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Object-Oriented Programming - HWS Department of Mathematics

1. x Eile Edit Diagram Code Settings Help aHsae x nG UML Diagrams ja A wX F a Kyjenent commu a 3 poe session map Map i common _ SecurityServer fe g seal canExecute Ses ionContext sessio fA connection cl l e i send Type m void 2 DEG execution ee a Sl ER E Worker T EE worker i receive Type racelya Bice Revevertiveas requests Queue Jro voa execut bss mig overview coiia send Type m void Worker receive data send Type my void sequence dia amp tools receive Type 2 Map H t loadWorker T ransaction job Worker L juse Case View Documentation WorkerManager 4 Ready Figure 3 9 Umbrello UML Modeller showing a Collaboration Diagram 3 3 6 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 cre
2. Generic Programming 8 1 Generic Programming in Java 1 2 ee 8 2 Array Lists amp sisan ec Gurnee age fo HE we Paige ala ee ad ae Gods 8 3 Parameterized Types 2 0 ec 8 4 The Java Collection Framework 00000005 8 5 Iterators and for each Loops 0 000 eee ee ee es 8 6 Equality and Comparison 0 000 eee eee ee ne 8 7 Generics and Wrapper Classes 0 000 eee eens O20 ASUS ete gA A a an ete cae Eier oe gave tir e amp Map Stas SSP attr vee Sy ecg ie ia Correctness and Robustness 9 1 Introduction sanoin oes dy be Seb We eo Rode GASP ee oe a BE Pe Se 9 1 1 Horror Stories o s oiea aas n a E a A e a E A E A Goji 9 12 Java to th Rescue 0 0 cee ee ns 9 1 8 Problems Remain in Java 0 00 eee ne 9 2 Writing Correct Programs 2 0 00 cee ee ee ns 9 2 1 Provably Correct Programs 0 0000 ee uae 9 2 2 Robust Handling of Input 2 0004 9 3 Exceptions andtry catch oaaae 9 3 1 Exceptions and Exception Classes 2008 4 9 3 2 The try Statement 0 000 cee een 9 3 3 Throwing Exceptions 0 0 000 ce eee ee ene 9 3 4 Mandatory Exception Handling 9 3 5 Programming with Exceptions 20008 9A sASSErtIONS suorat hia 0 deed Gs leet de Sh al Gs Be Ge BPS aS 10 Input and Output 10 1 Streams Readers and Writers 000 00004 10 1 1 Cha
3. end class PokerGame If game is a variable of type PokerGame then conceptually game contains its own 97 copy of the Player class In an an instance method of a PokerGame object a new Player object would be created by saying new Player just as for any other class A Player object could be created outside the PokerGame class with an expression such as game newAaPlayer Again however this is very 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 players 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 behave 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 5 2 1 Anonymous Inner Classes In some cases you might find yourself writing an inner 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 inner class An anonymous class is created with a variation of the newoperator that has the form new superclass or interface paramet
4. PairOfDice looks like a method call and that is no accident It is in fact a call to a special type of method called a constructor This might puzzle you since there is no such method in the class definition However every class has at least one con structor If the programmer doesn t write a constructor definition in a class then the system will provide a default constructor for that class 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 method with three differences 1 A constructor does not have any return type not even void 2 The name of the constructor must be the same as the name of the class in which it is defined 3 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 method body of the usual form a block of statements There are no restrictions on what statements can be used And it can have a 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 F
5. 1 You can use ArrayList lt ColoredRect gt anyplace where you could use a normal type to declare variables as the type of a formal parameter in a method or as the return type of a method ArrayList lt ColoredRect gt is not considered to be a separate class from ArrayList An object of type ArrayList lt ColoredRect gt actually belongs to the class ArrayList but the compiler restricts the type of objects that can be added to the list The only drawback to using parameterized types is that the base type cannot be a primitive type For example there is no such thing as ArrayList lt int gt However this is not such a big drawback as it might seem at first because of the wrapper types and autoboxing A wrapper type such as Double or Integer can be used as a base type for a parameterized type An object of type ArrayList lt Double gt can hold objects of type Double Since each object of type Double holds a value of type double it s almost like having a list of doubles If numlist is declared to be of type 171 ArrayList lt Double gt and if x is of type double then the value of x can be added to the list by saying numlist add new Double x Furthermore because of autoboxing the compiler will automatically do double to Double and Double to double type conversions when necessary This means that the compiler will treat numlist add x as begin equivalent to the statement numlist add new Double x
6. 40 2 21 Black Boxes se 225 hee Wn hk OA OOOO Re e Tin 40 2 2 2 Preconditions and Postconditions 41 2 2 3 APIs and Packages srr ady Sc een le ee a Na ee ates 42 2 3 Introduction to Error Handling sssssesssesecsese 46 24 VV AVADOC anara 6S EEE A E AEE S oie Neg sh shaw E N 49 2 5 Creating Jar Files spp eia a E tee ee ee reese ees 51 2 6 Creating Abstractions 0 200 cc eevee cvees 52 2 6 1 Designing theclasses 0 0 00 e eens 52 2 7 Example A Simple Card Game 0eeeee008 58 2 1 Abstraction ABSTRACTION IS A CENTRAL IDEA in computer science and an understanding of this important term is crucial to successful programming Abstraction is the purposeful suppression or hiding of some details of a process or artifact in order to bring out more clearly other aspects details or structures Timothy Budd Heres another definition from wikipedia 1This discussion is based on a wikipedia article on abstraction www wikipedia org 37 In computer science abstraction is a mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time In general philosophical terminology abstraction is the thought process wherein ideas are separated from objects Our minds work mostly with abstractions For example when thinking about a chair we do not have in mind a particular chair but an abstract idea of a chair the concept of a chair This wh
7. 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 So whatever object figure refers to that object must have 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 method or the return type of a method 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 95 5 2 Nested Classes A class seems like it should be a pretty important thing A class is a high level build ing 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 essen
8. this null Call the other constructor with parameter null x Constructor creates a panel to display 25 copies of a specified message x param messageString The message to be displayed If this is null x then the default message Java is displayed public RandomStringsPanel String messageString message messageString 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 30 font4 new Font Dialog Font PLAIN 36 font5 new Font Serif Font ITALIC 48 setBackground Color BLACK xx The paintComponent method is responsible for drawing the content x of the panel It draws 25 copies of the message string using a x random color font and position for each string 127 public void paintComponent Graphics g super paintComponent g Call the paintComponent method from the superclass JPanel This simply fills the entire panel with the background color black int width getWidth int height getHeight 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 5xMath 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
9. D gt I state Methods behavior Figure 1 1 An Object 1 2 Fundamentals of Objects and Classes We move now from the conceptual picture of objects and classes to a discussion of software classes and objects Objects are closely related to classes A class can contain variables and methods If an object is also a collection of variables and methods how do they differ from classes 1 2 1 Objects and Classes Objects In object oriented programming we create software objects that model real world ob jects Software objects are modeled after real world objects in that they too have state and behavior A software object maintains its state in one or more variables A variable is an item of data named by an identifier A software object implements its behavior with methods A method is a function associated with an object Definition An object is a software bundle of variables and related methods An object is also known as an instance An instance refers to a particular object For e g Karuna s bicycle is an instance of a bicycle It refers to a particular bicycle Sandile Zuma is an instance of a Student The variables of an object are formally known as instance variables because they contain the state for a particular object or instance In a running program there may be many instances of an object For e g there may be many Student objects Each of these objects will have their own instance variables and each object may h
10. 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 down wards 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 con trol is passed to the called object until that method has finished running or asyn chronous 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 pro gram control 3 3 5 Collaboration Diagrams Collaboration Diagrams show the interactions occurring between the objects partic ipating 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 interactions 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 repre sented by arrows showing the message name parameters and the sequence of the message Collaboration Diagrams are specially well suited to showing a specific pro gram flow or situation and are one of the best diagram types to quickly demonstrate or explain one process in the program logic 73 44 asf xmi Umbrello UML Modeller lt 2 gt Jo
11. So behind the scenes numlist add x is ac tually adding an object to the list but it looks a lot as if you are working with a list of doubles The ArrayList class is just one of several standard classes that are used for generic programming in Java We will spend the next few sections looking at these classes and how they are used and we ll see that there are also generic methods and generic interfaces All the classes and interfaces discussed in these sections 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 jmportajava 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 so it is often better to import the individual classes that you need 8 4 The Java Collection Framework 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 An ArrayList is an example of a collection A map associates objects in one set with objects in another set in the way that a dictionary associates definitions with words or a phone book associates phone numbers with names In Java collections and maps are represented by the parameterized interfaces Collection lt T gt and Map lt T S gt Her
12. 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 are also a lot of tutorials on the Internet which you can take as a starting point 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 Objectory Since UML has evolved some of these methods have been recast to take advantage of the
13. 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 whether 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 158 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 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 ImageIO read new File images blfv 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 o
14. 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 Cint Math random Cheight 20 Draw the message g drawString message x y end for end paintComponent end class RandomStringsPanel This class defines a panel which is not something that can stand on its own To 128 see it on the screen we have to use it in an applet or a frame Here is a simple applet class that uses a RandomStringsPanel as its content pane import javax swing JApplet x A RandomStringsApplet displays 25 copies of a string using random colors x fonts and positions for the copies The message can be specified as the x value of an applet param with name message If no param with name x message is present then the default message Java is displayed x The actual content of the applet is an object of type RandomStringsPanel public class RandomStringsApplet extends JApplet public void init String message getParameter message RandomStringsPanel content new RandomStringsPanel message setContentPane content Note that the message to be displayed in the applet can be set using an applet pa rameter when the applet is a
15. public void draw Graphics g if empty g drawRect x y Card width Card height 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 s 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 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 this means adding it to its pile and making the new Card the topCard 162 7 3
16. John Doe Change data in the object The value stored in stu is not changed It still refers to the same object Next suppose that obj is a variable that refers to an object Let s consider what happens when obj is passed as an actual parameter to a method The value of obj is assigned to a formal parameter in the method and the method is executed The method 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 method has a reference to the object it can change the data stored in the object After the method ends obj still points 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 void change Student s Zz 42 S name Fred The lines The lines xX 17 stu name Jane dontChange x change stu System out print1ln x System out println stu name outputs the value 17 outputs the value Fred The value of x is not The value of stu is not changed changed by the method but stu name is which is equivalent to This is equivalent to Z Xs s stu Zz 42 s name Fred 1 2 3 Access Control When writing new classes it s a good idea to pay attention to the issue of access control Recall that making a member of a class public makes it accessible from 27 anywher
17. ful Suppose that list is a variable of type ArrayList Then we have e list size This method 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 new ArrayList creates an ArrayList of size zero list add obj Adds an object onto the end of the list increasing the size by 1 The parameter obj can refer to an object of any type or it can be null list get N 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 of type IndexOutOfBoundsException occurs Calling this method is similar to referring to A N for an array A except you can t use list get N on the left side of an assignment statement list set N obj Assigns the object obj to position N in the ArrayList re placing 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 method is equivalent to the com mand A N obj for an array A 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 list remove N For a
18. message setBackground Color black on a black background message setFont new Font Serif Font BOLD 18 in a big bold font message setOpaque true Make sure background is filled in 6 6 3 JCheckBox A JCheckBox 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 140 Usually it s the user who sets the state of a JCheckBox but you can also set the state in your program using its setSelected boolean method If you want the checkbox showTime to be checked you would say showTime setSelected true To uncheck the box say showTime setSelected false You can determine 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 pro gram 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 is changed by the user 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
19. 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 ob ject This is called a dangling pointer error and it leads to problems when the 34 program tries to access an object that is no longer there Another type of error is a memory leak 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 collection 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 1 2 6 Everything is NOT an object Wrapper Classes and Autoboxing Recall that there are two kinds of types in JAVA primitive types and object types Classes In some object oriented languages everything is an object However in JAVA and in C the primitive types like int and double are not objects This decision was made for memory and processing efficiency it takes less memory
20. 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 implement this it is necessary to respond to 137 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 belonging to the standard class javax swing JButton A JButton object draws itself and 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 listeners to tell them that the button has been pushed Your program gets only the information it needs the fact that a button was pushed The standard components that are defined as part of the Swing graphical user interface API are defined by subclasses of the class JComponent which is itself a subclass of Component Note that this includes the JPanel class that we have already been working with extensiv
21. this name name More variables and methods In the constructor the instance variable called name is hidden by a formal pa rameter 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 param eter 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 method as an actual parameter In that case you can use this as the actual parameter For ex ample 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 4 6 2 The Special Variable super Java also defines another special variable named super for use in the definitions 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 wr
22. Just for fun I have used a Scanner to read the user s input import java io File import java util Scanner x This program lists the files in a directory specified by x the user The user is asked to type in a directory name x If the name entered by the user is not a directory a x message is printed and the program ends public class DirectoryList 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 scanner For reading a line of input from the user scanner new Scanner System in scanner reads from standard input System out print Enter a directory name directoryName scanner nextLine trim 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 printin 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 218 variable
23. The setEnabled and setText methods of a button are particu larly 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 6 6 2 JLabel JLabel is certainly the simplest type of component An object of type JLabel exists just to display a 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 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 JLabel is a subclass of JComponent you can use JComponent methods such as setForeground with labels If you want the background color to have any effect call setOpaque true on the label since otherwise the JLabel might not fill in its background For example JLabel message new JLabel Hello World JLabel CENTER message setForeground Color red Display red text
24. 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 ap proach can be effective Analyze the problem to discover the concepts that are in volved and create classes to represent those 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 3 1 3 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 sci ence 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 independently 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
25. and it should have a parameter with the same type as the variable A setter method for the variable title could be written public void setTitle String newTitle title newTitle It is actually very common to provide both a getter and a setter method for a private member variable Since this allows other classes both to see and to change the value of the variable you might wonder why not just make the variable public The reason is that getters and setters are not restricted to simply reading and writing the variable s value In fact they can take any action at all For example a getter method might keep track of the number of times that the variable has been accessed public String getTitle titleAccessCount Increment member variable titleAccessCount return title and a setter method might check that the value that is being assigned to the variable is legal public void setTitle String newTitle if newTitle null Don t allow null strings as titles title Untitled Use an appropriate default value instead else title newTitle Even if you can t think of any extra chores to do in a getter or setter method you might change your mind in the future when you redesign and improve your class If you ve used a getter and setter from the beginning you can make the modification to your class without affecting any of the classes that use your class The private member variable is not part of th
26. 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 follows 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 public class WireFrameModel other members of the WireFrameModel class static public class Line Represents a line from the point x1 y1 21 to the point x2 y2 z2 in 3 dimensional space double x1 yl z1 double x2 y2 z2 end class Line other members of the WireFrameModel class end WireFrameModel Inside the WireFrameModel class a Line object would be created with the con structor new Line Outside the class new WireFrameModel Line would be used 96 A static nested class has full access to the static members of the containing class even to the private members Similarly the containing class has full access to the members of the nested class 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 ma
27. 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 6 4 5 An Example Let s use some of the material covered in this section to write a subclass of JPanel for use as a drawing surface The panel can then be used in either an applet or a frame All the drawing will be done in the paintComponent method of the panel class The panel 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 message can be specified in the constructor if the default constructor is used the message is the string Java The panel works OK no matter what its size Here s an applet that uses the panel as its content pane The source for the panel class is shown below I use an instance variable called message to hold the message that the panel will display There are five instance vari ables of type Font that represent different sizes and styles of text These variables are initialized in the constructor and are used in the paintComponent method The paintComponent method for the panel simply draws 25 copies of the mes sage 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 setCo
28. imports the class javax swing JOptionPane to make it possible to refer to the JOptionPane class using its simple name If you want to display a message to the user in a GUI program this is a good way to do it Just use a standard class that already knows how to do the work And in fact JOptionPane is regularly used for just this purpose but as part of a larger program usually Of course if you want to do anything serious in a GUI program there is a lot more to learn To give you an idea of the types of things that are involved we ll look at a short GUI program that does the same things as the previous program open a window containing a message and an OK button and respond to a click on the button by ending the program but does it all by hand instead of by using the built in JOptionPane class Mind you this is not a good way to write the program but it will illustrate some important aspects of GUI programming in JAVA Here is the source code for the program I will explain how it works below but it will take the rest of the chapter before you will really understand completely 108 import java awt x import java awt event x import javax swing x public class HelloWorldGUI2 private static class HelloWorldDisplay extends JPanel public void paintComponent Graphics g super paintComponent g g drawString Hello World 20 30 private static class ButtonHandler implements ActionListener public v
29. it s usually necessary to type cast it to type Player by saying 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 every player make a move is 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 re quired because of Java s precedence rules The parentheses force the type cast to be performed before the makeMove method is called for each loops work for ArrayLists just as they do for arrays But note that since the items in an ArrayList are only known to be Objects the type of the loop control variable must be Object For example the for loop used above to let each Player make a move could be written as the for each loop for Object plrObj players Player plr Player plr0Obj plr makeMove In the body of the loop the value of the loop control variable plrObj is one of the objects from the list players This object must be type cast to type Player before it can be used 8 3 Parameterized Types THE MAIN DIFFERENCE BETWEEN true generic programmi
30. 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 null Constructors 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 instance variables and then returns a reference to the object This reference is the value of the expression and that value is stored by the assignment statement in the variable dice so that after the assignment state ment is executed dice refers to the newly created object Part of this expression
31. sce eRe este a ds ese E E ae ee Ee 124 6 4 5 AnExample 0 0 00 cee eee eee 126 6 5 Mouse Events 0 eee cece eee ere teenies 129 6 5 1 Event Handling 0 0000 eens 130 6 5 2 MouseEvent and MouseListener 131 6 5 3 Anonymous Event Handlers aana aaa 134 6 6 Basic Components sssessessesocsesesooeeoe 137 GOL IBUN n irea e ee eH Se Re h 139 6 6 2 JLabel sc 28s 6 A ed be e ee LR ete ye 140 6 6 3 J ChECK BOX 2 0 Fk od es eh A capa ie owe 140 6 6 4 JTextField and JTextArea 0 00005 141 6 7 Basie Layout iaar acs aye dice ccs ead dicate eat at wat Ps 143 6 7 1 Basic Layout Managers 2 0000 ee 144 6 7 2 A Simple Calculator 000000005 146 6 7 3 ALittleCardGame 00 000000 ae 148 6 8 Images and Resources 020 ccceeecsevense 152 6 8 1 WM ages ce i es SR ee RE he OEE OS 153 6 8 2 Image FileV O 0 0 0 es 155 6 1 Introduction The Modern User Interface WHEN COMPUTERS WERE FIRST INTRODUCED ordinary people including most program mers couldn t get near them They were locked up in rooms with white coated at tendants who would take your programs and data feed them to the computer and return the computer s response some time later When timesharing where the com puter switches its attention rapidly from one person to another was invented in the 1960s it became possible for several people to interac
32. week The options of an enum are called Enum 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 Book a E J 1 Figure 3 7 72 o asf xmi Umbrello UML Modeller lt 2 gt ee OTE Eile Edit Diagram Code Settings Help ASae XhG UML Diagrams e fo pies alinee xX amp connection cl fest 5 i i a execution Dispatcher Requests Queue WorkerManager Worker TA _ Dispatcher fe baal f i cy E worker receive Type x E worker t ioc f execution cia loadWorker job Transaction Worker pp overview colla preceive data i sequence dia ai amp tools e E List B Ey Map ES Queue Ps Use Case View lw Documentation a X ae y KEG Ready Figure 3 8 Umbrello UML Modeller showing a Sequence Diagram 3 3 4 Sequence Diagrams Sequence Diagrams show the message exchange i e method call between several Objects in a specific time delimited situation Objects are instances of classes Se quence Diagrams put special emphasis in the order and the times in which the mes sages to the objects are sent
33. y 0 answer setText Can t divide by zero else answer setText x y x y end actionPerformed The complete source code for this example can be found in SimpleCalc java 6 7 3 A Little Card Game For a final example let s look at something a little more interesting as a program The example is 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 You ve seen a text oriented version of the same game previously have also seen Deck Hand and Card classes that are used in the game program 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 The complete source code for this example is in the file HighLowGUI java The overall structure of the main panel in this example should be clear It has three buttons in a subpanel at the bottom of the main panel and a large drawing surface that displays the cards and a message The main panel uses a BorderLayout The drawing surface occupies the CENTER position of the border layout The subpanel that contains the buttons occupies the SOUTH position of th
34. 1 The Special Variablethis 000 88 4 6 2 The Special Variablesuper 0 0 0004 89 4 6 3 Constructors in Subclasses 2 2000 90 A CLASS REPRESENTS A SET OF OBJECTS which share the same structure and behay iors 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 instance 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 cen tral new idea in object oriented programming the idea that really distinguishes it from traditional programming is to allow classes to express the similarities among objects that share some but not all of their structure and behavior Such similarities can be expressed using inheritance and polymorphism 4 1 Extending Existing Classes 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 77 public class Subclass name extends Existing class name Changes and additions As an example suppose you want to write a progra
35. 4 1 3 3 which allows a 3 pixel gap be tween the rows where the gray background color of the panel is visible The gray bor der around the edges of the panel is added with the statement setBorder BorderFactory createEmptyBorder 5 5 5 5 The first row of the grid layout actually contains two components a JLabel dis playing the text x and a JTextField A grid layout can only only have one com ponent in each position In this case that component is a JPanel a subpanel that is nested inside the main panel This subpanel in turn contains the label and text field This can be programmed as follows xInput new JTextField 0 10 Create a text field to hold 10 chars JPanel xPanel new JPanel Create the subpanel xPanel add new JLabel x Add a label to the subpanel xPanel add xInput Add the text field to the subpanel mainPanel add xPanel Add the subpane to the main panel The subpanel uses the default FlowLayout layout manager so the label and text field are simply placed next to each other in the subpanel at their preferred size and are centered in the subpanel Similarly the third row of the grid layout is a subpanel that contains four buttons In this case the subpanel uses a GridLayout with one row and four columns so that the buttons are all the same size and completely fill the subpanel One other point of interest in this example is the actionPerformed method that responds
36. 4 Enums as Classes 0 0 ce eee eee erence nnse 101 THIS SECTION simply pulls together a few more miscellaneous features of object oriented programming in Java Read it now or just look through it and refer back to it later when you need this material You will need to know about the first topic interfaces almost as soon as we begin GUI programming 5 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 illustra tion below for example classE is shown as having both classA and classB as direct superclasses while classF has three direct superclasses 93 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 inher itance were not worth the cost in increased complexity However Java does have a feature that can be used to accomplish many of the same goals as multiple inheri tance interfaces We ve encountered the term interface before in connection with black boxes in general and methods in particular The interface of a method consists of the name of the method 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 method A method also has an implementation the block
37. A layout manager is an object associated with a con tainer that implements some policy for laying out the components in that container Different types of layout manager implement different policies In this section we will cover the three most common types of layout manager and then we will look at several programming examples that use components and layout Every container has an instance method setLayout that takes a parameter of type LayoutManager and that is used to specify the layout manager that will be responsible for laying out any components that are added to the container Com ponents are added to a container by calling an instance method named add in the container object There are actually several versions of the add method with differ ent parameter lists Different versions of add are appropriate for different layout managers as we will see below 143 6 7 1 Basic Layout Managers JAVA has a variety of standard layout managers that can be used as parameters in the setLayout method They are defined by classes in the package java awt Here we will look at just three of these layout manager classes FlowLayout BorderLayout and GridLayout A FlowLayout simply lines up components in a row across the container The size of each component is equal to that component s preferred size After laying out as many items as will fit in a row across the container the layout manager will move on to the next row The d
38. Agency a by Customer Ly amp flight booker use case dia Call center agent Customer Documentation a v ae F ja Ready Figure 3 1 Umbrello UML Modeller showing a Use Case Diagram 3 3 3 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 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 pr
39. Computer machines understand operations at the very low level such as moving some bits from one location of the memory to another location and producing the sum of two sequences of bits Programming languages allow this to be done at a higher level For example consider the high level expression program statement a 1 2 5 To a human this is a fairly simple and obvious calculation one plus two is three times five is fifteen However the low level steps necessary to carry out this eval uation and return the value 15 and then assign that value to the variable a are actually quite subtle and complex The values need to be converted to binary representation often a much more complicated task than one would think and the calculations decomposed by the compiler or interpreter into assembly instructions again which are much less intuitive to the programmer operations such as shifting a binary register left or adding the binary complement of the contents of one register to another are simply not how humans think about the abstract arithmetical opera tions of addition or multiplication Finally assigning the resulting value of 15 to the variable labeled a so that a can be used later involves additional behind the scenes steps of looking up a variable s label and the resultant location in physical or virtual memory storing the binary representation of 15 to that memory location etc
