in class that contains methods and objects used to get input from the keyboard print text on the screen and do file input output 1 0 System out is the name of the object we use to to display text on the screen When you invoke print and printi1n you invoke them on the object named System out Java 5 0 introduced a simplified reader class the Scanner class which accesses the input or a file and does limited conversion for the user For example to read input from the keyboard one usually connects to the input stream the object System in reads a line as a string at a time and breaks the string into tokens parts separated by some delimiter usually whitespace and then convert the tokens to various number formats With the Scanner class we construct a Scanner object with the stream System in as input The resulting tokens may then be converted into values of different types using the various next methods By default the scanner uses whitespace as the delimiter Whitespace may be blank spaces tabs or newlines The scanner can also use delimiters other than whitespace The Scanner class provides various next methods One for each primitive type Some of these are listed below 231 232 CHAPTER 11 BASIC INPUT OUTPUT e next returns the next token as a string e nextInt returns the next integer e nextDouble returns the next double e nextLong returns the
416. value in its declaration just like any other variable For example consider a class named PairOfDice An object of this class will represent a pair of dice It will contain two instance variables to represent the numbers showing on the dice and an instance method for rolling the dice 30 CHAPTER 2 OBJECT BASED PROGRAMMING public class PairOfDice public int diel 3 Number showing on the first die public int die2 4 Number showing on the second die public void roll Roll the dice by setting each of the dice to be a random number between 1 and 6 diel int Math random 6 1 die2 int Math random 6 1 end class PairOfDice The instance variables die1 and die2 are initialized to the values 3 and 4 respec tively These initializations are executed whenever a PairOfDice object is constructed It s important to understand when and how this happens There can be many PairOfDice objects Each time one is created it gets its own instance variables and the assignments diel 3 and die2 4 are executed to fill in the values of those variables To make this clearer consider a variation of the PairOfDice class public class PairOfDice public int die1 int Math random 6 1 public int die2 int Math random 6 1 public void roll diel int Math random 6 1 die2 int Math random 6 1 end class PairOfDice Here the dice are initialized to
417. vax swing public class RandomStrings extends JApplet String message The message to be displayed This can be set in an applet param with name message If no value is provided in the applet tag then the string Java is used as the default Font fontt font2 font3 font4 font5 The five fonts Display drawingSurface This is the component on which the drawing will actually be done It is defined by a nested class that can be found below public void init Called by the system to initialize the applet message getParameter message if message null message Java font1 new Font Serif Font BOLD 14 font2 new Font SansSerif Font BOLD Font ITALIC 24 font3 new Font Monospaced Font PLAIN 20 font4 new Font Dialog Font PLAIN 30 font5 new Font Serif Font ITALIC 36 drawingSurface new Display Create the drawing surface drawingSurface setBackground Color black setContentPane drawingSurface Since drawingSurface will fill the entire applet we simply replace the applet s content pane with drawingSurface end init 100 CHAPTER 5 APPLETS HTML AND GUI S class Display extends JPanel This nested class defines a JPanel used for displaying the content of the applet An object of this class is used as the content pane of the applet Note that since this is a nested non static class it has acc
418. ve horizontal gaps of H pixels between columns and vertical gaps of V pixels between rows use new GridLayout R C H V instead When you use a GridLayout it s probably good form to add just enough components to fill the grid However this is not required In fact as long as you specify a non zero value for the number of rows then the number of columns is essentially ignored The system will use just as many columns as are necessary to hold all the components that you add to the container If you want to depend on this behavior you should probably specify zero as the number of columns You can also specify the number of rows as zero In that case you must give a non zero number of columns The system will use the specified number of columns with just as many rows as necessary to hold the components that are added to the container Horizontal grids with a single row and vertical grids with a single column are very common For example suppose that button1 button2 and button3 are buttons and that you d like to display them in a horizontal row in a panel If you use a horizontal grid for the panel then the buttons will completely fill that panel and will all be the same size The panel can be created as follows JPanel buttonBar new JPanel buttonBar setLayout new GridLayout 1 3 Note The 3 here is pretty much ignored and you could also say new GridLayout 1 0 To leave gaps between the buttons you could use
419. ven time every CPU can be executing a different thread However the computer uses time sharing to give the illusion that several threads are being executed at the same time in parallel Time sharing means that the CPU executes one thread for a while then switches to another thread then to another and then back to the first thread typically about 100 times per second As far as users are concerned the threads might as well be running at the same time To say that Java is a multi threaded language means that a Java program can create one or more threads which will then run in parallel with the program This is a funda mental built in part of the language not an option or add on like it is in some languages Still programming with threads can be tricky and should be avoided unless it is really necessary Ideally even then you can avoid using threads directly by using well tested classes and libraries that someone has written for you 8 5 1 Animation In Swing One of the places where threads are used in GUI programming is to do animation An animation is just a sequence of still images displayed on the screen one after the other If the images are displayed quickly enough and if the changes from one image to the next are small enough then the viewer will perceive continuous motion To program an animation you need some way of displaying a sequence of images A GUI program already has at least one thread an event handling threa