40. 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 thing 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 xx Create a Hand object that is initially empty x precondition None x postcondition An empty hand object is created x public Hand xx Discard all cards from the hand making the hand empty x precondition None x postcondition The hand object is empty public void clear xx If the specified card is in the hand it is removed x param c the Card to be removed x precondition c is a Card object and is non null x postcondition The specified card is removed if it exists x public void removeCard Card c 54 xx Add the card c to the hand x param The Card to be added x precondition c is a Card object and is non null x postcondition The hand object contains the Card c and now has one more card x throws NullPointerException is thrown if c is nota Card or is null public void addCard Card c xx Remove the card in the specified position from the hand x param position the position of the card that is to be removed where positions start from zero precondition position is valid i e 0 lt position lt number cards postcondition The card in the specified position is removed and there
41. 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 their programs are correct 9 2 1 Provably Correct Programs In some cases it is possible to prove that a program is correct That is it is possible 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 the ideas and techniques that are used in proving that programs are correct 190 The fundamental ideas are process and state A state consists of all the informa tion relevant to the execution of a program at a given moment during its execution 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 p
42. Javadoc comments but I will explicitly label the preconditions and postconditions Many computer scientists think that new doc tags precondition and postcondi tion should be added to the Javadoc system for explicit labeling of preconditions and postconditions but that has not yet been done 2 2 3 APIs and Packages One of the important advantages of object oriented programming is that it promotes reuse When writing any piece of software a programmer can use a large and growing body of pre written software The JAVA SDK software development kit consists of thousands of classes that can be used by programmers So learning the JAVA language means also being able to use this vast library of classes Toolboxes 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 Toolbox a collection of well over a thousand different methods There are methods 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 methods for creating menus and for reacting to user selections from menus Aside from the user interface there are methods 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 stan
43. Programs that have many direct interrelationships between any two random parts of the program code are less modular than programs where those rela tionships occur mainly at well defined interfaces between modules e Information hiding separate the implementation and the function The prin ciple of information hiding is the hiding of design decisions in a computer pro gram that are most likely to change thus protecting other parts of the program 65 from change if the design decision is changed Protecting a design decision in volves 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 communicates with other classes via a small well defined interface We strive for cohesion each class performs one and only one task for readability reuse 3 2 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 Cun ningham 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 responsib
44. You can use javadoc in a command line interface similarly to the way that the javac and java commands are used Javadoc can also be applied in the Eclipse integrated development environment Just right click the class or package that you want to doc ument in the Package Explorer select Export and select Javadoc in the window that pops up Consult the documentation for more details 50 x This method computes the area of a rectangle given its width x and its height The length and the width should be positive numbers x param width the length of one side of the rectangle x param height the length the second side of the rectangle x return the area of the rectangle x throws IllegalArgumentException if either the width or the height is a negative number public static double areaOfRectangle double length double width if width lt O0 height lt 0 throw new IllegalArgumentException Sides must have positive length double area area width height return area 2 5 Creating Jar Files As the final topic for this chapter we look again at jar files Recall that a jar file is a java archive that can contain a number of class files When creating a program that uses more than one class it s usually a good idea to place all the classes that are required by the program into a jar file since then a user will only need that one file to run the program Jar files can also be used for stand alone
45. a few other components in the applet The labels themselves are components even though you can t interact with them The right half of the applet is a text area component which can display multiple lines of text and a scrollbar component appears alongside the text area when the number of lines of text becomes larger than will fit in the text area And in fact in JAVA terminology the whole applet is itself considered to be a component Checkbox was turned on Push Button Click Me Pressed return in TextField Checkbox Click me with contents Hello World Text Field Hello World Item Third Option selected Pop up Menu Third Option from pop up menu JAVA actually has two complete sets of GUI components One of these the AWT or Abstract Windowing Toolkit was available in the original version of JAVA The other which is known as Swing is included in JAVA version 1 2 or later and is used 106 in preference to the AWT in most modern JAVA programs The applet that is shown above uses components that are part of Swing When a user interacts with the GUI components in this applet an event is gen erated For example clicking a push button generates an event and pressing return while typing in a text field generates an event Each time an event is generated a message is sent to the applet telling it that the event has occurred and the applet responds according to its program In fact the program
46. addMouseListener new MouseAdapter public void mousePressed MouseEvent evt ae To see how this works in a real example let s write another version of the appli cation ClickableRandomStringsApp This version uses an anonymous class based on MouseAdapter to handle mouse events import java awt Component import java awt event MouseEvent import java awt event MouseListener import javax swing JFrame 135 public class ClickableRandomStringsApp public static void main String args JFrame window new JFrame Random Strings RandomStringsPanel content new RandomStringsPanel content addMouseListener new MouseAdapter Register a mouse listener that is defined by an anonymous subclass of MouseAdapter This replaces the RepaintOnClick class that was used in the original version public void mousePressed MouseEvent evt Component source Component evt getSource source repaint window setContentPane content window setDefaultCloseOperation JFrame EXIT_ON_CLOSE window setLocation 100 75 window setSize 300 240 window setVisible true Anonymous inner classes can be used for other purposes besides event handling For example suppose that you want to define a subclass of JPanel to represent a drawing surface The subclass will only be used once It will redefine the paintComponent method but will make no other changes to JPanel It might make sense
47. all the file operations described in this section 10 2 1 Reading and Writing Files 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 construc tor will throw an exception of type FileNotFoundException if the file doesn t exist It requires mandatory exception handling so you have to call the constructor in a try catch statement or inside a method that is declared to throw the exception 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 The FileNotFoundException class is a subclass of IOException so it would be acceptable to catch IOExceptions in the above try catch statement More gener ally 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
48. an event loop In this section we ll look at handling mouse events in JAVA and we ll cover the framework for handling events in general The next section will cover keyboard related events and timer events JAVA also has other types of events which are produced by GUI components 6 5 1 Event Handling 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 information 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 implement this MouseListener interface JAVA interfaces were cov ered previously To review briefly An interface in JAVA is just a list of instance methods A class can implement an interface by doing two t
49. and 100 pixels high The third line says that the upper left corner of the window will be 100 pixels over from the left edge of the screen and 100 pixels down from the top Once all this has been set up the window is actually made visible on the screen with the command window setVisible true It might look as if the program ends at that point and in fact the main method does end However the the window is still on the screen and the program as a whole does not end until the user clicks the OK button The content that is displayed in a JFrame is called its content pane In addition to its content pane a JFrame can also have a menu bar which is a separate thing that I will talk about later A basic JFrame already has a blank content pane you can ei ther add things to that pane or you can replace the basic content pane entirely In my sample program the line window setContentPane content replaces the origi nal blank content pane with a different component Remember that a component is just a visual element of a graphical user interface In this case the new content is a component of type JPanel JPanel is another of the fundamental classes in Swing The basic JPanel is again just a blank rectangle There are two ways to make a useful JPanel The first is to add other components to the panel the second is to draw something in the panel Both of these techniques are illustrated in the sample program In fact you will find two
50. and the event handling that will determine its behavior You can also do some initialization in the constructor for your class but there are certain aspects of the applet s environment that are set up after its constructor is called but before the init method is called so there are a few operations that will work in the init method but will not work in the constructor The other applet life cycle methods are start stop and destroy I will not use these methods for the time being and will not discuss them here except to mention that destroy is called at the end of the applet s lifetime and can be used as a place to do any necessary cleanup such as closing any windows that were opened by the applet With this in mind we can look at our first example of a JApplet It is of course an applet that says Hello World To make it a little more interesting I have added a button that changes the text of the message and a state variable currentMessage that holds the text of the current message This example is very similar to the stand alone application HelloWorldGUI2 from the previous section It uses an event handling class to respond when the user clicks the button a panel to display the message and another panel that serves as a container for the message panel and the button The second panel becomes the content pane of the applet Here is the source code for the applet again you are not expected to understand all the details
51. applications In fact it is possible to make a so called executable jar file A user can run an executable jar file in much the same way as any other application usually by double clicking the icon of the jar file The user s computer must have a correct version of JAVA installed and the computer must be configured correctly for this to work The configuration is usually done automatically when JAVA is installed at least on Windows and Mac OS The question then is how to create a jar file The answer depends on what pro gramming environment you are using There are two basic types of programming environment command line and IDE Any IDE Integrated Programming Environ ment for JAVA should have a command for creating jar files In the Eclipse IDE for example it s done as follows In the Package Explorer pane select the programming project or just all the individual source code files that you need Right click on the selection and choose Export from the menu that pops up In the window that ap pears select JAR file and click Next In the window that appears next enter a name for the jar file in the box labeled JAR file Click the Browse button next to this box to select the file name using a file dialog box The name of the file should end with jar If you are creating a regular jar file not an executable one you can hit Finish at this point and the jar file will be created You coul
52. 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 diel 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 rollq Call the roll method to roll the dice public PairOfDice int vall int val2 Constructor Creates a pair of dice that are initially showing the values vall and val2 diel vall 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 Now we have the option of constructing a PairOfDice object with new PairOfDice or with new PairOfDice x y where x and y are int valued expressions 31 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
53. asoca raaua ha a we eee 203 A PROGRAM IS CORRECT if it accomplishes the task that it was designed to per form It is robust if it can handle illegal inputs and other unexpected situations in a reasonable way 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 pro gram 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 particularly robust The question of correctness is actually more subtle than it might appear A pro grammer 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 185 mean that the program itself is correct What if the specification is incorrect or in complete A correct program should be a correct implementation of a complete and correct specification The question is whether the specification correctly expresses the i
54. at this time import java awt x import java awt event x import javax swing x A simple applet that can display the messages Hello World x and Goodbye World The applet contains a button and it x Switches from one message to the other when the button is x Clicked public class HelloWorldApplet extends JApplet private String currentMessage Hello World private MessageDisplay displayPanel private class MessageDisplay extends JPanel Defines the display panel public void paintComponent Graphics g super paintComponent g g drawString currentMessage 20 30 private class ButtonHandler implements ActionListener The event listener public void actionPerformed ActionEvent e if currentMessage equals Hello World currentMessage Goodbye World else currentMessage Hello World displayPanel repaint Paint display panel with new message 114 x The applet s init method creates the button and display panel and x adds them to the applet and it sets up a listener to respond to x clicks on the button public void init displayPanel new MessageDisplay JButton changeMessageButton new JButton Change Message ButtonHandler listener new ButtonHandler changeMessageButton addActionListener listener JPanel content new JPanel content setLayout new BorderLayout content add displayPanel BorderLayout CENTER content a
55. available to the applet when it runs An applet can use the predefined method getParameter to check for parameters specified in param tags The getParameter method has the following interface String getParameter String paramName The parameter paramName corresponds to the param name in a param tag If the specified paramName occurs in one of the param tags then get Parameter paramName returns the associated param value If the specified paramName does not occur in any param tag then getParameter paramName returns the value null Parameter names are case sensitive so you cannot use size in the param tag and ask for Size in getParameter The getParameter method is often called in the applet s init method It will not work correctly in the applet s constructor since it depends on in formation about the applet s environment that is not available when the constructor is called Here is an example of an applet tag with several params 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 applet gt The ShowMessage applet would presumably read these parameters in its init method which could go something like this 118 String message Instance variable message to be displayed String fontName Instance variable font to use for display int fontSize I
56. be static so the class exists only for creating objects This class definition says that any object that is an instance 23 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 called 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 state ment the type of a formal parameter or the return type of a method 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 memory In fact there is a special portion of memory called the heap where objects live Instead of holding an object itself a variable holds the information necessary to fi
57. 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 it again as a new stream Note that for most stream classes the close method can throw an IOException which must be handled PrintWriter overrides this method so that it cannot throw such exceptions If you forget to close a file the file will ordinarily be closed automat ically when the program terminates or when the file object is garbage collected but in the case of an output file some of the data that has been written to the file might be lost This can occur because data that is written to a file can be buffered that is the data is not sent immediately to the file but is retained in main memory in a buffer until a larger chunk of data is ready to be written This is done for efficiency The close method of an output stream will cause all the data in the buffer to be sent to the file Every output stream also has a flush method that can be called to force any data in the buffer to be written to the file without closing the file 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 an other file named result dat It is assumed that data dat contains only one number on each line Exception handling is used to check for problems along the way Al though the application
58. calling the repaint method of the panel Here is an applet version of the ClickableRandomStrings program for you to try when you click the applet a new set of random strings is displayed For the new version of the program we need an object that implements the MouseListener interface One way to create the object is to define a separate class such as import java awt Component import java awt event x An object of type RepaintOnClick is a MouseListener that will respond to a mousePressed event by calling the repaint method of the source of the event That is a RepaintOnClick object can be added as a mouse listener to any Component when the user clicks that component the component will be x repainted public class RepaintOnClick implements MouseListener public void mousePressed MouseEvent evt Component source Component evt getSource source repaint Call repaint on the Component that was clicked public void mouseClicked MouseEvent evt public void mouseReleased MouseEvent evt public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt This class does three of the four things that we need to do in order to handle mouse events First it imports java awt event for easy access to event related classes Second it is declared that the class implements MouseListener And third it pro vides definitions for the five methods that a
59. classes are part of the Java Collection Framework Each implements the interface List lt T gt and therefor the interface Collection lt T gt An object of type ArrayList lt T gt represents an ordered sequence of objects of type T stored in an array that will grow in size whenever necessary as new items are added An object of type LinkedList lt T gt also represents an ordered sequence of objects of type T but the objects are stored in nodes that are linked together with pointers Both list classes support the basic list operations that are defined in the interface List lt T gt and an abstract data type is defined by its operations not by its represen tation 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 oper ations 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
60. consists mainly of event handlers that tell the applet how to respond to various types of events In this ex ample the applet has been programmed to respond to each event by displaying a message in the text area The use of the term message here is deliberate Messages are sent to objects In fact JAVA GUI components are implemented as objects JAVA includes many prede fined classes that represent various types of GUI components Some of these classes are subclasses of others Here is a diagram showing some of Swing s GUI classes and their relationships JLabel JAbstractButton JComboBox JScrollbar JTextComponent JToggleButton JTextField JCheckBox JRadioButton Note that all GUI classes are subclasses directly or indirectly of a class called JComponent which represents general properties that are shared by all Swing com ponents Two of the direct subclasses of JComponent themselves have subclasses The classes JTextArea and JTextField which have certain behaviors in common are grouped together as subclasses of JTextComponent Also JButton and JToggleButton are subclasses of JAbstractButton which represents properties common to both but tons and checkboxes Just from this brief discussion perhaps you can see how GUI programming can make effective use of object oriented design In fact GUI s with their visible ob jects are probably a major factor contributing to the popularity of OOP 6 2 The Basic GUI App
61. crash the program as usual By the way note that the braces and are part of the syntax of the try catch statement They are required even if there is only one statement between the braces This is different from the other statements we have seen where the braces around a single statement are optional As an example suppose that str is a variable of type String whose value might or might not represent a legal real number Then we could say try double x x Double parseDouble str System out println The number is xX catch NumberFormatException e System out println Not a legal number If an error is thrown by the call to Double parseDouble str then the output statement in the try part is skipped and the statement in the catch part is executed It s not always a good idea to catch exceptions and continue with the program Often that can just lead to an even bigger mess later on and it might be better just to let the exception crash the program at the point where it occurs However sometimes it s possible to recover from an error For example suppose that we have the enumerated type enum Day MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SATURDAY SUNDAY and we want the user to input a value belonging to this type TextIO does not know about this type so we can only read the user s response as a string The method Day valueOf can be used to convert the user s response to a value of
62. dialog box where you can manage run configurations The name of the project and of the main class will be already be filled in Click the Arguments tab and enter ea in the box under VM Arguments The contents of this box are added to the java command that is used to run the program You can enter other options in this box including more complicated enableassertions options such as ea paintutils When you click the Run button the options will be applied Furthermore they will be applied whenever you run the program unless you change the run configuration or add a new configuration Note that it is possible to make two run configurations for the same class one with assertions enabled and one with assertions disabled 206 owe LQ Input and Output Contents 10 1 Streams Readers and Writers 2 cecececee 207 10 1 1 Character and Byte Streams 0040 207 10 1 2 PrntWriter s s 24 6 a woes BA Ba ee a eS 209 10 1 3 Data Streams occ rroa saag ae Se eA 210 10 1 4 Reading Text 2 ee 211 10 1 5 The Scanner Class 1 20 0 0 eee eee ee ee 212 10 2 Files okies Se ae erie ew ROS We Bac Beene Ge 213 10 2 1 Reading and Writing Files 04 214 10 2 2 Files and Directories 0 0 002 e eee eee 217 10 3 Programming With Files 0 wees eeeee 219 10 3 1 Copying a File 0 ee 219 10 1 Streams Readers and Writers Without the ability to interact
63. exception might match several of those catch clauses For example an exception of type Nul1PointerException would match catch clauses for NullPointerException RuntimeException Exception or Throwable In this case only the first catch clause that matches the exception is executed 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 state ment 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 exception handling does not solve the problem of 197 program robustness It just gives you a tool that will in many cases let you approach the problem in a more organized way I have still not completely specified the syntax of the try statement There is one additional element the possibility of a finally clause at the end of a try statement The complete syntax of the try statement can be described as try statements optional catch clauses optional finally clause Note that the catch clauses are also listed as optional The try statement can incl
64. filled A BorderLayout selects the sizes of its components as follows The NORTH and SOUTH components if present are shown at their preferred heights but their width is set equal to the full width of the container The EAST and WEST components are shown at their preferred widths but their height is set to the height of the container minus the space occupied by the NORTH and SOUTH components Finally the CENTER component takes up any remaining space the preferred size of the CENTER component is completely ignored You should make sure that the components that you put into a BorderLayout are suitable for the positions that they will occupy A horizontal slider or text field for example would work well in the NORTH or SOUTH position but wouldn t make much sense in the EAST or WEST position The default constructor new BorderLayout leaves no space between compo nents If you would like to leave some space you can specify horizontal and vertical gaps in the constructor of the 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 background color of the con tainer will show through in these gaps The default layout for the original content pane that comes with a JFrame or JApplet is a BorderLayout with no horizontal or vertical gap Finally we consider the GridLayout
65. generates an event of type ActionEvent If you want something to hap pen when the user changes the state you must register an ActionListener with the checkbox by calling its addActionListener method Note that if you change the state 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 call evt getSource in the actionPerformed method to find out which object generated the event Of course if you are only lis tening 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 public void actionPerformed ActionEvent evt Object source evt getSource if source cb1 boolean newState JCheckBox cb1 isSelected respond to the change of state else if source cb2 boolean newState JCheckBox cb2 isSelected respond to the change of state Alternatively you can use evt getActionCommand to retrieve the action com mand associated with the source For a JCheckBox the act
66. in which the precondition is checked by an if statement In the first part of the if statement 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 keyboard new Scanner System in System out println Enter your values for A B and C System out print A A keyboard nextDouble System out print B B keyboard nextDouble System out print C C keyboard nextDouble if A 0 amp amp BxB 4 AxC gt 0 disc BxB 4 AxC x B Math sqrt disc 2A System out println A solution of AxX X BX C 0 is x else if A 0 System out println The value of A cannot be zero else System out println Since BxB 4xAxC is less than zero the System out println equation A X X BxX C 0 has no solution 192 Whenever you write a program it s 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
67. interface Comparable lt T gt The method Collections sort will work for example for lists of String and for lists of any of the wrapper classes such as Integer and Double There is also a sorting method that takes a Comparator as its second argument Collections sort list comparator In this method 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 The sorting method that is used by Collections sort is the so called merge sort algorithm 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 order Collections reverse list will reverse the order of the elements so that the last element is moved to the beginning of the list the next to last element to the second position 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 statement 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
68. 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 same 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 157 Suit Piles JU Ue Ba Figure 7 1 Layout of the Solitaire Game from a tableau leaving a face down card on the top the latter card can be turned face up 7 2 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
69. 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 xx An object of type Card represents a playing card from a x Standard Poker deck including Jokers The card has a suit which can be spades hearts diamonds clubs or joker A spade heart diamond or club has one of the 13 values ace 2 3 4 5 6 7 8 9 10 jack queen or king Note that ace is considered to be the smallest value A joker can also have an associated value x 56 x this value can be anything and can be used to keep track of several x different jokers public class Card public final static int SPADES 0 Codes for the 4 suits public final static int HEARTS 1 public final static int DIAMONDS 2 public final static int CLUBS 3 public final static int ACE 1 Codes for the non numeric cards public final static int JACK 11 Cards 2 through 10 have their public final static int QUEEN 12 numerical values for their codes public final static int KING 13 x This card s suit one of the constants SPADES HEARTS DIAMONDS x CLUBS The suit cannot be changed after the card is x constructed private final int suit xx The card s value For a norm
70. it does offer This is my opinion at least The Scanner class was introduced in Java 5 0 to make it easier to read basic data types from a character input source It does not again in my opinion solve the problem completely but it is a big improvement The Scanner class is in the package java util Input methods are defined as instance methods in the Scanner class so to use the class you need to create a Scanner object The constructor specifies the source of the characters that the Scanner will read The scanner acts as a wrapper for the input source The source can be a Reader an InputStream a String or a File If a String is used as the input source the Scanner will simply read the characters in the string from beginning to end in the same way that it would process the same sequence of characters from a stream The File class will be covered in the next section For example you can use a Scanner to read from standard input by saying Scanner standardInputScanner new Scanner System in and if charSource is of type Reader you can create a Scanner for reading from char Source with Scanner scanner new Scanner charSource When processing input a scanner usually works with tokens A token is a mean ingful string of characters that cannot for the purposes at hand be further broken down into smaller meaningful pieces A token can for example be an individual word or a string of characters that represents a value of
71. it might even require a new design Maintenance usually involves redoing some of the work from previous stages 64 Large complex programming projects are only likely to succeed if a careful sys tematic approach is adopted during all stages of the software life cycle The sys tematic approach to programming using accepted principles of good design is called software engineering The software engineer tries to efficiently construct programs that verifyably meet their specifications and that are easy to modify if necessary There is a wide range of methodologies that can be applied to help in the system atic design of programs Most of these methodologies seem to involve drawing little boxes to represent program components with labeled arrows to represent relation ships among the boxes We have been discussing object orientation in programming languages which is relevant 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 pro gram 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 be coming classes or objects in the program design Similarly underline all the verbs
72. layout manager A grid layout lays out com ponents in a grid of equal sized rectangles This illustration shows how the compo nents would be arranged in a grid layout with 3 rows and 2 columns 145 If a container uses a GridLayout the appropriate add method for the container takes a single parameter of type Component for example cntr add comp Compo nents 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 The constructor for a GridLayout takes the form new GridLayout R C where R is the number of rows and C is the number of columns If you want to leave hori zontal gaps of H pixels between columns and vertical gaps of V pixels between rows then you need to use new GridLayout R C H V instead When you use a GridLayout it s probably good form to add just enough compo nents 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