420. vents associated with a component object comp the listener must be registered with the component by calling comp addMouseListener mListener The addMouseListener method is an instance method in the class Component and so can be used with any GUI component object In our first few examples we will listen for events on a JPanel that is being used as the drawing surface of a JApplet The event classes such as MouseEvent and the listener interfaces such as MouseListener are defined in the package java awt event This means that if you want to work with events you should include the line import java awt event at the beginning of your source code file Admittedly there is a large number of details to tend to when you want to use events To summarize you must 1 Put the import specification import java awt event at the beginning of your source code 2 Declare that some class implements the appropriate listener interface such as MouseListener 3 Provide definitions in that class for the subroutines from the interface 4 Register the listener object with the component that will generate the events by calling a method such as addMouseListener in the component Any object can act as an event listener provided that it implements the appropriate interface A component can listen for the events that it itself generates An applet can listen for events from components that are contained in the applet A spec
421. void roll Roll the dice by setting each of the dice to be a random number between 1 and 6 die1 int Math random 6 1 die2 int Math random 6 1 end class PairOfDice Now we have the option of constructing a PairOfDice object either with new PairOfDice or with new PairOfDice x y where x and y are int valued expressions This class once it is written can be used in any program that needs to work with one or more pairs of dice None of those programs will ever have to use the obscure incantation int Math random 6 1 because it s done inside the PairOfDice class And the programmer having once gotten the dice rolling thing straight will never have to worry about it again Here for example is a main program that uses the PairOfDice class to count how many times two pairs of dice are rolled before the two pairs come up showing the same value 2 2 CONSTRUCTORS AND OBJECT INITIALIZATION 33 public class RollTwoPairs public static void main String args PairOfDice firstDice Refers to the first pair of dice firstDice new PairOfDice PairOfDice secondDice Refers to the second pair of dice secondDice new PairOfDice int countRolls Counts how many times the two pairs of dice have been rolled int total1 Total showing on first pair of dice int total2 Total showing on second pair of dice countRolls 0 do Roll the two pairs of di
422. when an actual object reference is required A RuntimeException generally indicates a bug in the pro gram which the programmer should fix RuntimeExceptions and Errors share the prop 10 3 EXCEPTIONS AND THE TRY CATCH STATEMENT 217 erty that a program can simply ignore the possibility that they might occur Ignoring here means that you are content to let your program crash if the exception occurs For ex ample a program does this every time it uses an array reference like A i without making arrangements to catch a possible ArrayIndexOutOfBoundsException For all other excep tion classes besides Error RuntimeException and their subclasses exception handling is mandatory in a sense that I ll discuss below The following diagram is a class hierarchy showing the class Throwable and just a few of its subclasses Classes that require mandatory exception handling are shown in red Throwable InterruptedException EOFException ArraylndexOutOfBoundsException egalArgumentException NumberFormatException Figure 10 1 HelloWorldApplet2 IOException SocketException RuntimeException The class Throwable and some of its subclasses To catch exceptions in a Java program you need a try statement The idea is that you tell the computer to try to execute some commands If it succeeds all well and good But if an exception is thrown during the execution of those commands you can catch the excep
423. window or even to call System exit and end the program entirely It is even possible as we will see below to set up a listener to listen for the event and to program any response you want You can set the default response to a click in a frame s close box by calling void setDefaultCloseOperation int operation where the parameter operation is one of the constants e JFrame HIDE_ON_CLOSE just hide the window so it can be opened again e JFrame DISPOSE_ON_CLOSE destroy the window but don t end the program e JFrame EXIT_ON_CLOSE terminate the Java interpretor by calling System exit e JFrame DO_NOTHING_ON_CLOSE no automatic response your program is responsible for closing the window 118 CHAPTER 5 APPLETS HTML AND GUI S If a frame is opened by a main program and if the program has only one window it might make sense to use EXIT_ON_CLOSE However note that this option is illegal for a frame that is created by an applet since an applet is not allowed to shut down the Java interpreter In case you are curious here are the lines that I added to the end of the ShapeDrawFrame constructor setDefaultClose0peration EXIT_ON_CLOSE setLocation 20 50 setSize 550 420 show I also added a main routine to the class This means that it is possible to run ShapeDrawFrame as a stand alone application In a typical command line environment you would do this with the command java ShapeDrawFrame It has
424. x y we get a kind of picture of the set For a point in the Mandelbrot set the count always reaches the maximum since for such points zx zy never moves far away from zero For other points in general the closer the point is to the Mandelbrot set the more steps it will take With all that said here is an applet that computes a picture of the Mandelbrot set It will begin its computation when you press the Start button Eventually the color of every pixel in the applet will be computed but the applet actually computes the colors progressively filling the applet with smaller and smaller blocks of color until it gets down to the single pixel level The applet represents a region of the plane with 1 25 lt x lt 1 0 and 1 25 lt y lt 1 25 The Mandelbrot set is colored purple Points outside the set have other colors Try it Applet Reference see Applet Mandelbrot 8 5 TIMERS ANIMATION AND THREADS 177 Figure 8 6 Mandelbrot The algorithm for computing the colors in this applet is For square sizes 64 32 16 8 4 2 and 1 For each square in the applet Let a b be the pixel coords of the center of the square Let x y be the real numbers corresponding to a b Let 2x zy x y Let count 0 Repeat until count is 80 or zx zy is big Let new_zx ZX ZX zy zy x Let zy 2 zx zy y Let zx new_zx Let count count 1 Let color Color getHSBColor count 100 0F 0 0F 0 OF Fill t