73. mandatory exception handling The Scanner class has very nice look ahead capabilities You can query a scanner to determine whether more tokens are available and whether the next token is of a given type If scanner is of type Scanner e scanner hasNext returns a boolean value that is true if there is at least one more token in the input source e scanner hasNextInt scanner hasNextDouble and so on returns a boolean value that is true if there is at least one more token in the input source and that token represents a value of the requested type e scanner hasNextLine returns a boolean value that is true if there is at least one more line in the input source Although the insistence on defining tokens only in terms of delimiters limits the usability of scanners to some extent they are easy to use and are suitable for many applications 10 2 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 USB memory stick 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 o
74. message From this point of view the computer always exe cutes 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 84 respond to the same message in different ways 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 Suppose that I decide to add beveled rectangles to the types of shapes my program can deal with A beveled rectangle has a triangle cut off each corner Q Beveled Rects To implement beveled rectangles 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 re
75. 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 of course 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 statement 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 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 This would come even before any import directive in that file Furthermore the source code file would be placed in a folder with the same name as the package A class that is in a package automatically has access to other classes in the same package that is a class doesn t have to import the package in which it is defined In projects that define large numbers of classes it makes sense to organize those classes into packages It also makes sense for programmers to create new packages as toolboxes that provide functionality and API s for dealin
76. names in the collection with for String name namelist System out println name This for each loop could of course be written as a while loop using an iterator but the for each loop is much easier to follow 8 6 Equality and Comparison THERE ARE SEVERAL METHODS in the collection interface that test objects for equality 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 the boolean valued method equals Object for testing whether one object is equal to another This method is used by many but not by all collection classes for deciding whether two objects are to be considered the same In the Object class obj1 equals obj2 is
77. 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 This illustrates once again that you can create several instances of the same class 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 totall Total showing on first pair of dice int total2 Total showing on second pair of dice countRolls 0 do Roll the two pairs of dice 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 totall secondDice roll Roll the second pair of dice total2 secondDice diel secondDice die2 Get total System out println Second pair comes up total2 countRolls 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 Constructors are methods but they are methods of a special type They are cer tain
78. 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 Ra tional Software Corporation 3 3 1 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 In cludes Use Case Diagrams e Object Model Showcases the structure and substructure 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 Activity Diagrams and State Machine Diagrams UML is composed of many model elements that represent the different parts of a software 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 e Class Diagrams show classes and the relationships between them 67 e Sequence Diagrams show objects and a sequence of method calls they make to other objects e Collaboration Diagrams show objects and their relationship putting emphasis on the objects that participate in the message exchange e State Diagrams show states state changes and events in
79. not when the method returns When drawing an image that you have created in the com puter s memory or one that you are sure has already been completely loaded you can set the ImageObserver parameter to null There are a few useful variations of the drawImage method For example it is possible to scale the image as it is drawn to a specified width and height This is done with the command g drawlmage img x y width height imageObserver The parameters width and height give the size of the rectangle in which the image is displayed Another version makes it possible to draw just part of the image In the command 153 g drawlmage img dest_x1 dest_yl dest_x2 dest_y2 source_xl source_yl source_x2 source_y2 imageObserver the integers source x1 source_yl source x2 and source_y2 specify the top left and bottom right corners of a rectangular region in the source image The integers dest_x1 dest_yl dest_x2 and dest_y2 specify the corners of a region in the destination graph ics context The specified rectangle in the image is drawn with scaling if necessary to the specified rectangle in the graphics context For an example in which this is useful consider a card game that needs to display 52 different cards Dealing with 52 image files can be cumbersome and inefficient especially for downloading over the Internet So all the cards might be put into a single image ORE wy 22 wm Ste EEEE EE ERE TEE EE afi s De r
80. 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 Even 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 appro priate method The statement oneShape redraw is a message to the object re ferred to by oneShape Since that object knows what type of object it is it knows how it should respond to the
81. of the card s suit x return one of the strings Spades Hearts public String getSuitAsString switch suit case SPADES return Spades case HEARTS return Hearts case DIAMONDS return Diamonds case CLUBS return Clubs default return Null Diamonds xx Returns a String representation of the card s value x return for a regular card one of the strings x 3 10 Jack Queen or King public String getValueAsString 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 default return King x Returns a string representation of this card x its suit and its value Sample return values x are Queen of Hearts 10 of Diamonds public String toString return getValueAsString end class Card 2 7 Example A Simple Card Game Ace NON Clubs x including both Ace of Spades of getSuitAsString We will finish this section by presenting a complete program that uses the Card and Deck classes The program lets the user play a very simple card game called High Low A deck of cards is shuffled and one card is dealt from the deck and shown to the user The user predicts whether the next
82. 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 TextReader to read from the file you could say TextReader data try data new TextReader new FileReader file catch FileNotFoundException e handle the exception 10 3 Programming With Files N THIS SECTION we look at several programming examples that work with files using the techniques that were introduced previously 10 3 1 Copying a File As a first example we look at a simple command line program that can make a copy of a file Copying a file is a pretty common operation and every operating system already has a command for doing it 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 data from the InputStream to the OutputStream one byte at a time If source is the variable that refers to the InputStream then the method source read can be u
83. of type Shape by saying 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 program ming and of polymorphism for int i 0 i lt listOfShapes size i Shape s i th element of the list considered as a Shape s Shape listOfShapes get i s redraw What s drawn here depends on what type of shape s is The sample source code file ShapeDraw java uses an abstract Shape class and an ArrayList to hold a list of shapes The file defines an applet in which the user can add various shapes to a drawing area Once a shape is in the drawing area the user can use the mouse to drag it around You might want to look at this file even though you won t be able to understand all of it at this time Even the definitions of the shape classes are somewhat different 87 from those that I have described in this section For example the draw method has a parameter of type Graphics This parameter is required because of the way Java handles all drawing Ill return to this example in later chapters when you know more about GUI programming However it would still be worthwhile to look at the definition of the Shape class and its subclasses in the source code You might also check how an ArrayList is used to hold the list of shapes If you click one of the buttons along the bottom of t
84. or non static is simple If the nested 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 using a name of the form 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 an inner 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 inner class object is permanently associated with the containing class object and it has complete access to the members of the containing class object Looking at an example will help and will hopefully convince you that inner classes are really very natural Consider a class that represents poker games This class might include a nested class to represent the players of the game This structure of the PokerGame class could be public class PokerGame Represents a game of poker private 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
85. 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 arcAngle 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 mea sured clockwise To get an arc of a circle make sure that width is equal to height 125 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 component you could say g fillRect 0 0 getWidth getHeight fillOval int x int y int width int height Draws a filled in oval fillRoundRect int x int y int width int height int xdiam int ydiam Draws a filled in rounded rectangle fi113DRect int x int y int width int height boolean raised Draws a filled in three dimensional rectangle
86. require mandatory handling are several that can occur when using Java s input output methods This means that you can t even use these methods unless you understand something about exception handling 9 3 5 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 cluttered 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 When a program encounters an exceptional condition and has no way of han dling it immediately the program can throw an exception In some cases it makes sense to throw an exception belonging to one of Java s predefined classes such as IllegalArgumentException or IOException However if there is no standard class that adequately represents the exceptional 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 if the programmer does not want to require manda tory exception handling the new class will extend RuntimeException or one of its subclasses To create a new exception class that does require mandatory handling the programmer can extend one of the other subclasses o
87. string that you would find unrecogniz able 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 An 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 computer are simply copied to and from the streams For reading and writing human readable character data the main classes are the abstract classes Reader and Writer All character stream classes are subclasses of one of these If a number is to be written to a Writer stream the computer must translate it into a human readable sequence of characters that represents that num ber 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 th
88. system that the component needs to be redrawn The repaint method returns immediately without doing any painting itself The sys 120 tem will call the component s paintComponent method later as soon as it gets a chance to do so after processing other pending 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 resized or if it 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 program should store data about the state of the component in its instance vari ables These variables should contain all the information necessary to redraw the component completely 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 that method will use the new values of the
89. 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 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 187 in this section Pointers are a notorious source of programming errors In Java a variable of ob ject 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
90. the execution of sequential commands The following is the factorial function written in a functional language called Lisp defun factorial n if lt n 1 1 x n factorial n 1 Notice that it defines the factorial function rather than give the steps to calculate it The factorial of n is defined as 1 ifn lt 1 else itis n factorial n 1 Logic Programming Prolog PROgramming in LOGic is the most widely available language in the logic programming paradigm It is based on the mathematical ideas of relations and log ical inference Prolog is a declarative language meaning that rather than describing how to compute a solution a program consists of a data base of facts and logical relationships rules which describe the relationships which hold for the given appli cation Rather then running a program to obtain a solution the user asks a question When asked a question the run time system searches through the data base of facts and rules to determine by logical deduction the answer Logic programming was an attempt to make a programming language that en abled the expression of logic instead of carefully specified instructions on the com puter In the logic programming language Prolog you supply a database of facts and rules you can then perform queries on the database This is also an example of a declarative style of programming where we state or define what we know In the following example we declare fact
91. 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 new GridLayout 1 0 5 5 buttonBar add button1 buttonBar add buttonz2 buttonBar add button3 You might find this button bar to be more attractive than the one that uses the default FlowLayout layout manager 6 7 2 A Simple Calculator As our next example we look briefly at an example that uses nested subpanels to build a more complex user interface The program has two JTextFields where the user can enter two numbers four JButtons that the user can click to add subtract 146 multiply or divide the two numbers and a JLabel that displays the result of the operation Like the previous example this example uses a main panel with a GridLayout that has four rows and one column In this case the layout is created with the statement setLayout new GridLayout
92. the entire contents of all the sub packages of the java packages by saying importjava Some programmers think that using a wildcard in an import statement is bad style since it can make a large number of class names available that you are not going to use and might not even know about They think it is better to explicitly import each individual class that you want to use In my own programming I often use wildcards to import all the classes from the most relevant packages and use individual imports when I am using just one or two classes from a given package In fact any JAVA program that uses a graphical user interface is likely to use many classes from the java awt and java swing packages as well as from another package named java awt event and I usually begin such programs with import java awt x import java awt event x import javax swing 45 9 A program that works with networking might include import java net while one that reads or writes files might use import java io But 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
93. the panel 6 2 2 Components and Layout Another way of using a JPanel is as a container to hold other components JAVA has many classes that define GUI components Before these components can appear on the screen they must be added to a container In this program the variable named content refers to a JPanel that is used as a container and two other components are added to that container This is done in the statements content add displayPanel BorderLayout CENTER content add okButton BorderLayout SOUTH Here content refers to an object of type JPanel later in the program this panel becomes the content pane of the window The first component that is added to content is displayPanel which as discussed above displays the message Hello World The second is okButton which represents the button that the user clicks to close the window The variable okButton is of type JButton the JAVA class that represents push buttons The BorderLayout stuff in these statements has to do with how the two com ponents are arranged in the container When components are added to a container there has to be some way of deciding how those components are arranged inside the container This is called laying out the components in the container and the most common technique for laying out components is to use a layout manager A layout manager is an object that implements some policy for how to arrange the components in a container
94. the specified type and runs the Student constructor The constructor s job is to properly initialize the object The String type is another example of an object type Student and String are composite types and give us the same advantages as the built in types The ability to create our own types is a very powerful idea in modern languages When declaring variables we can assign initial values If you do not specify ini tial values the compiler automatically assigns one Instance variables of numerical type int double etc are automatically initialized to zero boolean variables are initialized to false and char variables to the Unicode character with code number zero The default initial value of object types is null Introduction to Enums JAVA comes with eight built in primitive types and a large set of types that are de fined by classes such as String But even this large collection of types is not suffi cient to cover all the possible situations that a programmer might have to deal with So an essential part of JAVA just like almost any other programming language is the ability to create new types For the most part this is done by defining new classes But we will look here at one particular case the ability to define enums short for enumerated types Enums are a recent addition to JAVA They were only added in Version 5 0 Many programming languages have something similar Technically an enum is considered to be a special
95. the value 1 if the user s input is illegal However this only does any good if the main program bothers to test the return value It is very easy to be lazy about checking for special return values every time a method is called And in this case using 1 as a signal that an error has occurred makes it impossible to allow negative return values Exceptions are a cleaner way for a method to react when it encounters an error 9 4 Assertions WE END THIS CHAPTER WITH A SHORT SECTION ON ASSERTIONS another feature of the Java programming language that can be used to aid in the development of correct and robust programs 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 for example if it depends on input from the user then it s a good idea to insert 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 In many cases of course instead of using an if statement to test whether a precon dition holds a programmer tries to write the program in a way that will guarantee that the precondition holds In that case the test should not be necessary and the
96. 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 value of N has become too large break N 3x N 1 System out println N 189 The problem here is not that the original algorithm for computing 3N 1 se quences was wrong The problem is that it just can t be correctly implemented using 32 bit integers Many programs ignore this type of problem But integer overflow errors have been responsible for their share of serious computer failures and a com pletely 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 over flow errors which occur when the result of a computation is outside the range of val ues that can be represented as a value of type double This range extends up to about 1 7 x 10 98 Numbers beyond this range do not wrap around to negative values In stead they are represented by special values that have no real numerical equivalent The special values Double POSITIVE_INFINITY and Double NEGATIVE_INFINITY rep resent numbers outside the range of legal values For example 20 x 10 is computed to be Double POSITIVE_INFINITY Another special value of type double Double NaN represents an illegal or undefined result N
97. 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 An actor is an external entity outside of the system that interacts with the sys tem 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 cus tomer 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 68 o Umbrello UML Modeller Al ol x Eile Edit Diagram Code Settings Help HSae xinG UML Diagrams e x i i Ajclass diagram Book Mant fom G jUse Case View DEG gt i St H fl Book flight gm aps x Book flight Book flight ty i tes r Il ee i A Y K Call center Book flight via phone Book flight at Travel
98. to define the subclass as an anonymous nested class As an example I present HelloWorldGUI4 java This version is a variation of HelloWorldGUI2 java that uses anonymous nested classes where the original program uses ordinary named nested classes import java awt x import java awt event x import javax swing x x A simple GUI program that creates and opens a JFrame containing x the message Hello World and an OK button When the user clicks x the OK button the program ends This version uses anonymous x classes to define the message display panel and the action listener x object Compare to HelloWorldGUI2 which uses nested classes public class HelloWorldGUI4 x The main program creates a window containing a HelloWorldDisplay x and a button that will end the program when the user clicks it 136 public static void main String args JPanel displayPanel new JPanel An anonymous subclass of JPanel that displays Hello World public void paintComponent Graphics g super paintComponent g g drawString Hello World 20 30 JButton okButton new JButton OK okButton addActionListener new ActionListener An anonymous class that defines the listener object public void actionPerformed ActionEvent e System exit 0 JPanel content new JPanel content setLayout new BorderLayout content add displayPanel BorderLayout CENTER content add okButton
99. type Day This will throw an exception of type IllegalArgumentException if the user s response is not the name of one of the values of type Day but we can respond to the error easily enough by asking the user to enter another response Here is a code segment that does this Converting the user s response to upper case will allow responses such as Monday or monday in addition to MONDAY 48 Scanner keyboard new Scanner System in Day weekday User s response as a value of type Day while true String response User s response as a String keyboard put Please enter a day of the week response keyboard nextLinen response response toUpperCase try weekday Day valueOf response break catch IllegalArgumentException e System out println response is not the name of a day of the week The break statement will be reached only if the user s response is acceptable and so the loop will end only when a legal value has been assigned to weekday 2 4 Javadoc Good programming means extensive comments and documentation At the very least explain the method of each instance variable and for each method explain its pur pose 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
100. 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 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 on demand 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 6 4 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 shows a 16 by 10 pixel component 121 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
101. 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 main I O abstractions are called streams Other I O abstractions such as files and channels also exist but in this section we will look only at streams Every stream represents either a source of input or a destination to which output can be sent 10 1 1 Character and Byte Streams 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 binary form the same way that data is represented 207 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 read ing a sequence of characters and interpreting them as a number The same number would be represented in the computer as a bit
102. you want to declare a variable named rectColor of type java awt Color You could say java awt Color rectColor This is just an ordinary variable declaration of the form type name variable name Of course using the full name of every class can get tiresome so JAVA makes it possible to avoid using the full names of a class by importing the class 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 abbreviate the full name java awt Color to just the simple name of the class Color Note that the import line comes at the start of a file and is not inside any class Although it is sometimes referred to as as a statement it is more properly called an import directive since it is not a statement in the usual sense Using this import directive would allow you to say Color rectColor to declare the variable Note that the only effect of the import directive 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 directive 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 The is a wildcard that matches every class in the package However it does not match sub packages you cannot import
103. 2 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 x import java awt x 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 public void paintComponent Graphics g super paintComponent g for int i 0 i lt 13 i allPiles i draw g 163 public static void main String args JFrame frame new JFrame frame setDefaultCloseOperation JFrame EXIT_ON_CLOSE frame setVisible true frame
104. 27 1 2 4 Creating and Destroying Objects 29 1 2 5 Garbage Collection 0 0 00002 eee 34 1 2 6 Everything is NOT an object 00040 35 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 Program ming 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 ob jects entities that have behaviors that hold information and that can interact with one another Programming consists of designing a set of objects that model the prob lem 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 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 1 1 What is Object Oriented Programming OBJECT ORIENTATION is a set of tools and methods that enable software engineers to build reliable user friendly maintaina
105. 6 control off its normal track When an error occurs during the execution of a program the default behavior is to terminate the program and to print an error message How ever Java makes it possible to catch such errors and program a response different from simply letting the program crash This is done with the try catch statement In this section we will take a preliminary incomplete look at using try catch to handle errors Exceptions The term exception is used to refer to the type of error that one might want to handle with a try catch An exception is an exception to the normal flow of control in the program The term is used in preference to error because in some cases an exception might not be considered to be an error at all You can sometimes think of an exception as just another way to organize a program Exceptions in Java are represented as objects of type Exception Actual excep tions are defined by subclasses of Exception Different subclasses represent differ ent types of exceptions We will look at only two types of exception in this section NumberFormatException and IllegalArgumentException A NumberFormatException can occur when an attempt is made to convert a string into a number Such conversions are done for example by Integer parseInt and Integer parseDouble Consider the method call Integer parseInt str where str is a variable of type String If the value of str is the string 42 then the meth
106. 8 nication The first try statement is there to make sure that we don t even try to communicate over the network unless we have successfully opened a connection The pseudocode in this example follows a general pattern that can be used to robustly obtain a resource use the resource and then release the resource 9 3 3 Throwing Exceptions There are times when it makes sense for a program to deliberately throw an excep tion 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 This can be done with a throw statement In this section we cover the throw statement more fully 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 ob ject if e refers to the object the error message can be retrieved by calling e getMessage You might find this example a bit odd because you might ex pect the system itself to
107. BorderLayout SOUTH JFrame window new JFrame GUI Test window setContentPane content window setSize 250 100 window setLocation 100 100 window setVisible true 6 6 Basic 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 lim iting 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 but ton 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 dis abled 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 released In fact it
108. Comparable interface and for defining several different orderings 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 8 7 Generics and Wrapper Classes AS NOTED ABOVE JAVA S GENERIC PROGRAMMING does not apply to the primitive types since generic data structures can only hold objects while values of primitive type are not objects However the wrapper classes make it possible to get around this restriction to a great extent Recall that each primitive type has an associated wrapper class class Integer for type int class Boolean for type boolean class Character for type char and so on An object of type Integer contains a value of type int The object serves as a wrapper for the primitive type value which allows it to be used in contexts where objects are required such as in generic data structures For example a list of Inte gers can be stored in a variable of type ArrayList lt Integer gt and interfaces such as Collection lt Integer gt and Set lt Integer gt are defined Furthermore class Integer defines equals compareTo and toString methods that do what you would expect that is that compare and write out the corresponding primitive type values in the usual way Similar remarks apply for all the wrapper classes Recall also that Java does automatic conversions between a primitive type and the corresponding wrapper
109. EA eae eee a es a E 4 6 thisand super lt a T hee eo 4 WE eS ale BS Se eA 4 6 1 The Special Variable this 0000 4 6 2 The Special Variable super 0 0000 eee 4 6 3 Constructors in Subclasses 2 2 000005 Interfaces Nested Classes and Other Details Bids Interfaces sch 8 cake he eS Sek Roath ec eS SES COO Ra 5 2 Nested Classes 0 5 2 1 Anonymous Inner Classes 0 0000 ee eens 5 3 Mixing Static and Non static 20 0 0 ee ee ee ee 5 3 1 Static Import 2 hn ete ee Ae he oie Se 5 4 Bintims as Classes 2 034 ose g a ee SS ee ee ee eae oe Graphical User Interfaces in JAVA 6 1 Introduction The Modern User Interface 6 2 The Basic GUI Application 2 0 00 0 eee ee ee 6 2 1 JFrame and JPanel 0 0000 eee ene 6 2 2 Components and Layout 0000 eee eens 6 2 3 Events and Listeners 0 0000 eee eens 6 38 AppletsandHTML 0 00 000 cee ee ee es 638k cJApplet i ae Hes ct Ane Hee G8 ile SO eee ee a RES eas te EM 6 3 2 Reusing Your JPanels 0 0 000 eee eee ne 6 3 3 Applets on Web Pages 0 000 eee eee ne 6 4 Graphics and Painting 0 0 ce ee AN A 6 41 Coordinates Pe iain a Go Bin eee BM i Ge i Ee ee 64 2 gt Colors otya aie nt chnst HP date tau SINE oR aed Sic OD able hah abe tien 6 43 Fonts 5 23 a AR ad ane Sort OG ond ae teal amp MG So i fev sl GAA Shapes os 2 55 mi
110. 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 baseline 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 124 e drawLine int x1 int yl 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 colors the single pixel with upper left corner at the point x y 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
111. JFrame EXIT_ON_CLOSE This says that when the user closes the window by clicking the close box in the title bar of the window the program should be terminated This is necessary because no other way is provided to end the program Without this line the default close operation of the window would simply hide the window when the user clicks the close box leaving the program running This brings up one of the difficulties of reusing the same panel class both in an applet and in a frame There are some things that a stand alone application can do that an applet can t do Terminating the program is one of those things If an applet calls System exit it has no effect except to generate an error Nevertheless in spite of occasional minor difficulties many of the GUI examples in this book will be written as subclasses of JPanel that can be used either in an applet or in a frame 6 3 3 Applets on Web Pages The lt applet gt tag can be 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 class The modifiers height and width are required to specify the size of the applet in pixels 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 cen
112. JPanels in the program content which is used to contain other components and displayPanel which is used as a drawing surface Let s look more closely at displayPanel displayPanel is a variable of type HelloWorldDisplay which is a nested static class inside the HelloWorldGUI2 class This class defines just one instance method paintComponent which overrides a method of the same name in the JPanel class 110 private static class HelloWorldDisplay extends JPanel public void paintComponent Graphics g super paintComponent g g drawString Hello World 20 30 The paintComponent method is called by the system when a component needs to be painted on the screen In the JPanel class the paintComponent method simply fills the panel with the panel s background color The paintComponent method in HelloWorldDisplay begins by calling super paintComponent g This calls the version of paintComponent that is defined in the superclass JPanel that is it fills the panel with the background color Then it calls g drawString to paint the string Hello World onto the panel The net result is that whenever a HelloWorldDisplay is shown on the screen it displays the string Hello World We will often use JPanels in this way as drawing surfaces Usually when we do this we will define a nested class that is a subclass of JPanel and we will write a paintComponent method in that class to draw the desired content in
113. ML commands in the comments to the web pages that it creates As an example you can add lt p gt to indicate the start of a new paragraph Generally in the absence of HTML commands blank lines and extra spaces in the comment are ignored In addition to HTML commands Javadoc comments can include doc tags which are processed as commands by the javadoc tool A doc tag has a name that begins with the character I will only discuss three tags G param return and throws These tags are used in Javadoc comments for methods to provide information about its parameters its return value and the exceptions that it might throw These tags are always placed at the end of the comment after any description of the method itself The syntax for using them is param parameter name description of parameter return description of return value throws exception class name description of exception The descriptions can extend over several lines The description ends at the next tag or at the end of the comment You can include a param tag for every parameter of the method and a throws for as many types of exception as you want to document You should have a return tag only for a non void method These tags do not have to be given in any particular order Here is an example that doesn t do anything exciting but that does use all three types of doc tag If you want to create Web page documentation you need to run the javadoc tool
114. Object Oriented Programming School of Computer Science University of KwaZulu Natal February 5 2007 Object Oriented Programming using Java Notes for the Computer Science Module Object Oriented Programming COMP200 Adapted from Introduction to Programming Using Java Version 5 0 December 2006 by David J Eck http math hws edu javanotes Adapted by Anban Pillay School of Computer Science University of KwaZulu Natal Durban February 2007 Contents 1 Introduction to Objects 1 1 What is Object Oriented Programming 2 0004 1 1 1 Programming Paradigms 02 200005 1 1 2 Object Orientation as a New Paradigm The Big Picture 1 2 Fundamentals of Objects and Classes 0 20004 1 2 1 Objects and Classes 2 a a EAA ee es 1 2 2 Class Members and Instance Members 1 2 3 Aecess Control aci ale Bee hele ewe hs Rake aa eats 1 2 4 Creating and Destroying Objects 204 1 2 5 Garbage Collection 0 0 00 ee eee ee eee 1 2 6 Everything is NOT an object oaa The Practice of Programming 21 sADStRACtION sa gerria ea 3b rok Shox ted a SAR nts Sok hth 2 1 1 Control Abstraction 0 0 ee 2k27 Data Abstraction awenn ene n Sane eee ee eB A 2 1 3 Abstraction in Object Oriented Programs 2 2 Methods as an Abstraction Mechanism 0 2251 Black Boxes rsa s bd doe Boe be yal Qe ate he 2 2 2 Preco
115. Pile 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 165 166 Generic Programming Contents 8 1 Generic Programming in Java 000ceeeeeee 168 8 2 Array Lists esws Slee ia a a ee ees ee ein 168 8 3 Parameterized Types 0 00 cece eecvcccces 170 8 4 The Java Collection Framework 2 6 172 8 5 Iterators and for each Loops 22eeeeeee08 174 8 6 Equality and Comparison 000 cee eeeeee 176 8 7 Generics and Wrapper Classes 2 2eeeeeee008 179 8 8 Lists cock are eee 2 Sea e ee Selec Rae Be OS ee Se 179 A DATA STRUCTURE IS A COLLECTION OF DATA ITEMS considered as a unit For exam ple a list is a data structure that consists simply of a sequence of items Data struc tures play an important part in programming Various standard data structures have been developed including lists sets and trees Most programming libraries provide built in data structures that may be used with very little effort from the programmer Java has the Collection Framework that provides standard data structures for use by programmers Generic programming refers to writing code that will work for many types of data The source code presented there for working with dynamic arrays of integers works only for data of type int But the source code for dynamic arrays of double String JButton
116. The type parameter has no effect at run time and is not even known at run time The type information is said to be erased at run time This type erasure introduces a certain amount of weirdness For example you can t test if list instanceof ArrayList lt String gt because the instanceof operator is evaluated at run time and at run time only the plain ArrayList ex ists Even worse you can t create an array that has base type ArrayList lt String gt using the new operator as in new ArrayList lt String gt N This is because the new operator is evaluated at run time and at run time there is no such thing as ArrayList lt String gt only the non parameterized type ArrayList exists at run time Fortunately most programmers don t have to deal with such problems since they turn up only in fairly advanced programming Most people who use the Java Collec tion Framework will not encounter them and they will get the benefits of type safe generic programming with little difficulty 8 2 ArrayLists IN THIS SECTION we discuss ArrayLists that are part of the Collection Framework Arrays in JAVA have two disadvantages they have a fixed size and their type must be must be specified when they are created The size of an array is fixed when it is created In many cases however the number of data items that are actually stored in the array varies with time Consider 168 the following examples An array that stores the lin
117. UBS This enumerated type represents the four possible suits for a playing card and it could have been used in the example Card java Furthermore in addition to its list of values an enumerated type can contain some of the other things that a regular class can contain including methods and additional member variables Just add a semicolon at the end of the list of values and then add definitions of the methods and variables in the usual way For example we might make an enumerated type to represent the possible values of a playing card It might be useful to have a method that returns the corresponding value in the game of Blackjack As another example suppose that when we print out one of 101 the values we d like to see something different from the default string representation the identifier that names the constant In that case we can override the toString method in the class to print out a different string representation This would gives something like public enum CardValue ACE TWO THREE FOUR FIVE SIX SEVEN EIGHT NINE TEN JACK QUEEN KING x Return the value of this CardValue in the game of Blackjack x Note that the value returned for an ace is 1 public int blackJackValue if this JACK this QUEEN this KING return 10 else return 1 ordinal x Return a String representation of this CardValue using numbers x for the numerical cards and names for the
118. aN stands for Not a Number For ex ample 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 is this special non a number value by calling the boolean valued method 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 infi nite number of digits after the decimal point A value of type double is only accu rate 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 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 So you see that not all possible errors are avoided or detected automatically in Java Furthermore even when an error is detected automatically the system s de fault response is to report the error and terminate the program This is hardly robust behavior So a Java programmer still needs to learn techniques for avoiding and dealing with errors These are the main topics of the rest of this chapter 9 2 Writing Correct Programs CORRECT PROGRAMS DON T
119. ace and face cards public String toString switch this this is one of the enumerated type values case ACE ordinal number of ACE return Ace case JACK ordinal number of JACK return Jack case QUEEN ordinal number of QUEEN return Queen case KING ordinal number of KING return King default it s a numeric card value int numericValue 1 ordinal return numericValue end CardValue blackjackValue and toString are instance methods in CardValue Since CardValue JACK is an object belonging to the class CardValue you can off course call CardValue JACK blackjackValue Suppose that cardVal is declared to be a variable of type CardValue so that it can refer to any of the values in the enumer ated type We can call cardVal blackjackValue to find the Blackjack value of the CardValue object to which cardVal refers and System out println cardVal will implicitly call the method cardVal toString to obtain the print representation of that CardValue One other thing to keep in mind is that since CardValue is a class the value of cardVal can be null which means it does not refer to any object Remember that ACE TWO KING are the only possible objects of type CardValue so in an instance methods in that class this will refer to one of those values Recall that the instance method ordinal is defined in any enumerated type and gives the position of the enumerated ty
120. acter oriented classes the I O operations read and write characters rather than bytes The return value of read is 1 if the end of the input stream has been reached Otherwise the return value must be type cast to type char to obtain the character that was read In practice you will ordinarily use higher level I O operations pro vided by sub classes of Reader and Writer as discussed below 10 1 2 PrintWriter 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 capabilities 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 meth ods 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 sub classes 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 209 PrintWriter printableCharSink new PrintWriter charSink When you output data to printableCharSink using the high level output meth ods in Pri
121. actually im plemented 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 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 col lected 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 older Windows computers had so many memory leaks that the computer would run out of memory after a few days of use and would have to be restarted Many programs have be
122. 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 end 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 argu ments 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 x Makes a copy of a file The original file and the name of the x copy must be given as command line arguments In addition the x first command line argument can be f if present the program x will overwrite an existing file if not the program will report x an error and end if the output file already exists The number x of bytes that are copied is reported public class CopyFile public static void main String args String sourceName Name of the source file specified on the command line String copyName Name of the copy specified on the command line InputStream sourc
123. al cards this is one of the values x 1 through 13 with 1 representing ACE The value cannot be changed x after the card is constructed private final int value xx Creates a card with a specified suit and value x param theValue the value of the new card For a regular card non joker the value must be in the range 1 through 13 with 1 representing an Ace You can use the constants Card ACE Card JACK Card QUEEN and Card KING For a Joker the value can be anything param theSuit the suit of the new card This must be one of the values Card SPADES Card HEARTS Card DIAMONDS Card CLUBS or Card JOKER throws IllegalArgumentException if the parameter values are not in the x permissible ranges public Card int theValue int theSuit if theSuit SPADES amp amp theSuit HEARTS amp amp theSuit DIAMONDS amp amp theSuit CLUBS throw new IllegalArgumentException Illegal playing card suit if theValue lt 1 theValue gt 13 throw new IllegalArgumentException IIlegal playing card value value theValue suit theSuit x xx Returns the suit of this card x returns the suit which is one of the constants Card SPADES Card HEARTS Card DIAMONDS Card CLUBS public int getSuit return suit xx Returns the value of this card x return the value which is one the numbers 1 through 13 x public int getValue return value 57 xx Returns a String representation
124. alue gameInProgress false message Too bad You lose on ties else if cardCt 4 gameInProgress false message You win You mace three correct guesses else message Got it right Try for cardCt repaint end doHigher The paintComponent method of the CardPanel class uses the values in the state variables to decide what to show It displays the string stored in the message vari able 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 deck 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 method decides where to draw each card and calls this method to do the drawing You can check out all the details in the source code HighLowGUI java One further note on the programming of this example The source code defines HighLowGUI as a subclass of JPanel The class contains a main method so that it can be run as a stand alone application the main method simply opens a window that uses a panel of type JPanel as its content pane In addition I decided to write an applet version of the program as a static nested class named Applet inside the HighLowGUI class Since this is a nested cl
125. am creates one or more such components and displays them on the computer screen Very often that s all it does Once a GUI component has been created it follows its own programming programming that tells it how to draw itself on the screen and how to respond to events such as being clicked on by the user A GUI program doesn t have to be immensely complex We can for example write a very simple GUI Hello World program that says Hello to the user but does it by opening a window where the greeting is displayed import javax swing JOptionPane public class HelloWorldGUI1 public static void main String args JOptionPane showMessageDialog null Hello World When this program is run a window appears on the screen that contains the mes sage Hello World The window also contains an OK button for the user to click after reading the message When the user clicks this button the window closes and the program ends By the way this program can be placed in a file named HelloWorldGUI1 java compiled and run just like any other JAVA program Now this program is already doing some pretty fancy stuff It creates a window it draws the contents of that window and it handles the event that is generated when the user clicks the button The reason the program was so easy to write is that all the work is done by showMessageDialog a static method in the built in class JOptionPane Note the source code
126. among all the files available to the computer It contains full information about which directory the file is in and what the file s name is A relative path name tells the computer how to locate the file starting from the current directory 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 It is possible for a Java program to find out the absolute path names for two important directories the current directory and the user s home directory The names of these directories are system properties and they can be read using the method calls e System getProperty user dir returns the absolute path name of the cur rent directory as a String e System getProperty user home returns the absolute path name of the user s home directory as a String To avoid some of the problems caused by differences in path names between plat forms 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 obje
127. 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 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 relation ships 3 3 2 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 Diagrams 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 sys tem 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
128. an value that is true if object is in the collection Note that object is not required to be of type T since it makes sense to check whether object is in the collection no matter what type object has For testing equality null is considered to be equal to itself The criterion for testing non null objects for equality can differ from one kind of collection to another coll remove object removes object from the collection if it occurs in the collection and returns a boolean value that tells you whether the object was found Again object is not required to be of type T coll containsAl1 coll2 returns a boolean value that is true if every object in col12 is also in the coll The parameter can be any collection coll addAl1l coll2 adds all the objects in coll2 to coll The parameter coll2 can be any collection of type Collection lt T gt However it can also be more general For example if T is a class and S is a sub class of T then coll2 can be of type Collection lt S gt This makes sense because any object of type S is automatically of type T and so can legally be added to coll coll removeAl1 col12 removes every object from coll that also occurs in the collection col12 col1l2 can be any collection coll retainAl1l coll2 removes every object from coll that does not occur in the collection coll2 It retains only the objects that do occur in col12 coll2 can be any collection coll toArray returns an array of t
129. and 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 making subclasses Access modifiers such as public and private are used to control access to mem bers of a class There is one more access modifier protected that comes into the picture when subclasses are taken into consideration When protected is applied as an access modifier to a method or member variable in a class that member can be used in subclasses direct or indirect of the class in which it is defined but it cannot be used in non subclasses There is one exception A protected member can also be accessed by any class in the same package as the class that contains the protected member Recall that using no access modifier makes a member accessible to classes in the same package and nowhere else Using the protected modifier is strictly more liberal than using no modifier at all It allows access from classes in the same package and from subclasses that are not in the same package When you declare a method or member variable to be protected you are saying that it is part of the implementation of the class rather than part of the public inter face of the class However you are allowing subclasses to use and modify that part of the implementation For example consider a PairOfDice class that has instance variables diel and die2 to represent the numbers appea
130. 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 75 Fo screen shots xmi Umbrello UML Modeller ok Eile Edit Diagram Code Settings Help PECOIEDE UML Diagrams fay Views amp jLogical View jactivity diagram f i P class diagram class diagram_1 i Business Process Make Reservation sequence diagram authentification failed i Gy state diagram Use Case View customer identified Take reservation wish Check available space S cancel gt ok book reservation Reservation canceled Confirm reservation to customer i COOOFEAE no space available Documentation Process sucessfully finished Figure 3 11 Umbrello UML Modeller showing an Activity Diagram 76 Inheritance Polymorphism and Abstract Classes Contents 4 1 Extending Existing Classes 00 cee eeeeee 77 4 2 Inheritance and Class Hierarchy 66 80 4 3 Example Vehicles 0 cee ececrvcessvecee 81 44 Polymorphism 0 eee www eee terre eens 83 4 5 Abstract Classes 2 2 2c cee ewer er ee eee eee 86 4 6 this and Super i 5 66 32608 scans oe Wee wh 88 4 6
131. ass its full name is HighLowGUI Applet and the class file produced when the code is compiled is HighLowGUI Applet class This class is used for the applet version of the program shown above The lt applet gt tag lists the class file for the applet as code HighLowGUI Applet class This is admittedly an unusual way to organize the program and it is probably more natu ral to have the panel applet and stand alone program defined in separate classes However writing the program in this way does show the flexibility of JAVA classes Simple dialogs 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 hope fully reads the message and dismisses the dialog by clicking the OK button A message dialog can be shown by calling the static method void JOptionPane showMessageDialog Component parentComp String message 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 automat
132. ass method directly from the class whereas you must invoke instance methods on a particular instance e g The methods in the Math class are static and can be invoked without creating an instance of the Math class for e g we can say Math sqrt x Consider a simple class whose job is to group together a few static member vari ables for example a class could be used to store information about the person who is using the program class UserData static String name static int age In programs that use this class there is one copy 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 int 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 There is nothing much 22 in the class 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 vari ables 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 ag
133. ass will have a constructor 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 55 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 It might be better to use an enumerated type but for now we will stick to integer valued constants Pll return to the question of using enumerated types in this example at the end of the chapter The possible values of a card are the numbers 1 2 18 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 A Card object can be constructed knowing the value and the suit of the card For example we can call the constructor with statements such as cardl 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 Pve made these private so that they cannot be changed from outside the class and I ve pro vided getter methods getSuit and g
134. at student Objects of type Student can be created with statements such as std new Student John Smith std1 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 the 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 indi rectly when it calls the constructor I ve provided a method getName that can be used from outside the class to find out the name of the student But I haven t provided any setter method or other way to change the name Once a student object is created it keeps the same name as long as it exists 1 2 5 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 t
135. ated 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 74 M screen shots xmi Umbrello UML Modeller ol x Eile Edit Diagram Code Settings Help POECILE UML Diagrams Server States g attra OC Ea listen Request arrived Processing done Documentation destroy End b 4 4 b Ready Figure 3 10 Umbrello UML Modeller showing a State Diagram 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 exe cution 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 Component 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
136. ave different values stored in their instance variables For e g each Student object will have a different number stored in its StudentNumber variable Encapsulation Object diagrams show that an objects variables make up the center or nucleus of the object Methods surround and hide the object s nucleus from other objects in the program Packaging an object s variables within the protective custody of its methods is called encapsulation This discussion is based on the Object oriented Programming Concepts section of the Java Tuto rial by Sun MicroSystems 16 Message lt gt Object A aD Object B Figure 1 2 A Message Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two benefits to software developers e Modularity The source code for an object can be written and maintained in dependently of the source code for other objects Also an object can be easily passed around in the system You can give your bicycle to someone else and it will still work e Information hiding An object has a public interface that other objects can use to communicate with it The object can maintain private information and meth ods that can be changed at any time without affecting other objects that depend on it Messages Software objects interact and communicate with each other by sending messages to each other When object A wants object B to perform
137. awable void draw Graphics g g setColor Color red g fi11Rect 10 10 100 100 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 sep arate class file If the name of the main class is MainClass for example then the 98 names of the class files for the anonymous nested classes will be MainClass 1 class MainClass 2 class MainClass 3 class and so on 5 3 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 methods Or it can be used as a factory for making objects The non static variables and methods 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 sepa rate 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 methods An instance method belongs to an object not to the class itself and there can be many objects with their own versions of the insta
138. awn by the video card 152 Since it s just a set of numbers the data for an image doesn t have to be stored in a frame buffer It can be stored elsewhere in the computer s memory It can be stored in a file on the computer s hard disk Just like any other data file an image file can be downloaded over the Internet Java includes standard classes and methods that can be used to copy image data from one part of memory to another and to get data from an image file and use it to display the image on the screen 6 8 1 Images The class java awt Image represents an image stored in the computer s memory There are two fundamentally different types of Image One kind represents an im age read from a source outside the program such as from a file on the computer s hard disk or over a network connection The second type is an image created by the program I refer to this second type as an off screen canvas An off screen canvas is region of the computer s memory that can be used as a drawing surface It is possible to draw to an offscreen image using the same Graphics class that is used for drawing on the screen An Image of either type can be copied onto the screen or onto an off screen can vas using methods that are defined in the Graphics class This is most commonly done in the paintComponent method of a JComponent Suppose that g is the Graph ics object that is provided as a parameter to the paintComponent method and that im
139. ayList lt String gt object We will look at various classes that implement the list and set interfaces in the next section But before we do that we ll look briefly at some of the general operations that are available for all collections The interface Collection lt T gt specifies methods for performing some basic opera 172 tions on any collection of objects Since collection is a very general concept oper ations 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 con taining various types of objects Suppose that coll is an object that implements the interface Collection lt T gt for some specific non primitive type T Then the following operations which are specified in the interface Collection lt T gt are defined for coll coll size returns an int that gives the number of objects in the collection coll isEmpty returns a boolean value which is true if the size of the collec tion is 0 coll clear removes all objects from the collection coll add tobject adds tobject to the collection The parameter must be of type T if not a syntax error occurs at compile time This method returns a boolean value which tells you whether the operation actually modified the collection For example adding an object to a Set has no effect if that object was already in the set coll contains object returns a boole
140. behaviors of windows already programmed in In particular it comes 109 with the basic properties shared by all windows such as a titlebar and the ability to be opened and closed Since a JFrame comes with these behaviors you don t have to program them yourself This is of course one of the central ideas of object oriented programming What a JFrame doesn t come with of course is content the stuff that is contained in the window If you don t add any other content to a JFrame it will just display a large blank area You can add content either by creating a JFrame object and then adding the content to it or by creating a subclass of JFrame and adding the content in the constructor of that subclass The main program above declares a variable window of type JFrame and sets it to refer to a new window object with the statement JFrame window new JFrame GUI Test The parameter in the constructor GUI Test specifies the title that will be dis played in the titlebar of the window This line creates the window object but the window itself is not yet visible on the screen Before making the window visible some of its properties are set with these statements window setContentPane content window setSize 250 100 window setLocation 100 100 The first line here sets the content of the window The content itself was created earlier in the main program The second line says that the window will be 250 pixels wide
141. ble well documented reusable software 1This discussion is based on Chapter 2 of An Introduction to Object Oriented Programming by Tim othy Budd 11 systems that fulfills the requirements of its users It is claimed that object orientation provides software developers with new mind tools to use in solving a wide variety of problems Object orientation provides a new view of computation A software system is seen as a community of objects that cooperate with with each other by passing messages in solving a problem An object oriented programming laguage provides support for the following object oriented concepts Objects and Classes Inheritance Polymophism and Dynamic binding 1 1 1 Programming Paradigms Object oriented programming is one of several programming paradigms Other pro gramming paradigms include the imperative programming paradigm as exemplified by languages such as Pascal or C the logic programming paradigm Prolog and the functional programming paradigm exemplified by languages such as ML Haskell or Lisp Logic and functional languages are said to be declarative languages We use the word paradigm to mean any example or model This usage of the word was popularised by the science historian Thomas Kuhn He used the term to describe a set of theories standards and methods that together represent a way of organising knowledge a way of viewing the world Thus a programming paradigm is a Way of c
142. blic class PairOfDice public int diel public int die2 3 Number showing on the first die 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 diel and die2 are initialized to the values 3 and 4 respec tively These initializations are executed whenever a PairOfDice object is constructed It is important to understand when and how this happens Many PairOfDice objects may exist Each time one is created it gets its own instance variables and the assign ments 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 int Math random 6 1 int Math random 6 1 public int diel public int die2 public void roll diel int Math random 6 1 die2 int Math random 6 1 end class PairOfDice Here the dice are initialized to 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 29 pairs of dice can have different initial values For initialization of static member variables
143. boxes and 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 and JFrame be long to subclasses of the class javax swing JComponent which is itself a subclass of java awt Component Every component is responsible for drawing itself If you want to use a standard component you only have to add it to your applet or frame You don t have to worry about painting it on the screen That will happen automatically since it already knows how to draw itself 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 I will always use a subclass of JPanel when I need a drawing surface of this kind 119 as I did for the MessageDisplay class in the example HelloWorldApplet java in the previous section A JPanel like any JComponent draws its content in the method public void paintComponent 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 this class and use it in your applet or frame When the time comes for your component to be drawn on t
144. bs 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 shuffle and dealCard Cards can be added to and removed from hands This gives two candidates for instance methods in a Hand class addCard and removeCard Cards are relatively passive things but we need to be able to determine their suits and values We will discover more instance methods as we go along The Deck Class 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 method with a return type of Card since the caller needs to know what card is being dealt It has no parameters when you deal the next card from the deck you don t provide any information to the deck you just get the next card whatever it is 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 so the deck can throw an exception in that case But this raises another questio
145. 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 fromIndex 1 A toIndex 1 into ascending order You can use Arrays sort A 0 N 1 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 imple ment 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 183 184 Chapter Correctness and Robustness Contents 9 1 Introduction sese See we ie levee nike Ps ee Be a 186 9 1 1 Horror Stories e 6b sa a Oa a EEKEREN 186 9 1 2 Java to the Rescue 3 4 44 soa cs bobo ae sa PRE aaa ee 187 9 1 3 Problems Remain in Java 0000050005 189 9 2 Writing Correct Programs ssessseesecsesseeeee 190 9 2 1 Provably Correct Programs aoaaa aaae 190 9 2 2 Robust Handling of Input aoaaa 193 9 3 Exceptions and try catch 0 00 ee eee eee eens 194 9 3 1 Exceptions and Exception Classes 194 9 3 2 The try Statement 0 00000 eee eae 196 9 3 3 Throwing Exceptions 0 0005 ee ee 199 9 3 4 Mandatory Exception Handling 200 9 3 5 Programming with Exceptions 201 9 4 AssertionS