425. xamples data dat Assuming that Hard Drive is the name of a disk drive this would be an absolute path name on a computer using Macintosh OS9 e examples data dat a relative path name under UNIX Examples is the name of a directory that is contained within the current directory and data data is a file in that directory The corresponding relative path names for Windows and Macintosh would be examples data dat and examples data dat Similarly the rules for determining which directory is the current directory are differ ent for different types of computers It s reasonably safe to say though that if you stick to using simple file names only and if the files are stored in the same directory with the program that will use them then you will be OK To avoid some of the problems caused by differences between platforms Java has the class java io File An object belonging to this class represents a file More precisely an object of type File represents a file name rather than a file as such The file to which the name refers might or might not exist Directories are treated in the same way as files so a File object can represent a directory just as easily as it can represent a file A File object has a constructor new File String that creates a File object from a path name The name can be a simple name a relative path or an absolute path For example new File data dat creates a File object that refers to a file
426. xtArea String contents int lines int columns The parameter lines specifies how many lines of text should be visible in the text area This determines the preferred height of the text area The text area can actually contain any number of lines the text area can be scrolled to reveal lines that are not currently visible It is common to use a JTextArea as the Center component of a BorderLayout In that case it isn t useful to specify the number of lines and columns since the TextArea will expand to fill all the space available in the center area of the container The JTextArea class adds a few useful procedures to those inherited from JTextComponent 162 CHAPTER 8 ADVANCED GUI PROGRAMMING void append String text add the specified text at the end of the current contents line breaks can be inserted by using the special character n void insert String text int pos insert the text starting at specified position void replaceRange String text int start int end delete the text from position start to position end and then insert the specified text in its place void setLineWrap boolean wrap If wrap is true then a line that is too long to be displayed in the text area will be wrapped onto the next line The default value is false A JTextField generates an ActionEvent when the user presses return while typing in the JTextField The JTextField class includes an addActionListener method that can b
427. y check its arguments It notifies the caller of a problem by using an exception or a special return value The programmer should try to avoid exceptions consider error recovery and avoidance Access Modifiers The fields of an object are accessible by all the methods of that object A variable has a scope where it is accessible from and a lifetime when it exists The variables defined in a method are called local variables and are accessible only within the method and exist only while the method is being executed an exception is static variables whose lifetime is the program s execution they are always available A method or field can be made private public or protected private means it is accessible by no other classe and public that it is accessible by all other classes We will discuss protected later Note that within the code for the class one can access all fields of all objects of that type even if they are private For example if one created one s own version of an integer one could write public class MyInteger private int val public boolean isGreaterThan MyInteger other return this val gt other val The this refers to the current object in this case it could be omitted return val gt other val Constants are usually uppercase for example public final static int MAX_SIZE 100 There are also static methods Consider for example a Coord class that stores a point A static method might be used to
428. y part of a package If a class is not specifically placed in a package then it is put in something called the default package which has no name All the examples that you see in these notes are in the default package 1 3 Javadoc Good programming means extensive comments and documentation At the very least explain the function of each instance variable and for each method explain its purpose parameters returns where applicable You should also strive for a consistent layout and for expressive variable names A program that is well documented is much more valuable than the same program without the documentation Java comes with a tool called javadoc that can make it easier to produce the documentation is a readable and organized format JavaDoc is a program that will automatically extract generate an HTML help page from code that is properly commented In particular it is designed produce a help file that for a class lists the meth ods constructors and public fields and for each method explains what it does together with pre conditions post conditions the meaning of the parameters exceptions that may be thrown and other things Javadoc is especially useful for documenting classes and packages of classes that are meant to be used by other programmers A programmer who wants to use pre written classes shouldn t need to search through the source code to find out how to use them If the documentation in the source code is in the correct
429. ystem For example when an applet appears on a page in a Web browser the system means the Web browser It is up to the browser program to create the applet object and to add it to a Web page The Web browser in turn gets instructions about what is to appear on a given Web page from the source document for that page For an applet to appear on a Web page the source document for that page must specify the name of the applet and its size This specification like the rest of the document is written in a language called HTML Here is some HTML code that instructs a Web browser to display a HelloWorldApplet lt center gt lt applet code HelloWorldApplet class width 200 height 50 gt lt applet gt lt center gt and here is the applet that this code displays Applet Reference see Applet HelloWorldApplet The message is displayed in a rectangle that is 200 pixels in width and 50 pixels in height You shouldn t be able to see the rectangle as such since by default an applet has a background color that is the same as the color of the Web page on which it is displayed This might not actually be the case in your browser The Applet class defines another method that is essential for programming applets the init method This method is called just after the applet object has been created and before it appears on the screen Its purpose is to give the applet a chance to do any necessary initialization Again this met

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