146. card from the deck will be higher or lower than the current card If the user predicts correctly then the next card from the deck becomes the current card and the user makes another prediction This continues until the user makes an incorrect prediction The number of correct predictions is the user s score 58 My program has a method that plays one game of HighLow This method has a return value that represents the user s score in the game The main method lets the user play several games of HighLow At the end it reports the user s average score Note that the method that plays one game of HighLow returns the user s score in the game as its return value This gets the score back to the main program where it is needed Here is the program import java util Scanner x x This program lets the user play HighLow a simple card game x that is described in the output statements at the beginning of x the main method After the user plays several games x the user s average score is reported public class HighLow Scanner keyboard new Scanner System in public static void main String args System out println This program lets you play the simple card game System out println HighLow A card is dealt from a deck of cards System out println You have to predict whether the next card will be System out println higher or lower Your score in the game is the System out println number of c
147. ce that it is the interface that is platform independent the implementation varies from one platform to another A JAVA system on a particular computer includes implementations of all the standard API methods A JAVA program includes only calls to those methods When the JAVA interpreter executes a program and encounters a call to one of the standard methods it will pull up and execute the implementation of that method 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 JAVA s Standard Packages Like all methods in JAVA the methods in the standard API are grouped into classes To provide larger scale organization classes in JAVA can be grouped into packages which were introduced briefly in Subection2 6 4 You can have even higher levels of grouping since packages can also contain other packages In fact the entire stan dard JAVA API is implemented in several packages One of these which is named 43 java contains several non GUI packages as well as the original AWT graphics user interface classes Another package javax was added in JAVA version 1 2 and con tains the classes used by the Swing graphical user interface and other additions to the API A package can contain both classes and other packages A package that is con tained in another package is sometimes called a sub packag
148. ck as a nested subclass although it could just as easily be a separate class The main point is that this program uses the same RandomStringsPanel class that was used in the original program which did not re spond to mouse clicks The mouse handling has been bolted on to an existing class without having to make any changes at all to that class import java awt Component import java awt event MouseEvent import java awt event MouseListener import javax swing JFrame 133 Displays a window that shows 25 copies of the string Java in random colors fonts and positions The content of the window is an object of type RandomStringsPanel When the user clicks the window the content of the window is repainted with the x strings in newly selected random colors fonts and positions public class ClickableRandomStringsApp public static void main String args JFrame window new JFrame Random Strings RandomStringsPanel content new RandomStringsPanel content addMouseListener new RepaintOnClick Register mouse listener window setContentPane content window setDefaultCloseOperation JFrame EXIT_ON_CLOSE window setLocation 100 75 window setSize 300 240 window setVisible true private static class RepaintOnClick implements MouseListener public void mousePressed MouseEvent evt Component source Component evt getSource source repaint public void mo
149. common properties of all its subclasses A class that is not abstract is said to be concrete You can create objects belonging to a concrete class but not to an abstract class A variable whose type is given by an abstract class can only refer to objects that belong to concrete subclasses of the abstract class 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 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 computer syntactically that they are abstract by adding the modifier abstract to their definitions For an abstract method the block of code that gives the implemen tation 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 public abstract class Shape Color color color of shape voi
150. contained inside this x panel and then adds those components to this panel public HelloWorldPanel displayPanel new MessageDisplay Create the display subpanel JButton changeMessageButton new JButton Change Message The button ButtonHandler listener new ButtonHandler changeMessageButton addActionListener listener setLayout new BorderLayout Set the layout manager for this panel add displayPanel BorderLayout CENTER Add the display panel add changeMessageButton BorderLayout SOUTH Add the button Once this class exists it can be used in an applet The applet class only has to create an object of type HelloWorldPanel and use that object as its content pane import javax swing JApplet public class HelloWorldApplet2 extends JApplet public void init HelloWorldPanel content new HelloWorldPanel setContentPane content 116 Similarly its easy to make a frame that uses an object of type HelloWorldPanel as its content pane import javax swing JFrame public class HelloWorldGUI3 public static void main String args JFrame window new JFrame GUI Test HelloWorldPanel content new HelloWorldPanel window setContentPane content window setSize 250 100 window setLocation 100 100 window setDefaultCloseOperation JFrame EXIT_ON_CLOSE window setVisible true One new feature of this example is the line window setDefaultCloseOperation
151. ct 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 named data dat in the current directory Another constructor has two parameters new File File String The first is a File object that refers to the directory that contains the file The second can be the name of the file or a relative path from the directory to the file 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 method returns true if the file named by the File object already exists You can use this method if you want to avoid overwriting the contents of an existing file when you create a new FileWriter e file isDirectory This boolean valued method returns true if the File ob ject refers to a directory It returns false if it refers to a regular file or if no file with the given name exists 217 e file delete Deletes the file if it exists Returns a boolean value to indicate whether the file was successfully deleted e file list If the File object refers to a directory this method returns an array of type String containing the names of the files in that directory Oth erwise it returns null Here for example is a program that will list the names of all the files in a di rectory specified by the user
152. d by writing to a DataOutputStream java io provides the class DataInputStream You can wrap any InputStream in aDataInputStream object to provide it with the ability to read data of various types from the byte stream The methods in theDataInputStream for reading binary data are called readDouble readInt and so on Data writ ten 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 computer The cross platform compatibility of binary data is a major aspect of Java s platform independence In some circumstances you might need to read character data from an InputStream or write character data to an OutputStream This is not a problem since characters like all data are represented as binary numbers However for character data it is convenient to use Reader and Writer instead of InputStream and OutputStream To make this possible you can wrap a byte stream in a charac ter stream If byteSource is a variable of type InputStream and byteSink is of type OutputStream then the statements Reader charSource Writer charSink new InputStreamReader byteSource new OutputStreamWriter byteSink create character streams that can be used to read character data from and write char acter data to the byte streams In particular the standard input stream System in which is of type InputStream for historica
153. d can generate an exception of a type that requires mandatory handling The statement could be a throw statement which throws the exception directly or it could be a call to a method that can throw the exception 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 in this case the exception is handled within the method so that any caller of the method will never see the exception The second way is to declare that the method can throw the exception This is done by adding a throws clause to the method heading which alerts any callers to the possibility that an exception might be generated when the 200 method is executed The caller will in turn be forced either to handle the exception in a try statement or to declare the exception in a throws clause in its own header Exception handling is mandatory for any exception class that is not a subclass of either Error or RuntimeException Exceptions that require mandatory handling generally represent conditions that are outside the control of the programmer For ex ample they might represent bad input or an illegal action taken by the user There is no way to avoid such errors so a robust program has to be prepared to handle them The design of Java makes it impossible for programmers to ignore the possibility of such errors Among the exceptions that
154. d do this for exam ple if the jar file contains an applet but no main program To create an executable file hit the Next button twice to get to the Jar Manifest Specification screen At the bottom of this screen is an input box labeled Main class You have to enter the name of the class that contains the main method that will be run when the jar file is executed If you hit the Browse button next to the Main class box you can select the class from a list of classes that contain main methods Once you ve selected the main class you can click the Finish button to create the executable jar file It is also possible to create jar files on the command line The JAVA Development 51 Kit includes a command line program named jar that can be used to create jar files If all your classes are in the default package like the examples in this book then the jar command is easy to use To create a non executable jar file on the command line change to the directory that contains the class files that you want to include in the jar Then give the command jar cf JarFileName jar class where JarFileName can be any name that you want to use for the jar file The in x class is a wildcard that makes class match every class file in the current directory This means that all the class files in the directory will be included in the jar file If you want to include only certain class files you can name them
155. d postconditions as a way of specifying the contract of a method As the terms are being used here a precondition of a method is just a precon dition of the code that makes up the definition of the method and the postcondition of a method 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 do Scanner keyboard new Scanner System in System out println Enter A B and C BxB 4 A xC must be gt 0 System out print A A keyboard nextDouble System out print B B keyboard nextDouble System out print C C keyboard nextDouble if A 0 BxB 4 AxC lt 0 System out println Your input is illegal Try again while A 0 BxB 4 xAxC lt 0 disc BxB 4xAxC x B Math sqrt disc 2A After the loop ends we can be sure that B B 4 A C gt Oandthat A 0 The preconditions for the last two lines are fulfilled so the postcondition that x is a solution of the equation A 72 B x x C 0 is 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 algo rithm is correct but the program is not a perfect implementation of the algorithm Here is another variation
156. d 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 ares more instance variables and methods end of class Shape 86 Once you have declared the class to be abstract it becomes illegal to try to create actual objects of type Shape and the computer will report a syntax error if you try to do so Recall that a class that is not explicitly declared to be a subclass of some other class is automatically made a subclass of the standard class Object That is a class declaration with no extends part such as public class myClass is ex actly equivalent to public class myClass extends Object This means that class Object is at the top of a huge class hierarchy that includes every other class Semantially Object is an abstract class in fact the most abstract class of all Curiously however it is not declared to be abstract syntactially which means that you can create objects of type Object What you would do with them however I have no idea Since every class is a subclass of Object a variable of type Object can refer to any object whatsoever of any type Java has several standard data structures that are designed to hold Object s but since every object is an instance of class Object these data st
157. dard things that a computer has to do Microsoft Windows provides its own set of methods for programmers to use and they are quite a bit different from the methods used on the Mac Linux has several different GUI toolboxes for the programmer to choose from 42 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 language things like variables assignment statements if statements and loops To these the programmer can add existing toolboxes full of methods 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 as signed tasks without worrying about the particular steps they go through to accom plish those tasks The innovative part of programming 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 methods are in the toolbox what parameters they use and what tasks they perform This information c
158. dd changeMessageButton BorderLayout SOUTH setContentPane content You should compare this class with HelloWorldGUI2 java from the previous section One subtle difference that you will notice is that the member variables and nested classes in this example are non static Remember that an applet is an object A single class can be used to make several applets and each of those applets will need its own copy of the applet data so the member variables in which the data is stored must be non static instance variables Since the variables are non static the two nested classes which use those variables must also be non static Static nested classes cannot access non static member variables in the containing class Remember the basic rule for deciding whether to make a nested class static If it needs access to any instance variable or instance method in the containing class the nested class must be non static otherwise it can be declared to be static You can try out the applet itself Click the Change Message button to switch the message back and forth between Hello World and Goodbye World 6 3 2 Reusing Your JPanels Both applets and frames can be programmed in the same way Design a JPanel and use it to replace the default content pane in the applet or frame This makes it very easy to write two versions of a program one which runs as an applet and one which runs as a frame The idea is to create a subclass o
159. dded to an HTML document Remember that to use the applet on a Web page include both the panel class file RandomStringsPanel class and the applet class file RandomStringsApplet class in the same directory as the HTML document or alternatively bundle the two class files into a jar file and put the jar file in the document directory Instead of writing an applet of course we could use the panel in the window of a stand alone application You can find the source code for a main program that does this in the file RandomStringsApp java 6 5 Mouse Events EVENTS ARE CENTRAL TO PROGRAMMING for a graphical user interface A GUI program doesn t have a main method that outlines what will happen when 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 information Different types of events are represented by objects belonging to different class
160. de SB Scant Week RL A eS Sohn ees 6 45 An Exampl s sanoi 8b 68 Ses Hey alk Seat ee he as 6 5 Mouse Events 0 aa Aa EA e ee 6 5 21 Event Handling 003 2258 Gn bodes Ge Oi Soe ES Bak Mass 6 5 2 MouseEvent and MouseListener 200 4 6 5 3 Anonymous Event Handlers 2 0004 6 6 BasicComponents 2 2 00 e a 0 6 lt I Button eane eis aise Soke who Grae SP ag Gy wee ae a ee Gp edn aah es 6 6 2 SJ E pel 2 xsi es Bae ee AL he Gove Bi ang snk RE ate te 2G Rak ed Be God 6 6 38 JCheckBox 0 0 0 ce a 6 6 4 JTextField and JTextArea 2 0 0 0 0 eee ne 7 6 7 Basic Layout 3 03 ac tee amp Be ok wie ed ete Bee Wa eh ak eh 6 7 1 Basic Layout Managers 0 000 eee ene 6 7 2 A Simple Calculator 0 0 0 00 cee ee ee Ge A Little Card Game estice toe ae ini awe ae eS ee 6 8 Images and Resources 0 2c eb oae reh 68 1 Images s 6240 0 whore ee eG eee Gah Pola Ame ee ee A 6 8 2 Image File VO 2 en A Solitaire Game Klondike 7 1 Klondike Solitaire 2 aa ee ee eee T2 Card Gamesi 06 jo Pa bo aa a Sods fo ee oo veg EAS He BE sd AE 7 2 1 The CardNames Interface 0 0 000 ee uae 7 2 2 TheDeckclass 2 0 0 0 2 cee ee ee 7 3 Implementation of Klondike 0 00000004 7 3 1 The CardPile class the base class 0004 7 3 2 Phe Solitaire Class ee ls ae aS a he ole See 7 3 3 Completing the Implementation
161. de will remove all null values from any collection of type Collection lt JButton gt as long as that collection supports removal of values Iterator lt JButton gt iter coll iterator while iter hasNext JButton item iter next if item null iter remove Note by the way that when Collection lt T gt Iterator lt T gt or any other param eterized type is used in actual code they are always used with actual types such as String or JButton in place of the formal type parameter T An iterator of type Iterator lt String gt is used to iterate through a collection of Strings an iterator of type Iterator lt JButton gt is used to iterate through a collection of JButtons and so on An iterator is often used to apply the same operation to all the elements in a collection In many cases it s possible to avoid the use of iterators for this purpose by using a for each loop A for each loop can also be used to iterate through any collection For a collection coll of type Collection lt T gt a for each loop takes the form for T x coll for each object x of type T in coll process x 175 Here x is the loop control variable Each object in coll will be assigned to x in turn and the body of the loop will be executed for each object Since objects in coll are of type T x is declared to be of type T For example if namelist is of type Collection lt String gt we can print out all the
162. defined to be the same as obj1 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 shown below Without the equals method in this class methods such as contains and remove in the interface Collection lt Card gt will not work as expected 176 public class Card Class to represent playing cards int suit Number from 0 to 3 that codes for the suit spades diamonds clubs or hearts int value Number from 1 to 13 that represents the value public boolean equals Object obj try 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 catch Exception e This will catch the NullPointerException that occurs if obj is null and the ClassCastException that occurs if obj is not of type Card In these cases obj is not equal to this Ca
163. different types of layout manager implement different policies One type of layout manager is defined by the BorderLayout class In the program the statement content setLayout new BorderLayout creates a new BorderLayout object and tells the content panel to use the new ob ject as its layout manager Essentially this line determines how components that are added to the content panel will be arranged inside the panel We will cover lay out managers in much more detail later but for now all you need to know is that adding okButton in the BorderLayout SOUTH position puts the button at the bottom 111 of the panel and putting the component displayPanel in the BorderLayout CENTER position makes it fill any space that is not taken up by the button This example shows a general technique for setting up a GUI Create a container and assign a layout manager to it create components and add them to the container and use the container as the content pane of a window or applet A container is itself a component so it is possible that some of the components that are added to the top level container are themselves containers with their own layout managers and components This makes it possible to build up complex user interfaces in a hierarchical fashion with containers inside containers inside containers 6 2 3 Events and Listeners The structure of containers and components sets up the physical appearance of a GUI but it doesn t say an
164. ditions are known facts upon which we can build further deductions about the behavior of the program A postcondition of a program as a whole is simply 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 dou ble disc BxB 4xAxC x B Math sqrt disc 2A The quadratic formula from high school mathematics assures us that the value assigned to x is a solution of the equation Ax Bx 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 Bx B 4 x AxC 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 AxC gt Qis 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 191 We ve encountered preconditions and postconditions once before That section introduced preconditions an
165. draw method We say that an object implements an interface if it belongs to a class that implements the interface For example any object of type Line implements the Drawab1e interface While a class can extend only one other class it can implement any number of interfaces In fact a class can both extend one other 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 some thing 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 methods in an interface are abstract methods which must be imple mented 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 inter face and if Line and FilledCircle are classes that implement Drawable then you could say Drawable figure 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
166. draw 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 mes sage 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 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 ob ject 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 the method itself is in the object of type Rectangle 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
167. e T and S stand for any type except for the primitive types We will discuss only collections in this course 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 The defining property of a set is that no object can occur more than once in a set the elements of a set are not necessarily thought of as being in any particular or der The ideas of lists and sets are represented as parameterized interfaces List lt T gt and Set lt T gt These are sub interfaces of code Collection lt T gt That is any object that implements the interface List lt T gt or Set lt T gt automatically implements Collection lt T gt as well The interface Collection lt T gt specifies general operations that can be applied to any collection at all List lt T gt and Set lt T gt add additional oper ations that are appropriate for lists and sets respectively Of course any actual object that is a collection list or set must belong to a concrete class that implements the corresponding interface For example the class ArrayList lt TI gt implements the interface List lt T gt and therefore also implements Collection lt T gt This means that all the methods that are defined in the list and collection interfaces can be used with for example an Arr
168. e Both the java pack age and the javax package contain sub packages One of the sub packages of java for example is called awt Since awt is contained within java its full name is ac tually java awt This package contains classes that represent GUI components such as buttons and menus in the AWT the older of the two JAVA GUI toolboxes which is no longer widely used However java awt also contains a number of classes that form the foundation for all GUI programming such as the Graphics class which pro vides methods for drawing on the screen the Color class which represents colors and the Font class which represents the fonts that are used to display characters on the screen Since these classes are contained in the package java awt their full names are actually java awt Graphics java awt Color and java awt Font I hope that by now you ve gotten the hang of how this naming thing works in JAVA Simi larly javax contains a sub package named javax swing which includes such classes as javax swing JButton javax swing JMenu and javax swing JFrame The GUI classes in javax swing together with the foundational classes in java awt are all part of the API that makes it possible to program graphical user interfaces in JAVA The java package includes several other sub packages such as java io which provides facilities for input output java net which deals with network communi cation and java util which provides a variety of uti
169. e 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 x 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 sourceName args 1 copyName args 2 force true else if args length 2 sourceName args 0 copyName args 1 force false else System out println Usage java CopyFile lt source file gt lt copy name gt System out println or java CopyFile f lt source file gt lt copy name gt return 220 x Create the input stream If an error occurs end the program x try source new FileInputStream sourceName catch FileNotFoundException e System out println Can t find file sourceName return x If the output file already exists and the f option was not specified print an error message and end the program x File file new File copyName if file exists amp amp force false System out print1n Output file exists Use the f option to replace it return x Create the output stream If an error occurs end the pro
170. e 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 dynami cally 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 and the variables that the object contains are called instance variables The methods that the object contains are called instance methods For example if the PlayerData class is used to create an object then that object is an instance of the PlayerData 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 The source code for methods are defined in the class yet it s better to think of the instance methods as belonging to the object not to the class The non static methods in the class merely specify the instance methods that every object created from the class will contain For example a draw method in two different objects do the same thing in the sense that they both draw something But there is a real difference between the two methods the things that they draw can be different You might say
171. e including from other classes On the other hand a private member can only be used in the class where it is defined In the opinion of many programmers almost all member variables should be de clared private This gives you complete control over what can be done with the variable Even if the variable itself is private you can allow other classes to find out what its value is by providing a public accessor method that returns the value of the variable For example if your class contains a private member variable title of type String you can provide a method public String getTitle return title that returns the value of title By convention the name of an accessor method for a variable is obtained by capitalizing the name of variable and adding get in front of the name So for the variable title we get an accessor method named get Title or getTitle Because of this naming convention accessor methods are more often referred to as getter methods A getter method provides read access to a variable You might also want to allow write access to a private variable That is you might want to make it possible for other classes to specify a new value for the vari able This is done with a setter method If you don t like simple Anglo Saxon words you can use the fancier term mutator method The name of a setter method should consist of set followed by a capitalized copy of the variable s name
172. e we say that Integer parseInt str throws an exception of type NumberFormatException when the value of str is illegal When an exception is thrown it is possible to catch the exception and prevent it from crashing the program This is done with a try catch statement In somewhat simplified form the syntax for a try catch is 47 try statements 1 catch exception class name variable name statements 2 The exception class name in the catch clause could be NumberFormatException IllegalArgumentException or some other exception class When the computer ex ecutes this statement it executes the statements in the try part If no error occurs during the execution of statements 1 then the computer just skips over the catch part and proceeds with the rest of the program However if an exception of type exception class name occurs during the execution of statements 1 the computer immediately jumps to the catch part and executes statements 2 skipping any re maining statements in statements 1 During the execution of statements 2 the variable name represents the exception object so that you can for example print it out At the end of the catch part the computer proceeds with the rest of the pro gram the exception has been caught and handled and does not crash the program Note that only one type of exception is caught if some other type of exception occurs during the execution of statements 1 it will
173. e border layout and the other three positions of the layout are empty The drawing surface is defined by a nested class named CardPanel which is a subclass of JPanel I have chosen to let the drawing surface object do most of the work of the game It listens for events from the three buttons and responds by taking the appropriate actions The main panel is defined by HighLowGUI itself which is another subclass of JPanel The constructor of the HighLowGUI class creates all the other components sets up event handling and lays out the components 148 public HighLowGUI The constructor setBackground new Color 130 50 40 setLayout new BorderLayout 3 3 BorderLayout with 3 pixel gaps CardPanel board new CardPanel Where the cards are drawn add board BorderLayout CENTER JPanel buttonPanel new JPanel The subpanel that holds the buttons buttonPanel setBackground new Color 220 200 180 add buttonPanel BorderLayout SOUTH JButton higher new JButton Higher higher addActionListener board The CardPanel listens for events buttonPanel add higher JButton lower new JButton Lower lower addActionListener board buttonPanel add lower JButton newGame new JButton New Care newGame addActionListener board buttonPanel add newGame setBorder BorderFactory createLineBorder new Color 130 50 40 3 end constructor The programming of th
174. e correctness and robustness are important and especially difficult is in the processing of input data whether that data is typed in by the user read from a file or received over a network Sometimes it s useful to be able to look ahead at what s coming up in the input without actually reading it For example a program might need to know whether the next item in the input is a number or a word For this purpose the Scanner class has various hasNext methods These includes hasNextBoolean hasNextInteger hasNextLine and hasNextDouble For example the hasNextInteger method 193 returns true if the input s next token is an integer Thus you can check if the expected input is available before actually reading it 9 3 Exceptions and try catch 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 exceptional circumstances without crashing One approach to writing robust programs is to an ticipate the problems that might arise and to include tests in the program for each possible problem For example a program will crash if it tries to use an array ele ment Ali when i is not within the declared range of indices for the array A A robust program must anticipate the possibility of a bad index and guard against it One way to do this is to write the program in a way that ensures that the index is in the legal range Another way
175. e drawing surface class CardPanel is a nice example of thinking in terms of a state machine See Subection6 5 4 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 approach that produced the original text oriented game in Subection5 4 3 is not appropriate here Trying to think about the game in terms of a process that goes step by step from beginning to end is more likely to confuse you than to help you 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 less 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 CardPanel class uses a boolean instance variable named gameInProgress for this purpose The state of the game can change whenever the user clicks on a button CardPanel 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 t
176. e 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 2 6 1 Designing the classes When designing object oriented software a crucial first step is to identify the objects that will make up the application One approach to do this is to identify the nouns in the problem description These become candidates for objects Next we can identify verbs in the description these suggest methods for the objects Consider the following description of a card game 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 52 If we look for 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 ver
177. e objects have the same values To do that you would need to ask whether stdl test1 std2 testl amp amp stdl test2 std2 test2 amp amp stdl test3 std2 test3 amp amp stdl 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 26 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 The fact that variables hold references to objects 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 final Student stu new Student stu name
178. e of text 142 6 7 Basic Layout COMPONENTS ARE THE FUNDAMENTAL BUILDING BLOCKS of a graphical user interface But you have to do more with components besides create them 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 difficult problem especially if you don t assume a fixed size for the drawing area JAVA has a solution for this as well Components are the visible objects that make up a GUI Some components are containers which can hold other components Containers in JAVA are objects that belong to some subclass of java awt Container The content pane of a JApplet or JFrame is an example of a container The standard class JPanel which we have mostly 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 Because a JPanel is itself a component you can add a JPanel to another JPanel This makes complex nesting of components possible JPanels can be used to organize complicated user interfaces as shown in this illustration Three panels shown in color containing six other components shown in gray 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
179. e public interface of your class only the public getter and setter methods are If you haven t used get and set from the beginning you ll have to contact everyone who uses your class and tell them Sorry guys you ll have to track down every use that you ve made of this variable and change your code 28 1 2 4 Creating and Destroying Objects 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 Initializing Instance Variables An instance variable can be assigned an initial 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 pu
180. ead a JPEG file is only an approximation of the picture that was saved Some information in the picture has been lost Allowing some information to be lost makes it possible to compress the image into a lot fewer bits than would otherwise be necessary Usually the approximation is quite good It works best for 155 photographic images and worst for simple line drawings The PNG format on the other hand is lossless meaning that the picture in the file is an exact duplicate of the picture that was saved A PNG file is compressed but not in a way that loses information The compression works best for images made up mostly of large blocks of uniform color it works worst for photographic images GIF is an older format that is limited to just 256 colors in an image it has mostly been superseded by PNG Suppose that image is a BufferedImage The image can be saved to a file simply by calling ImageIO write image format file where format is a String that specifies the image format of the file and file is a File that specifies the file that is to be written The format string should ordinarily be either PNG or JPEG although other formats might be supported ImageIO write is a static method in the ImagelI0 class It returns a boolean value that is false if the image format is not supported That is if the specified image format is not supported then the image is not saved but no exception is thrown This means that you should al
181. eader wrapper can be created for reader with BufferedReader in new BufferedReader reader This can be combined with the InputStreamReader class that was mentioned above to read lines of text from an InputStream For example we can apply this to System in 211 BufferedReader in BufferedReader for reading from standard input in new BufferedReader new InputStreamReader System in try String line in readLine while line null amp amp line length gt 0 processOneLineOfInput line line in readLine catch IOException e This code segment reads and processes lines from standard input until either an empty line or an end of stream is encountered An end of stream is possible even for interactive input For example on at least some computers typing a Control D gen erates an end of stream on the standard input stream The try catch statement is necessary because the readLine method can throw an exception of type IOException which requires mandatory exception handling an alternative to try catch would be to declare that the method that contains the code throws IOException Also remem ber that BufferedReader InputStreamReader and IOException must be imported from the package java io 10 1 5 The Scanner Class Since its introduction Java has been notable for its lack of built in support for basic input and for its reliance on fairly advanced techniques for the support that
182. eas 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 4 6 1 The Special Variable this A static member of a class has a simple name which can only be used inside the class 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 defined Instance members also have full names but remem ber 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 con tains the
183. ect gt sets rects to refer to a newly created list that can only hold objects belonging to the class ColoredRect or to a subclass The funny looking ArrayList lt ColoredRect gt is being used here in the same way as an ordinary class name don t let the lt ColoredRect gt confuse you it s just part of the name of the type When a state ments such as rects add x occurs in the program the compiler can check whether x is in fact of type ColoredRect If not the compiler will report a syntax error When an object is retrieved from the list the compiler knows that the object must be of type ColoredRect so no type cast is necessary You can say simply ColoredRect rect rects get i You can even refer directly to an instance variable in the object such as rects get i color This makes using ArrayList lt ColoredRect gt very similar to using ColoredRect with the added advantage that the list can grow to any size Note that if a for each loop is used to process the items in rects the type of the loop control variable can be ColoredRect and no type cast is necessary For example when using ArrayList lt ColoredRect gt as the type for the list rects the code for drawing all the rectangles in the list could be rewritten as for ColoredRect rect rects g setColor rect color g fillRect rect x rect y rect width rect height g setColor Color BLACK g drawRect rect x rect y rect width 1 rect height
184. ed 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 83 Now you might see a problem here with the method redraw The problem is that each different type of shape is drawn differently The method 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 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
185. ed a arerri carTakeg agag Ire ides selena TAL eng Solitaire Card dmz f2 Q iarna allhilez ardhe er s decktite esise Mzeardhi j Hle Lune 2 acel gt bealea verre nt PEAN alact tea Y iwir ne saezh int Ptzoleau eoleble a Cadi damea Ir a sug aen tea F Se airs a dreat Bisat Ar e mint A yus y zi ag Ir moweTli kedi a aelig queer ar m uzE er JC a Tipo Seen ar mouveEs edi a ani F sande ire myuefiessedi a ca mouse zz J a Jiti pz WIumsv zti a uri gc Figure 7 2 Class diagram for the Solitaire app 7 3 1 The CardPile class the base class package solitaire import java awt Graphics import java util LinkedList import java util List public abstract class CardPile protected List pile protected int x protected int y xx x Make an Empty Pile public CardPile int x int y pile new LinkedList this x x this y y public boolean empty return pile isEmpty 161 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 ty lt y Card height public void addCard Card aCard pile add aCard
186. edges of the component you can say g drawRect 0 0 getWidth 1 getHeight 1 where g is a graphics context for the component If you use g drawRect 0 0 getWidth getHeight then the right and bottom edges of the rectangle will be drawn outside the component drawOval int x int y int width int height Draws the outline of an oval The oval is one that just fits inside the rectangle specified by x y width and height If width equals height the oval is a circle 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 specified 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 diameter 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 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
187. efault layout for a JPanel is a FlowLayout that is a JPanel uses a FlowLayout unless you specify a different layout manager by calling the panel s setLayout method The components in a given row can be either left aligned right aligned or cen tered within that row and there can be horizontal and vertical gaps between compo nents If the default constructor new FlowLayout is used then the components on each row will be centered and both the horizontal and the vertical gaps will be five pixels The constructor public FlowLayout int align int hgap int vgap can be used to specify alternative alignment and gaps The possible values of align are FlowLayout LEFT FlowLayout RIGHT and FlowLayout CENTER Suppose that cntr is a container object that is using a FlowLayout as its layout manager Then a component comp can be added to the container with the statement cntr add comp The FlowLayout will line up all the components that have been added to the con tainer in this way They will be lined up in the order in which they were added For example this picture shows five buttons in a panel that uses a FlowLayout Button Number 1 Button Number 2 Button Number 3 Button Number 4 Button Number 5 Note that since the five buttons will not fit in a single row across the panel they are arranged in two rows In each row the buttons are grouped together and are centered in the row The butto
188. ely Many useful methods are defined in the Component and JComponent classes and so can be used with any Swing component We begin by looking at a few of these methods Suppose that comp is a variable that refers to some JComponent Then the following methods can be used e comp getWidth and comp getHeight are methods that give the current size of the component in pixels One warning When a component is first cre ated 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 comp setEnabled true and comp setEnabled false can be used to enable and disable the component When a component is disabled its appearance might change and the user cannot do anything with it The boolean valued method comp isEnabled can be called to discover whether the component is enabled comp setVisible true and comp setVisible false can be called to hide or show the component comp setFont font sets the font that is used for text displayed on the compo nent See Subection6 3 3 for a discussion of fonts comp setBackground color and comp setForeground color set the back ground and foreground colors for the component comp setOpaque true tells the component that the area occupied by the com ponent should be filled with the component s background color before the con tent of the component is painted By default
189. en found to suffer from buffer overflow errors Buffer overflow errors often make the news because they are responsible for many network security problems When one computer receives data from 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 soft ware then the only thing that has happened 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 ex ecuting 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 maliciou
190. ent 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 the command g setFont bigBoldFont will set the current font of g to bigBoldFont The new font will be used for any text that is drawn after the setFont command is given 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 that can be set with the setFont font instance method 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 6 4 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 component 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 commands 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
191. eption 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 Here the computer tries to execute the statements in the try clause If no error oc curs both of the catch clauses are skipped If an ArrayIndexOutOfBoundsException occurs the computer executes the body of the first catch clause and skips the sec ond one If a NullPointerException occurs it jumps to the second catch clause and executes that 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 O 0O M 1 1 M 0O 1 M 1 0 System out println The determinant of M is determinant catch RuntimeException err System out println Sorry an error has occurred System out println The error was err The catch clause in this try statement will catch any exception belonging to class RuntimeException or to any of its subclasses 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 Because of subclassing when there are multiple catch clauses in a try statement it is possible that a given
192. er list methods and variables This constructor defines a new class without giving it a name and it simulta neously creates an object that belongs to that class This form of the newoperator 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 If an interface is used as a base the parameter list is empty Otherwise it contains parameters for a constructor in the superclass Anonymous classes are often used for handling events in graphical user inter faces and we will encounter them several times in the chapters on GUI program ming For now we will look at one not very plausible example Consider the Drawable interface which is defined earlier in this section Suppose that we want a Drawable object that draws a filled red 100 pixel square Rather than defining a new 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 Dr
193. er by negative zero gives Double NEGATIVE_INFINITY You also get Double POSITIVE_INFINITY whenever the mathematical value of an expres sion is greater than Double MAX_VALUE For example 1e200 1e200 is considered to be infinite The value Double NaN is even more interesting NaN stands for Not a Number and it represents an undefined value such as the square root of a negative number or the result of dividing zero by zero Because of the existence of Double NaN no mathematical operation on real numbers will ever throw an excep tion it simply gives Double NaN as the result You can test whether a value x of type double is infinite or undefined by calling the boolean valued static methods Double isInfinite x and Double isNaN It s especially important to use Double isNaN to test for undefined values because Double NaN has really weird behavior when used with relational operators such as In fact the values of x Double NaN and x Double NaN are both false no matter what the value of x so you really can t use these expressions to test whether x is Double NaN 36 Chapter The Practice of Programming Contents 2 1 Apstraction iga anpor e ea AGE iw cea a he tus is Loh oh oe E E 0 ew 37 2 1 1 Control Abstraction 0 0 0002 ee eee 38 2 1 22 DataAbstraction 0000 eee eee ee 39 2 1 3 Abstraction in Object Oriented Programs 39 2 2 Methods as an Abstraction Mechanism
194. er of correct predictions the user has made At the end of the game this will be the user s score char guess The user s guess H if the user predicts that the next card will be higher L if the user predicts that it will be lower deck shuffle Shuffle the deck into a random order before starting the game correctGuesses 0 currentCard deck dealCard System out println The first card is the currentCard while true Loop ends when user s prediction is wrong x Get the user s prediction H or L or h or l x System out print Will the next card be higher H or lower L do guess keyboard next charAt 0 guess Character toUpperCase guess if guess H amp amp guess L System out print Please resoond with H or L while guess H amp amp guess L x Get the next card and show it to the user x nextCard deck dealCard System out println The next card is nextCard 60 x Check the user s prediction x if nextCard getValue currentCard getValue System out println The value is the same as the previous card System out println You lose on ties Sorry break End the game else if nextCard getValue gt currentCard getValue if guess H System out println Your prediction was correct correctGuesses else Sys
195. ere might still be some translation Characters are stored in the com puter 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 Byte streams can be useful for direct machine to machine communication and they can sometimes be useful for storing data in files especially when large amounts of data need to be stored efficiently such as in large databases However binary data is fragile in the sense that its meaning is not self evident When faced with a long series of zeros and ones you have to know what information it is meant to represent and how that information is encoded before you will be able to interpret it Of course the same is true to some extent for character data which is itself coded into binary form But the binary encoding of character data has been standardized and is well understood and data expressed in character form can be made meaningful to human readers The current trend seems to be towards increased use of character data represented in a way that will make its meaning as self evident as possible I should note that the original version of Java did not have character streams and that for ASCII encoded character data byte streams are largely interchangeable with character streams In fac
196. error This helps the user see where the error occurred and let s the user start typing the correction immediately The JTextField class has a constructor public JTextField int columns where columns is an integer that 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 and because the preferred width is not always re spected the actual number of characters visible in the text field might not be 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 fill whatever space is available 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 public JTextField String contents public JTextField String contents int columns The constructors for a JTextArea are public JTextArea public JTextArea int rows int columns public JTextArea String contents public JTextArea String contents int rows int columns The parameter rows specifies how many lines of text should be visible in the text area This determines the preferred height of the text area just as columns deter mines the preferred width However the text area can actually contain any number of lines the text area can be scrolled to reveal l
197. es after the other FullName Return a positive value to indicate this return 1 else Last names are the same so base the comparison on the first names using compareTo from class String return firstName compareTo other firstName other methods Its odd to declare the class as classFullName implements Comparable lt FullName gt with FullName repeated as a type parameter in the name of the interface How ever it does make sense It means that we are going to compare objects that belong to the class Ful1Name to other objects of the same type Even though this is the only 178 reasonable thing to do that fact is not obvious to the Java compiler and the type parameter in Comparable lt FullName gt is there for the compiler 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 ob ject must implement the interface Comparator lt T gt where T is the type of the ob jects that are to be compared The interface Comparator lt T gt defines the method public int compare T obj1 T obj2 This method compares two objects of type T and returns a value that is negative or positive or zero depending on whether obj1 comes before obj2 or comes after obj2 or is considered to be the same as obj2 for the purposes of this comparison Comparators are useful for comparing objects that do not implement the
198. es 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 source of the event that is the component on which the user clicked the x y coordinates of the point in the component 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 method By writing that method the programmer says what should happen when the event occurs 129 As a JAVA programmer you get a fairly high level view of events There is a lot of processing that goes on between the time that the user presses a key or moves the mouse and the time that a method in your program is called to respond to the event Fortunately 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 method there is a sort of main method running somewhere that executes a loop of the form while the program is still running Wait for the next event to occur Call a method 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 method that runs
199. es for example MouseListener are defined in the package java awt event This means that if you want to work with events you either include the line import java awt event at the beginning of your source code file or import the individual classes and inter faces 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 or individual im ports 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 methods 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 appropri ate interface A component can listen for the events that it itself generates A panel can listen for events from components that are contained in the panel A special class can be created just for the purpose of defining a listening object Many people con sider it to be good form to use anonymous inner classes to define listening objects You will see all of these patterns in examples in this textbook 6 5 2 MouseEvent and MouseListener The MouseListener interface specifies five different instance methods public void mousePressed MouseEve
200. es 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 drawing The graphics context continues to use the color until you explicitly change it with another setColor command If you want to know what the current drawing color is you can call the method 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 Generally the component is filled with the background color before anything else is drawn although some components are transparent meaning that the background color is ignored When a new graphics context is created for a component the cur rent 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 Foreground and background colors can be set by the instance methods setForeground c and setBackground c which are defined in the Component class and therefore are available for use with any component This can be useful even for standard components if you want them to use colors that are different from the de faults 6 4 3 Fonts A font represents a particular
201. es of text in a word processing program An array that holds the list of computers that are currently downloading a page from a Web site An array that contains the shapes that have been added to the screen by the user of a drawing program Clearly we need some way to deal with cases where the number of data items in an array is not fixed Specifying the type when arrays are created means that one can only put primi tives or objects of the specified into the array for example an array of int can only hold integers One way to work around this is to declare Object as the type of an ar ray In this case one can place anything into the array because 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 An ArrayList serves much the same pupose as arrays do It allows you to store objects of any type The ArrayList class is in the package java util so if you want to use it in a program you should put the directive import java util ArrayList at the beginning of your source code file The ArrayList class always has a definite size and it is illegal to refer to a po sition 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 use
202. es that require mandatory exception handling are shown in red 195 Throwable InterruptedException IOException EOFException ArrayIndexOutOfBoundsException IllegalArgumentException NumberfFormatException The class Throwable includes several instance methods that can be used with any exception object If e is of type Throwable or one of its subclasses then e getMessage is a method that returns a String that describes the exception The method e toString which is used by the system whenever it needs a string rep resentation of the object returns a String that contains the name of the class to which the exception belongs as well as the same string that would be returned by e getMessage And e printStackTrace writes a stack trace to standard output that tells which methods were active when the exception occurred A stack trace can be very useful when you are trying to determine the cause of the problem Note that if an exception is not caught by the program then the system automatically prints the stack trace to standard output SocketException The class Throwable and some of its subclasses 9 3 2 The try Statement To catch exceptions in a Java program you need a try statement The try statements that we have used so far had a syntax similar to the following example try double determinant M O 0O M 1 1 M 0O 1 M 1 0 System out println The determinant of M is determ
203. etValue 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 I 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 some thing that Pll probably have to do in many programs it makes sense to include support for it in the class So Pve provided instance methods getSuitAsString and getValueAsString to return string representations of the suit and value of a card Finally ve defined the instance method toString to return a string with both the value and suit such as Queen of Hearts Recall that this method will be used whenever a Card needs to be converted into a String such as when the card is concatenated onto a string with the operator Thus the statement System out println Your card is the card is equivalent to System out println Your card
204. etc Without control abstraction a programmer would need to specify all the register binary level steps each time she simply wanted to add or multiply a couple of num bers and assign the result to a variable This duplication of effort has two serious negative consequences 38 e a it forces the programmer to constantly repeat fairly common tasks every time a similar operation is needed and e b it forces the programmer to program for the particular hardware and in struction set 2 1 2 Data Abstraction Data abstraction is the enforcement of a clear separation between the abstract prop erties of a data type and the concrete details of its implementation The abstract properties are those that are visible to client code that makes use of the data type the interface to the data type while the concrete implementation is kept entirely private and indeed can change for example to incorporate efficiency improvements over time The idea is that such changes are not supposed to have any impact on client code since they involve no difference in the abstract behaviour For example one could define an abstract data type called lookup table where keys are uniquely associated with values and values may be retrieved by specifying their corresponding keys Such a lookup table may be implemented in various ways as a hash table a binary search tree or even a simple linear list As far as client code is concerned the abstract properties
205. executed is the one in the same object 85 4 5 Abstract Classes 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 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 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 itself will never actually 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
206. extUniquelID variable 99 public class Student private String name Student s name private int ID Unique ID number for this student public double testl1 test2 test3 Grades on three tests private static int nextUniquelID 0 keep track of next available unique ID number Student String theName Constructor for Student objects provides a name for the Student and assigns the student a unique ID number name theName nextUniquelID ID nextUniquelID 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 public double getAverage Compute average test grade return testl test2 test3 3 end of class Student The initialization nextUniqueID 0 is done once when the class is first loaded Whenever a Student object is constructed and the constructor says nextUniqueID 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 i
207. f 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 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 constructor in the superclass Exception The class inherits the getMessage and printStackTrace methods from its superclass off course If e refers to an object of type ParseError then the method call e getMessage will retrieve the error mes sage 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 201 syntax of a string A ParseError would indicate presumably that some string that is being processed by the program 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 exam ple throw new ParseError Encountered an illegal negative number or word jis not a va
208. f JPanel that represents the content pane for your program all the hard programming work is done in this panel class An object of this class can then be used as the content pane either in a frame or in an applet Only a very simple main program is needed to show your panel in a frame and only a very simple applet class is needed to show your panel in an applet so it s easy to make both versions As an example we can rewrite HelloWorldApplet by writing a subclass of JPanel That class can then be reused to make a frame in a standalone application This class is very similar to HelloWorldApplet but now the initialization is done in a constructor instead of in an init method 115 import java awt x import java awt event x import javax swing x public class HelloWorldPanel extends JPanel private String currentMessage Hello World private MessageDisplay displayPanel private class MessageDisplay extends JPanel Defines the display panel public void paintComponent Graphics g super paintComponent g g drawString currentMessage 20 30 private class ButtonHandler implements ActionListener The event listener public void actionPerformed ActionEvent e if currentMessage equals Hello World currentMessage Goodbye World else currentMessage Hello World displayPanel repaint Paint display panel with new message x The constructor creates the components that will be
209. f System out printlin If you are going to work extensively with the Mathclass you can preface your class definition with import static java lang Math x This would allow to say sqrtinstead of Math sqrt log instead of Math log PI instead of Math PI and so on Note that the static import directive requires a package name even for classes in the standard package java lang One consequence of this is that you can t do a static import from a class in the default package 5 4 Enums as Classes Enumerated types are actually classes and each enumerated type constant is a pubic final static member variable in that class even though they are not de clared with these modifiers The value of the variable is an object belonging to the enumerated type class There is one such object for each enumerated type constant and these are the only objects of the class that can ever be created It is really these objects that represent the possible values of the enumerated types The enumerated type constants are actually variables that refer to these objects When an enumerated type is defined inside another class it is a nested class inside the enclosing class In fact it is a static nested class whether you declare it to be static or not But it can also be declared as a non nested class in a file of its own For example we could define the following enumerated type in a file named Suit java public enum Suit SPADES HEARTS DIAMONDS CL
210. f 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 digit 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 flipd repaint public void paintComponent Graphics g super paintComponent g c draw g 20 10 end of class Panel 159 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 7 2 1 The CardNames Interface The CardNames class is an interface defining names public interface CardNames public static final int heart public static final int diamond public static final int club 2 public static final int spade public static
211. final int ace 1 public static final int jack 11 public static final int 12 public static final int king 13 public static final int red public static final int black 0 1 3 Q 0 g gt D Il Its a convenience class that allows us to use these names in a consistent man ner Thus we can use the name CardNames ace throughout the program consis tently i e Different parts of the program will mean the same thing when they say CardNames ace 7 2 2 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 7 3 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 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 160 F Car Pile awdeoud vat Take wank emstyd i ade psp 6f sele pIa de O labterd Q SimiPile Q New arith 6B bei kee es T alertieg oF sunr 9 nearne f seekriei carTakeg a cantons m awy sr aTaked D n
212. g is of type Image Then the statement g drawImage img x y this will draw the image img in a rectangular area in the component The integer valued pa rameters x and y give the position of the upper left corner of the rectangle in which the image is displayed and the rectangle is just large enough to hold the image The fourth parameter this is the special variable that refers to the JComponent it self This parameter is there for technical reasons having to do with the funny way Java treats image files For most applications you don t need to understand this but here is how it works g drawImage does not actually draw the image in all cases It is possible that the complete image is not available when this method is called this can happen for example if the image has to be read from a file In that case g drawlmage merely initiates the drawing of the image and returns immedi ately Pieces of the image are drawn later asynchronously as they become available The question is how do they get drawn That s where the fourth parameter to the drawImage method comes in The fourth parameter is something called an Ima geObserver When a piece of the image becomes available to be drawn the system will inform the ImageObserver and that piece of the image will appear on the screen Any JComponent object can act as an ImageObserver The drawIlmage method re turns a boolean value to indicate whether the image has actually been drawn or
213. g 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 However I will not be creating any packages in this textbook For the purposes of this book you need to know about packages mainly so that you will be able to import the standard packages These packages are always available to the programs that you write You might wonder where the standard classes are actually located Again that can depend to some extent on the version of JAVA that you are using but in the standard JAVA 5 0 they are stored in jar files in a subdirectory of the main JAVA installation directory A jar or JAVA archive file is a single file that can contain many classes Most of the standard classes can be found in a jar file named classes jar In fact JAVA programs are generally distributed in the form of jar files instead of as individual class files Although we won t be creating packages explicitly every class is actually part of a package Ifa 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 this book are in the default package 2 3 Introduction to Error Handling IN ADDITION TO THE CONTROL STRUCTURES that determine the normal flow of control in a program Java has a way to deal with exceptional cases that throw the flow of 4
214. gers but if a programmer declares a method as returning a timestamp rather than merely an integer this documents part of the meaning of the method e Abstraction Types allow programmers to think about programs at a higher level not bothering with low level implementation For example programmers can think of strings as values instead of as a mere array of bytes There are fundamentally two types in JAVA primitive types and objects types i e any variable you declare are either declared to be one of the primitive types or an object type int double and char are the built in primitive types in JAVA The primitive types can be used in various combinations to create other composite types Every time we define a class we are actually defining a new type For example the Student class defined above introduces a new type We can now use this type like any other type we can declare variables to be of this type and we can use it as a type for parameters of methods 18 Before a variable can be used it must be declared A declaration gives a variable a name a type and an initial value for e g int x 8 declares x to be of type int All objects that we declare also have to be of a specified type the type of an object is the class from which it is created Thus when we declare objects we state the type like so Student st new Student This statement declares the variable st to be of type Student This statement creates a new object of
215. gher method 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 x x Called by actionPerformmed when user clicks Higher button x Check the user s prediction Game ends if user guessed x wrong or if the user has made three correct predictions void doHigher 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 Gare to start a new game repaint return hand addCard deck dealCard Deal a card to the hand int cardCt hand getCardCount Card thisCard hand getCard cardCt 1 Card just dealt Card prevCard hand getCard cardCt 2 The previous card 150 if thisCard getValue lt prevCard getValue gameInProgress false message Too bad You lose else if thisCard getValue prevCard getV
216. gram x try copy new FileOutputStream copyName catch IOException e System out println Can t open output file copyName return x 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 If any error occurs print an error message Also print a message if the file has been copied successfully byteCount 0 try while true int 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 Li e System out println Error end main end class CopyFile 221
217. gramming languages classifies values and expressions into types For e g String s and int s are types A type basically specifies the allowed values and allowed operations on values of that type Definition A type is a set of values together with one or more operations that can be applied uniformly to all these values A type system basically gives meaning to collections of bits Because any value simply consists of a set of bits in a computer the hardware makes no distinction between memory addresses instruction code characters integers and floating point numbers Types inform programs and programmers how they should treat those bits For example the integers are a type with values in the range 2 147 483 648 to 2 147 483 647 and various allowed operations that include addition subtraction mod ulus etc The use of types by a programming language has several advantages e Safety Use of types may allow a compiler to detect meaningless or invalid code For example we can identify an expression Hello World 3 as invalid because one cannot divide a string literal by an integer Strong typing offers more safety Optimization Static type checking may provide useful information to a com piler The compiler may then be able to generate more efficient code Documentation Types can serve as a form of documentation since they can illustrate the intent of the programmer For instance timestamps may be a subtype of inte
218. h Day d in Day values which conveys the meaning much better and is similar to the syntax used in other programming languages for similar types of loops It s helpful to think of the colon in the loop as meaning in 1 2 2 Class Members and Instance Members A class definition is made of members or components A class can define variables or fields and methods Variables and methods can be static or non static i e they are defined with or without the keyword static e g static double lastStudentNumber a static member variable field double studentNumber a non static variable static void printLastNumber a static member method void printNumber a non static method The non static members of a class variables and methods are also known as instance variables and methods while the non static members are also known as class variables and class methods Each instance of a class each object gets its own copy of all the instance variables defined in the class When you create an instance of a class the system allocates enough memory for the object and all its instance variables In addition to instance variables classes can declare class variables A class vari able contains information that is shared by all instances objects of the class If one object changes the variable it changes for all other objects of that type e g A Student number generator in a NewStudent class You can invoke a cl
219. hat hold references to the object What should be done with an object if there are no vari ables 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 In the first line a reference to a newly created 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 its 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
220. hat 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 de pending 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 149 values for the beginning 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 x Called by the CardPanel constructor and called by actionPerformed if x the user clicks the New Game button Start a new game void doNewGame 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 the deck and hand to use for this game hand new Hand deck shuffle hand addCard deck dealCard Deal the first card into the hand message Is the next card higher or lower gameInProgress true repaint end doNewGame 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 doHi
221. hat 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 Once the position has been found we use the same list iterator to place the item in that position The idea is to start at the beginning of the list and to move the iterator forward past all the items that are smaller than the item that is being inserted At that point the iterators add method can be used to insert the item To be more definite suppose that stringList is a variable of type List lt String gt Assume that that the strings that are already in the list are stored in ascending order and that newltem is a string that we would like to insert into the list The following code will place newItem in the list in its correct position so that the modified list is still in ascending order ListIterator lt String gt iter stringList listIterator Move the iterator so that it points to the position where newltem should be inserted into the list If newltem 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 String item iter next if newItem compareTo item lt 0 newltem should come BEFORE item in the list Move the iterator back one space so that it points to the c
222. he other hand the command DO20I 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 substitution of a period for a comma 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 DO20I If FORTRAN required variables to be declared the compiler would have complained that the variable DO20I 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
223. he screen the system will call its paintComponent to do the drawing That is the code that you put into the paintComponent method will be executed whenever the panel needs to be drawn on the screen by writing this method you determine the picture that will be displayed in the panel 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 Instance 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 JPanel The Graphics class is an abstract class which means that it is impossi ble 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 pa rameter to that method is a graphics context for drawing on the component Second every component has an instance method called getGraphics This method re turns a graphics context that can be used for drawing on the component outside its paintComponent method The official line is tha
224. hes any IOExceptions internally and sets the value of an internal error flag if one occurs The checkError method can be used to check the error flag This allows you to use PrintWriter methods without worrying about catching exceptions On the other hand to write a fully robust program you should call checkError to test for possible errors whenever you used a PrintWriter 10 1 3 Data Streams When you use a PrintWriter to output data to a stream the data is converted into the sequence of characters that represents the data in human readable form Sup pose 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 values 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 DataOutputStream has methods writeDouble double x for outputting val ues of type double writeInt int x for outputting 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 classname you could say DataOutputStream dataSink new DataOutputStream byteSink to wrap byteSink in a DataOutputStream dataSink 210 For input of machine readable data such as that create
225. hings First the class must be declared to implement the interface as in class MyListener implements MouseListener OR class MyApplet extends JApplet implements MouseListener Second 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 specified methods It must also belong to a class that is specifically declared to implement the interface 130 Many events in JAVA are associated with GUI components 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 events associated with a Component object comp the listener must be registered with the component by call ing comp addMouseListener mListener The addMouseListener method is an instance method in 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 a drawing surface The event classes such as MouseEvent and the listener interfac
226. his applet a shape will be added to the screen in the upper 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 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 the 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 method that implements dragging for example works only with variables of type Shape As the Shape is being dragged the dragging method 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 4 6 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 id
227. hould not need to know anything about the larger systems in which the box will be used In a way a black box divides the world into two parts the inside implementation and the outside The interface is at the boundary connecting those two parts You should not think of an interface as just the physical connection between the box and the rest of the world The interface also includes a specification of what the box does and how it can be controlled by using the elements of the physical interface It s not enough to say that a TV set has a power switch you need to specify that the power switch is used to turn the TV on and off To put this in computer science terms the interface of a method has a semantic as well as a syntactic component The syntactic part of the interface tells you just what you have to type in order to call the method The semantic component specifies exactly what task the method will accomplish To write a legal program you need to know the syntactic specification of the method To understand the purpose of the method and to use it effectively you need to know the method s semantic specification I will refer to both parts of the interface syntactic and semantic collectively as the contract of the method The contract of a method says essentially Here is what you have to do to use me and here is what I will do for you guaranteed When you write a method the comments that you write for the method should ma
228. how can we even hope to come up with a single generic method that will work for collections that are stored in wildly differ ent 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 iterators that are im plemented in different ways 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 programming for the first time but you should under stand that they solve a difficult problem in an elegant way The interface Collection lt T gt defines a method that can be used to obtain an it erator 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 iter ator 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 a parameterized interface named Iterator lt T gt If coll implements the interface Collection lt T gt for some specific type T then coll iterator returns an iterator 174 of type Iterator lt TI gt with the same type T as its type parameter The interface Iterator lt T gt defines just three methods If iter refers to an object that implement
229. ic void actionPerformed ActionEvent evt which is called to notify the object of an action event e 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 pa rameter evt contains information about the event This information can be re trieved by calling methods in the ActionEvent class In particular evt getActionCommand returns a String giving the command associated with 139 the button By default this command is the text that is displayed on the button but it is possible to set it to some other string The method evt getSource re turns 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 Several useful methods are defined in the JButton class For example stopGoButton setText Stop changes the text dis played on the button to Stop And stopGoButton setActionCommand sgb changes the action command associated to this button for action events Of course JButtons have all the general Component methods such as setEnabled and setFont
230. ically 151 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 showInputDialog Component parentComp String question Again the question can include newline characters The dialog box will contain an input box 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 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 Component parentComp 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 dialog wa
231. icient 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 that you need can be implemented most efficiently We ll see specific examples of this in the next two sections 8 5 iterators and for each Loops THE INTERFACE Collection lt T gt defines a few basic generic algorithms but suppose you want to write your own generic algorithms Suppose for example you want to do something 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 Collections can be represented in any of these forms and many others besides With such a variety of traversal mechanisms
232. if statement can be avoided The problem is that programmers are not perfect In spite of the programmer s intention the program might contain a bug that screws up the precondition So maybe it s a good idea to check the precondition at least during the debugging phase of program development Similarly a postcondition is a condition that is true at a certain point in the pro gram as a consequence of the code that has been executed before that point Assum ing that the code is correctly written a postcondition is guaranteed to be true but here again testing whether a desired postcondition is actually true is a way of check ing for a bug that might have screwed up the postcondition This is somthing that might be desirable during debugging 203 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 or postcondition that should hold at that point in the pro gram When the computer encounters an assertion during the execution of the pro gram it evaluates the condition If the condition is false the program is terminated Otherwise the program continues normally This allows the programmer s belief that the condition is true to be tested if if it not true that indicates that the part of the program that preceded the assertion contai
233. ile data eof false Read until end of file double inputNumber data getlnDouble numbers add inputNumber Output the numbers in reverse order for int i numbers size 1 i gt 0 i result println numbers get i System out println Done catch IOException e Some problem reading the data from the input file System out println Input Error e getMessage finally Finish by closing the files whatever else may have happened data close result close end of main end of class 216 10 2 2 Files and Directories 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 directory 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 also known as the 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 direc tory 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
234. ilities of the class 4 The names of other classes that the class will collaborate with to fulfill its re sponsibilities 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 Card class will collaborate with are listed on the right The idea is that class 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 66 3 3 The Unified Modelling Language
235. implements the interface Comparable lt String gt and define compareTo in a reasonable way and in this case the return value of compareTo is zero if and only if the two strings that are being compared are equal 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 177 x Represents a full name consisting of a first name and a last name public class FullName implements Comparable lt FullName gt private String firstName lastName Nom null first and last names public FullName String first String last Constructor if first null last null throw new IllegalArgumentException Names must be non null firstName first lastName last public boolean equals Object obj try FullName other FullName obj Type cast obj to type FullName return firstName equals other firstName amp amp lastName equals other lastName catch Exception e return false if obj is null or is not of type FirstName public int compareTo FullName other if lastName compareTo other lastName lt 0 If lastName comes before the last name of the other object then this FullName comes before the other FullName Return a negative value to indicate this return 1 if lastName compareTo other lastName gt 0 If lastName comes after the last name of the other object then this FullName com
236. 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 specifications are written to understand and fine tune applications that were al ready well 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 Usu ally this phase is divided into two sub phases 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 e Documentation An important and often overlooked task is documenting the internal design of software for the purpose of future maintenance and enhance ment Documentation is most important for external interfaces Maintenance Maintaining and enhancing software to cope with newly discov ered problems or new requirements can take far more time than the initial development of the software Not only may it be nece
237. in fact a car On the other hand the assignment statement myCar myVehicle would be il legal because myVehicle could potentially refer to other types of vehicles that are not cars This is similar to a problem we saw previously The computer will not al low you to assign an int value to a variable of type short because not every int is a short Similarly it will not allow you to assign a value of type Vehicle to a variable of type Car because not every vehicle is a car As in the case of int s and shorts the solution here is to use type casting If for some reason you hap pen 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 numberOfDoors else if myVehicle instanceof Truck System out println Type of vehicle Truck Truck t t Truck myVehicle System out printIn Number of axels t numberOfAxels 82 el
238. 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 incorrect 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 ra diation while undergoing radiation treatments by a mis programmed computerized radiation 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 Gemi
239. inant catch ArrayIndexOutOfBoundsException e System out println M is the wrong size to have a determinant e printStackTrace Here 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 exception of type ArrayIndex0utOfBoundsException occurs then the computer jumps immediately to the catch clause of the try statement 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 Before the body of the catch clause is executed the object that represents the exception is assigned to the variable e which is used in this example to print a stack trace 196 However the full syntax of the try statement allows more than one catch clause This makes it possible to catch several different types of exceptions with one try state ment In the example above in addition to the possibility of an ArrayIndexOutOfBoundsException there is a possible Nul1PointerException which will occur if the value of M is null We can handle both exceptions by adding a second catch clause to the try statement try double determinant M O 0O M 1 1 M O 1 M 1 0 System out println The determinant of M is determinant catch ArrayIndexOutOfBoundsExc
240. individually separated by spaces Things get more complicated if your classes are not in the default package In that case the class files must be in subdirectories of the directory in which you issue the jar file Making an executable jar file on the command line is a little more complicated There has to be some way of specifying which class contains the main method This is done by creating a manifest file The manifest file can be a plain text file con taining a single line of the form Main Class ClassName where ClassName should be replaced by the name of the class that contains the main method For example if the main method is in the class MosaicDrawFrame then the manifest file should read Main Class MosaicDrawFrame You can give the manifest file any name you like Put it in the same directory where you will issue the jar command and use a command of the form jar cmf ManifestFileName JarFileName jar class to create the jar file The jar command is capable of performing a variety of different operations The first parameter to the command such as cf or cmf tells it which operation to perform By the way if you have successfully created an executable jar file you can run it on the command line using the command java jar For example java jar JarFileName jar 2 6 Creating Abstractions IN THIS SECTION we look at some specific examples of object oriented design in a do main that is simpl
241. ines 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 methods to those already inherited from JTextComponent e g the instance method append moreText where moreText is of type String adds the specified text at the end of the current content of the text area When using append or setText to add text to a JTextArea line breaks can be inserted in the text by using the newline character n And setLineWrap wrap where wrap is of type boolean tells what should happen when a line of text is too long to be displayed in the text area If wrap is true then any line that is too long will be wrapped onto the next line if wrap is false the line will simply extend outside the text area and the user will have to scroll the text area horizontally to see the entire line The default value of wrap is false When the user is typing in a JTextField and presses return an ActionEvent is generated If you want to respond to such events you can register an ActionListener with the text field using the text field s addActionListener method Since a JTextArea can contain multiple lines of text pressing return in a text area does not generate an event is simply begins a new lin
242. ing 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 additional code to provide additional behavior As an example suppose you have a PairOfDice class that includes a roll method Suppose that you want a subclass GraphicalDice to represent a pair of dice drawn on the computer screen The roll 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 which calls the method in the superclass But in addition to that the roll 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 public 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 a
243. input methods inherited from the basic Reader class you will probably want to wrap your FileReader in a Scanner or in some other wrapper class 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 catch statement 214 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 This will be done without any warning To avoid overwriting a file that already exists you can check whether a file of the same name already exists before trying to create the stream as discussed later in this section An IOException might occur in the PrintWriter constructor 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 oper ating system that you are finished using it You can close a file by
244. instance member To get at an instance variable or method from outside the 88 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 vari able in the same object then this x can be used as a full name for that variable If otherMethod 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 to the object that contains the method One common use of thisis 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
245. ion command is by default the label of the checkbox 6 6 4 JTextField and JTextArea The JTextField and JTextArea classes represent components that contain text that can be edited by the user A JTextField holds a single line of text while a JTextArea can hold 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 Both classes are subclasses of an abstract class JTextComponent which defines their common properties JTextField and JTextArea have many methods in common The setText in stance method which takes a parameter of type String can be used to change the text that is displayed in an input component The contents of the component 141 can be retrieved by calling its getText instance method which returns a value of type String If you want to stop the user from modifying the text you can call setEditable false Call the same method with a parameter of true to make the input component user editable again The user can only type into a text component when it has the input focus The user can give the input focus to a text component by clicking it with the mouse but sometimes it is useful to give the input focus to a text field programmatically You can do this by calling its requestFocus method For example when I discover an error in the user s input I usually call requestFocus on the text field that contains the
246. is an interface that extends the interface Iterator lt TI gt The method list listIterator returns an object of type ListIterator lt T gt A ListIterator has the usual Iterator methods hasNext next and remove but it also has methods hasPrevious previous and add obj that 181 make it possible to move backwards in the list and to add an item at the current po sition of the iterator 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 TEREE i If iter is of type ListIterator lt T gt then 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 passes 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 obj that adds the specified object to the list at the current position of the iterator where obj must be of type T This can be between two existing items or at the beginning of the list or at the end of the list As an example of using a ListIterator suppose t
247. 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 215 import java io import java util ArrayList x x Reads numbers from a file named data dat and writes them to a file x named result dat in reverse order The input file should contain x exactly one real number per line public class ReverseFile public static void main String args TextReader data Character input stream for reading data PrintWriter result Character output stream for writing data ArrayList lt Double gt numbers An ArrayList for holding the data numbers new ArrayList lt Double gt try Create the input stream data new TextReader new FileReader data dat catch FileNotFoundException e 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 Error e data close Close the input file return End the program try Read numbers from the input file adding them to the ArrayList wh
248. is one less card in the hand throws IllegalArgumentException if the position does not exist in the hand public void removeCard int position x xx Return the number of cards in the hand x return int the number of cards in the hand x precondition none x postcondition No change in state of Hand public int getCardCount xx Gets the card in a specified position in the hand x Note that this card is not removed from the hand x param position the position of the card that is to be returned x return Card the Card at the specified position x throws IllegalArgumentException if position does not exist x precondition position is valid i e 0 lt position lt number cards x postcondition The state of the Hand is unchanged public Card getCard int position xx Sorts the cards in the hand in suit order and in value order x within suits Note that aces have the lowest value 1 x precondition none x postcondition Cards of the same suit are grouped together and within a suit the cards are sorted by value public void sortBySuit xx Sorts the cards in the hand so that cards are sorted into x order of increasing value Cards with the same value x are sorted by suit Note that aces are considered x to have the lowest value x precondition none x postcondition Cards are sorted in order of increasing value public void sortByValue The Card Class The cl
249. is to test whether the index value is legal before using it in the array 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 impos sible 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 pos sible problems can turn what would otherwise be a straightforward program into a messy tangle of if statements 9 3 1 Exceptions and Exception Classes We have already seen that Java like its cousin C provides a neater more struc tured 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 would rather not have clutter up your elegant algorithm When an exception occurs during the execution of a program we say that the ex ception is thrown When this happens the normal flow of the program is thrown off track and the program is in danger of crashing H
250. is wrapped in the object you can call the method d doubleValue Similarly you can wrap an int in an object of type Integer a boolean value in an object of type Boolean and so on As an example of where this would be useful the collection classes that will be studied in Chapter 10 can only hold objects If you want to add a primitive type value to a collection it has to be put into a wrapper object first In JAVA 5 0 wrapper classes have become easier to use JAVA 5 0 introduced automatic conversion between a primitive type and the corresponding wrapper class For example if you use a value of type int in a context that requires an object of type 35 Integer the int will automatically be wrapped in an Integer object For example you can say Integer answer 42 and the computer will silently read this as if it were Integer answer new Integer 42 This is called autoboxing It works in the other direction too For example if d refers to an object of type Double you can use d in a numerical expression such as 2xd The double value inside d is automatically unboxed and multiplied by 2 Autoboxing and unboxing also apply to method calls For example you can pass an actual parameter of type int to a method that has a formal parameter of type Integer In fact autoboxing and unboxing make it possible in many circumstances to ignore the difference between primitive types and objects The wrapper classes contain a few other things that de
251. 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 to create a class named B as a subclass of a class named A you would write class B extends A additions to and modifications of stuff inherited from class A classB classe class D Several classes can be declared as subclasses of the same superclass The sub classes which might be referred to as sibling classes share some structures and behaviors namely the ones they inherit from their common superclass The super class 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 gen erations of classes This is shown in the diagram where class E is a subclass of 80 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 This whole set of classes forms a small class hierarchy 4 3 Example Vehicles 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
252. iting 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 89 modifications it can only be used to refer to methods and variables in the superclass Let s say that the class you are writing contains an instance method doSomething Consider the method 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 hidden by things in the subclass For example super x always refers to an in stance 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 itself and one defined as part of the superclass The variable in the 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 us
253. ivate 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 visibility 69 b gt Umbrello UML Modeller File Edit Diagram Code Settings Help UML Diagrams a Views amp jLogical View incomming requests Eomeri m E getCode int setCode c int J 2 SP SE obs Documentation Figure 3 2 Umbrello UML Modeller showing a Class Diagram attri int attr2 string operation1 p bool double operation2 Figure 3 3 Visual representation of a Class in UML 70 Figure 3 4 Visual representation of a generalization in UML 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 program ming 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 con
254. ke the contract very clear I should admit that in practice methods contracts are often inadequately specified much to the regret and annoyance of the programmers who have to use them You should keep in mind that methods are not the only example of black boxes in programming For example a class is also a black box We ll see that a class can have a public part representing its interface and a private part that is entirely inside its hidden implementation All the principles of black boxes apply to classes as well as to methods 2 2 2 Preconditions and Postconditions When working with methods as building blocks it is important to be clear about how a method interacts with the rest of the program A convenient way to express the contract of a method is in terms of preconditions and postconditions The precondition of a method is something that must be true when the method is called if the method is to work correctly For example for the built in method Math sqrt x a precondition is that the param eter 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 Al of the caller of the method If you call a method 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 method A p
255. kind of class In this section we will look at enums in a simplified form In practice most uses of enums will only need the simplified form that is presented here An enun is a type that has a fixed list of possible values which is specified when the enum is created In some ways an enum is similar to the boolean data type which has true and false as its only possible values However boolean is a primitive type while an enum is not The definition of an enum types has the simplified form enum enum type name list of enum values This definition cannot be inside a method You can place it outside the main method of the program The enum type name can be any simple identifier This identifier becomes the name of the enum type in the same way that boolean is the name of the boolean type and String is the name of the String type Each value in the list of enum values must be a simple identifier and the identifiers in the list are separated by commas For example here is the definition of an enum type named Season whose values are the names of the four seasons of the year enum Season SPRING SUMMER AUTUMN WINTER By convention enum values are given names that are made up of upper case let ters but that is a style guideline and not a syntax rule Enum values are not vari ables Each value is a constant that always has the same value In fact the possible values of an enum type are usual
256. king 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 The full name of the class file for the Line class will be WireFrameModel Line class Non static nested classes are referred to as inner classes Inner classes are not in practice very different from static nested classes but a non static nested class is actually associated with an object rather than to the class in which it is nested This can take 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 inner classes just as 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 inner 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 even to those that are declared private The two copies of the inner class in two 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
257. l class How can this be The mousePressed method in class RepaintOnClick looks at the source of the event and calls its repaint method If we have registered the RepaintOnClick object as a listener on a RandomStringsPanel then it is that panel that is repainted But the listener ob ject could be used with any type of component and it would work in the same way Similarly RandomStringsPanel contains no reference to the RepaintOnClick class in fact RandomStringsPanel was written before we even knew anything about mouse events The panel will send mouse events to any object that has registered with it as a mouse listener It does not need to know anything about that object except that it is capable of receiving mouse events The relationship between an object that generates an event and an object that responds to that event is rather loose The relationship is set up by registering one object to listen for events from the other object This is something that can poten tially be done from outside both objects Each object can be developed independently with no knowledge of the internal operation of the other object This is the essence of modular design Build a complex system out of modules that interact only in straightforward easy to understand ways Then each module is a separate design problem that can be tackled independently To make this clearer consider the application version of ClickableRandomStrings I have included RepaintOnCli
258. l reasons can be wrapped in a Reader to make it easier to read character data from standard input Reader charIn new InputStreamReader System in As another application the input and output streams that are associated with a network connection are byte streams rather than character streams but the byte streams can be wrapped in character streams to make it easy to send and receive character data over the network 10 1 4 Reading Text Still the fact remains that much I O is done in the form of human readable charac ters In view of this it is surprising that Java does not provide a standard character input class that can read character data in a manner that is reasonably symmet rical with the character output capabilities of PrintWriter There is one basic case that is easily handled by a standard class The BufferedReader class has a method public String readLine throws IOException that reads one line of text from its input source If the end of the stream has been reached the return value is null When a line of text is read the end of line marker is read from the input stream but it is not part of the string that is returned Different input streams use different characters as end of line markers but the readLine method can deal with all the common cases Line by line processing is very common Any Reader can be wrapped in a BufferedReader to make it easy to read full lines of text If reader is of type Reader then a BufferedR
259. le 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 164 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 constructor 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 deck Pile The table pile is displayed differently from the other piles the cards over lap 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 is similar to the one in DiscardPile We must check if any SuitPile 4 of them or any Table
260. lets effectively you have to learn at least a little about creating Web pages Web pages are written using a language called HTML HyperText Markup Language 6 3 1 JApplet The JApplet class in package javax swing can be used as a basis for writing applets in the same way that JFrame is used for writing stand alone applications The basic JApplet class represents a blank rectangular area Since an applet is not a stand alone application this area must appear on a Web page or in some other environment that knows how to display an applet Like a JFrame a JApplet contains a content pane and can contain a menu bar You can add content to an applet either by adding content to its content pane or by replacing the content pane with another component In my examples I will generally create a JPanel and use it as a replacement for the applet s content pane To create an applet you will write a subclass of JApplet The JApplet class de fines several instance methods that are unique to applets These methods are called by the applet s environment at certain points during the applet s life cycle In the JApplet class itself these methods do nothing you can override these methods in a subclass The most important of these special applet methods is public void init An applet s init method is called when the applet is created You can use the init method as a place where you can set up the physical structure of the 113 applet
261. lication THERE ARE TWO BASIC TYPES of GUI program in JAVA stand alone applications and applets An applet is a program that runs in a rectangular area on a Web page Applets are generally small programs meant to do fairly simple things although there is nothing to stop them from being very complex Applets were responsible for a lot of the initial excitement about JAVA when it was introduced since they could do things that could not otherwise be done on Web pages However there are now easier ways to do many of the more basic things that can be done with applets and 107 they are no longer the main focus of interest in JAVA Nevertheless there are still some things that can be done best with applets and they are still fairly common on the Web A stand alone application is a program that runs on its own without depending on a Web browser You ve been writing stand alone applications all along Any class that has a main method defines a stand alone application running the program just means executing this main method However the programs that you ve seen up till now have been command line programs where the user and computer interact by typing things back and forth to each other A GUI program offers a much richer type of user interface where the user uses a mouse and keyboard to interact with GUI components such as windows menus buttons check boxes text input boxes scroll bars and so on The main method of a GUI progr
262. lid file name throw new ParseError The word If the throw statement does not occur in a try statement that catches the error then the method that contains the throw statement must declare that it can throw a ParseError by adding the clause throws ParseError to the method 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 method 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 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 was destroyed int where_x where_y Location where ship was destroyed ShipDestroyed String message Ship s int x int y Constructor creates a ShipDestroyed object carrying an error message plus 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 He
263. lity classes The most basic package is called java lang This package contains fundamental classes such as String Math Integer and Double 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 methods in those classes This is not a complete picture since it shows only a very few of the many items in each element Math Subroutines nested in classes nested in two layers of packages The full name of sqrt is java lang Math sart The official documentation for the standard JAVA 5 0 API lists 165 different pack ages including sub packages and it lists 3278 classes in these packages Many of these are rather obscure or very specialized but you might want to browse through the documentation to see what is available 44 Even an expert programmer won t be familiar with the entire API or even a ma jority of it In this book you ll only encounter several dozen classes and those will be sufficient for writing a wide variety of programs Using Classes from Packages Let s say that you want to use the class java awt Color in a program that you are writing Like any class java awt Color is a type which means that you can use it declare variables and parameters and to specify the return type of a method One way to do this is to use the full name of the class as the name of the type For example suppose that
264. llows you to extend the behavior of the rol1 method even if you don t know how the method is implemented in the superclass 4 6 3 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 constructors in the subclass you have to define new ones from scratch If 90 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 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 method even though super is not a method and you can t call constructors the same way you call other methods anyway As an example as
265. long expression that both defines an un named class and cre ates 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 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 useful only 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
266. lor to select that color for drawing It then chooses random x y coordi nates 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 panel Similarly the formula for y allows the string to extend beyond the top and bottom of the applet Here is the complete source code for the RandomStringsPanel import java awt Color import java awt Font import java awt Graphics import javax swing JPanel 126 x This panel 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 lt p gt This panel is meant to be used as the content pane in either an applet or a frame public class RandomStringsPanel extends JPanel private String message The message to be displayed This can be set in the constructor If no value is provided in the constructor then the string Java is used private Font fontl font2 font3 font4 font5 The five fonts x Default constructor creates a panel that displays the message Java public RandomStringsPanel
267. lue in the list That is Season SPRING ordinal is the int value 0 Season SUMMER ordinal is 1 while 2 is Season AUTUMN ordinal and 3 is Season WINTER ordinal is You can use the ordinal method with a variable of type Season such as vacation ordinal in our example You should appreciate enums as the first example of an important concept cre ating new types Here is an example that shows enums being used in a complete program public class EnumDemo Define two enum types definitions go OUTSIDE The main routine enum Day SUNDAY MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SATURDAY enum Month JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC public static void main String args Day tgif Declare a variable of type Day Month libra Declare a variable of type Month tgif Day FRIDAY Assign a value of type Day to tgif libra Month OCT Assign a value of type Month to libra System out print My sign is libra since was born in System out println libra Output value will be OCT System out print That s the System out print libra ordinal System out println th month of the year System out println Counting from 0 of course System out print Isn t it nice to get to System out printin tgif Output value will be FRIDAY System out printin tgif is the tgif ordinal th day of the week Can concatenate enum va
268. lues onto Strings 20 Enums and for each Loops Java 5 0 introduces a new enhanced form of the for loop that is designed to be convenient for processing data structures A data structure is a collection of data items considered as a unit For example a list is a data structure that consists simply of a sequence of items The enhanced for loop makes it easy to apply the same processing to every element of a list or other data structure However one of the applications of the enhanced for loop is to enum types and so we consider it briefly here The enhanced for loop can be used to perform the same processing on each of the enum constants that are the possible values of an enumerated type The syntax for doing this is for enum type name variable name enum type name values statement or for enum type name variable name enum type name values statements If MyEnum is the name of any enumerated type then MyEnum values is a method call that returns a list containing all of the values of the enum values is a static member method in MyEnum and of any other enum For this enumerated type the for loop would have the form for MyEnum variable name MyEnum values statement The intent of this is to execute the statement once for each of the possible values of the MyEnum type The variable name is the loop control variable In the statement it represents the enumerated type value that is cur
269. ly not instance methods since they don t belong to objects Since they are re sponsible for creating objects they exist before any objects have been created They are more like static member methods but they are not and cannot be declared to be static In fact according to the JAVA language specification they are technically 32 not members of the class at all In particular constructors are not referred to as methods Unlike other methods 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 computes 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 method 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 method 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 declara
270. ly referred to as enum constants 19 Note that the enum constants of type Season are considered to be contained in Season which means following the convention that compound identifiers are used for things that are contained in other things the names that you actually use in your program to refer to them are Season SPRING Season SUMMER Season AUTUMN and Season WINTER Once an enum type has been created it can be used to declare variables in exactly the same ways that other types are used For example you can declare a variable named vacation of type Season with the statement Season vacation After declaring the variable you can assign a value to it using an assignment statement The value on the right hand side of the assignment can be one of the enum constants of type Season Remember to use the full name of the constant including Season For example vacation Season SUMMER You can print an enum value with the statement System out print vacation The output value will be the name of the enum constant without the Season In this case the output would be SUMMER Because an enum is technically a class the enum values are technically objects As objects they can contain methods One of the methods in every enum value is ordinal When used with an enum value it returns the ordinal number of the value in the list of values of the enum The ordinal number simply tells the posi tion of the va
271. m 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 Instead 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 are two different names for 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 The test if stdl std2 tests 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 the instance variables in th
272. m that plays the card game Blackjack You can use the Card Hand and Deck classes developed previously How ever 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 accord ing 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 e The value of a numeric card such as a three or a ten is its numerical value e The value of a Jack Queen or King is 10 e 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 x Computes and returns the value of this hand in the game x of Blackjack public int getBlackjackValue int val The value computed for the hand boolean ace This will be set to true if the int val ace hand contains an ace cards Number of cards in the hand 0 false cards getCardCount for int i 0 i lt cards i Add the value of the i th card in the hand Card card The i th card int ca
273. ment Car 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 behavior 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 is an object of type Caror Truck or Motorcycle is automatically an object of type Vehicle too 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 is that an object of type Car can be assigned to a vari able of type Vehicle i e 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 re members 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
274. mplete very quickly ImageIO read on the other hand can take some time to execute 156 A Solitaire Game Klondike In this chapter will build a version of the Solitaire game We ll use the case study investigate the object oriented concepts of encapsulation inheritance and polymor phism The game is inspired by Timothy Budd s version in his book AN INTRODUC TION TO OBJECT ORIENTED PROGRAMMING 7 1 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 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
275. n 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 methods in the Deck class xx Constructor Create a shuffled deck of cards x precondition None x postcondition A deck of 52 shuffled cards is created x public Deck x Shuffle all cards in the deck into a random order x precondition None x postcondition The existing deck of cards with the cards in random order public void shuffle xx Returns the size of the deck x return the number of cards that are still left in the deck x precondition None x postcondition The deck is unchanged x public int size 53 xx Determine if this deck is empty x return true if this deck has no cards left in the deck x precondition None x postcondition The deck is unchanged x public boolean isEmpty xx Deal one card from this deck x return a Card from the deck x precondition The deck is not empty x postcondition The deck has one less card public Card deal This is everything you need to know in order to use the Deck class Of course it doesn t tell us ho
276. n 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 method that searches for the object obj in the ArrayList If the object is found in the list then the position number where it is found is returned If the object is not found then 1 is returned 169 For example suppose 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 newP layer 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 method players get i to get the value stored at position i in the ArrayList The return type of this method is Object In this case the object that is returned by the method is actually of type Player In order to do anything useful with the returned value
277. n of type IOException is thrown Since IOException 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 method 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 In stead you ll use subclasses of InputStream that add more convenient input methods to InputStreanm s rather primitive capabilities Similarly the OutputStream class de fines a primitive output method for writing one byte of data to an output stream The method is defined as public void write int b throws IOException The param eter is of type int rather than byte but the parameter value is type cast to type byte before it is written this effectively discards all but the eight low order bytes of b Again in practice you will almost always use higher level output operations defined in some subclass of OutputStream The Reader and Writer classes provide identical low level read and write meth ods As in the byte stream classes the parameter of the write c method in Writer and the return value of the read method in Reader are of type int but in these char
278. n to the java command that is used to run the program For example if the class that contains the main program is RootFinder then the command java enableassertions RootFinder will run the program with assertions enabled The enableassertions option can be abbreviated to ea so the command can alternatively be written as java ea RootFinder In fact it is possible to enable assertions in just part of a program An option of the form ea class name enables only the assertions in the specified class Note that 205 6649 there are no spaces between the ea the and the name of the class To enable all the assertions in a package and in its sub packages you can use an option of the form ea package name 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 ea For example to run a Java program named MegaPaint with assertions enabled for every class in the packages named paintutils and drawing you would use the command java ea paintutils ea drawing MegaPaint If you are using the Eclipse integrated development environment you can specify the ea option by creating a run configuration Right click the name of the main program class in the Package Explorer pane and select Run As from the pop up menu and then Run from the submenu This will open a
279. n when either of these preconditions is violated When an illegal condition is found in a method throwing an exception is often a rea sonable response If the program that called the method 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 A throws clause in a method heading can declare several different types of excep tions separated by commas For example void processArray int A throws NullPointerException ArrayIndexOutOfBoundsException 9 3 4 Mandatory Exception Handling In the preceding example declaring that the method root can throw an IllegalArgumentException is just a courtesy to potential readers of this method This is because handling of IllegalArgumentExceptions is not mandatory A method can throw an IllegalArgumentException without announcing the possibil ity And a program that calls that method is free either to catch or to ignore the exception just as a programmer can choose either to catch or to ignore an exception of type Nul 1PointerException For those exception classes that require mandatory handling the situation is dif ferent If a method can throw such an exception that fact must be announced in a throws clause in the method 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 the body of a metho
280. nce 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 Randon class for rolling the dice To do this a PairOfDice object needs access to an object of type Random But there is no need for each PairOfDice object to have a separate Randomobject In fact it would not even be a good idea Because of the way ran dom 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 import java util Random public class PairOfDice code private static Random randGen new Random public int diel 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 diel randGen nextInt 6 1 die2 randGen nextInt 6 1 end class PairOfDice As another example let s rewrite the Student class I 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 n
281. nd 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 after it has been created In this section we will look at a few of the basic standard components that are available in Swing In the next section we will consider the problem of laying out components in containers 6 6 1 JButton An object of class JButton is a push button that the user can click to trigger some action You ve already seen buttons but we consider them in much more detail here To use any component effectively there are several aspects of the corresponding class that you should be familiar with For JButton as an example I list these aspects explicitly e Constructors The JButton class has a constructor that takes a string as a parameter This string becomes the text displayed on the button For example constructing the JButton with stopGoButton new JButton Go creates a button object that will display the text Go but remember that the button must still be added to a container before it can appear on the screen e 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 as an ActionListener Listeners An object that wants to handle events generated by buttons must implement the ActionListener interface This interface defines just one method pub
282. nd the object in memory This information is called a reference or pointer to the object In effect a reference 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 In a program objects are created using 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 to say that the object is the value of the variable std 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 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 variables can be referred to as std name std testl1 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 System o
283. ndition error message where condition is a boolean valued expression and error message is a string or an expression of type String The word assert is a reserved word in Java which cannot be used as an identifier An assertion statement can be used anyplace in Java where a statement is legal If a program is run with assertions disabled an assertion statement is equiva lent to an empty statement and has no effect When assertions are enabled and an assertion statement is encountered in the program the condition in the assertion is evaluated If the value is true the program proceeds normally If the value of the condition is false then an exception of type java lang AssertionError is thrown and the program will crash unless the error is caught by a try statement If the assert statement includes an error message then the error message string becomes the message in the AssertionError So the statement assert condition error message is similar to if condition false throw new AssertionError error message except that the if statement is executed whenever the program is run and the assert statement is executed only when the program is run with assertions enabled The question is when to use assertions instead of exceptions The general rule is to use assertions to test conditions that should definitely be true if the program is written correctly Assertions are useful for testing a program to see whether o
284. nditions and Postconditions 008 4 2 2 8 APIsand Packages 2 e e cee eee ee ns 2 3 Introduction to Error Handling 2 000000 24 Javadoc asd s She ee ns Reel he ee Se SES ee ea ee 2 5 Creating Jar Files 0 ee 2 6 Creating Abstractions 1 0 e ee 2 6 1 Designing the classes 0 0 cee ee ns 2 7 Example A Simple Card Game 0000 ee uae Tools for Working with Abstractions 3 1 Introduction to Software Engineering 2 0008 3 1 1 Software Engineering Life Cycles 04 4 3 1 2 Object oriented Analysis and Design 3 1 3 Object Oriented design 0 0 000 eee ee es 3 2 Class Responsibility Collaboration cards 0 4 3 3 The Unified Modelling Language 0 0008 cae 3 3 1 Modelling 3 ec sedge Ae SE AP BS ee OR A a 3 3 2 Use Case Diagrams 2 2 ee ee es 3 3 38 Class Diagrams 0 0 3 3 4 Sequence Diagrams 00 eee ee ens 3 3 5 Collaboration Diagrams 00 002 eens 3 3 6 State Diagrams 66 5 4 ee ode ee eee A Sot ee 4 Inheritance Polymorphism and Abstract Classes 4 1 Extending Existing Classes 2 200000 ee eens 4 2 Inheritance and Class Hierarchy 00 0000s 4 3 Example Vehicles 2 ae i i e aa E E E a E E E a ia 4 4 Polymorphism sco sk se ee we he a a Bh ee es ALA 4 5 Abstract Classes u d eri ios Seats poh ode
285. necting 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 types 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 be tween 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 In UML associations are represented as lines connecting the classes participat ing 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 e Aggregations Aggregations are a special type of associations in which the two participating classes don t have an equal status but make a whole part re lationship An Aggregation describes how the class that takes the role of the 71 Figure 3 5 Visual represe
286. ned a bug One nice thing about 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 Although early versions of Java did not have assertions an assertion facility sim ilar to the one in C C has been available in Java since version 1 4 As with the C C version Java assertions can be turned on during debugging and turned off during normal execution In Java however assertions are turned on and off at run time rather than at compile time An assertion in the Java source code is always included in the compiled class file When the program is run in the normal way these assertions are ignored since the condition in the assertion is not evaluated in this case there is little or no performance penalty for having the assertions in the program When the program is being debugged it can be run with assertions en abled as discussed below and then the assertions can be a great help in locating and identifying bugs An assertion statement in Java takes one of the following two forms assert condition or assert co
287. ng and the ArrayList examples in the previous subsection is the use of the type Object as the basic type for objects that are stored in a list This has at least two unfortunate consequences First it makes it necessary to use type casting in almost every case when an element is retrieved from that list Second since any type of object can legally be added to the list there is no way for the compiler to detect an attempt to add the wrong type 170 of object to the list the error will be detected only at run time when the object is retrieved from the list and the attempt to type cast the object fails Compare this to arrays An array of type BaseType can only hold objects of type BaseType An attempt to store an object of the wrong type in the array will be detected by the compiler and there is no need to type cast items that are retrieved from the array back to type BaseType To address this problem Java 5 0 introduced parameterized types ArrayList is an example Instead of using the plain ArrayList type it is possible to use ArrayList lt BaseType gt where BaseType is any object type that is the name of a class or of an interface BaseType cannot be one of the primitive types ArrayList lt BaseType gt can be used to create lists that can hold only objects of type BaseType For example ArrayList lt ColoredRect gt rects declares a variable named rects of type ArrayList lt ColoredRect gt and rects new ArrayList lt ColoredR
288. nge or an IndexOutOfBoundsException is thrown list set index obj stores the object obj at position number index in the list replacing the object that was there previously The object obj must be of 180 type T 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 where obj must be of type T The number of items in the list increases by one and items that come after position index move up one position to make room for the new item The value of index must be in the range 0 to list size inclusive If index is equal to list size then obj is added at the end of the list e list remove index removes the object at position number index and returns that object as the return value of the method Items after this position move up one space in the list to fill the hole and the size of the list decreases by one The value of index must be in the range 0 to list size 1 e 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 The object obj can be of any type not just of type T If obj occurs more than once in the list the index of the first occurrence is returned These methods are defined both in class ArrayList lt T gt and in class LinkedList lt T gt although some of them get and set are only efficient for A
289. nging to the class ButtonHandler which is defined as a nested class inside Hel loWor1dGUI2 private static class ButtonHandler implements ActionListener public void actionPerformed ActionEvent e System exit 0 This class implements the ActionListener interface a requirement for listener ob jects that handle events from buttons The event handling method is named actionPerformed as specified by the ActionListener interface This method con tains the code that is executed when the user clicks the button in this case the code is a call to System exit which will terminate the program There is one more ingredient that is necessary to get the event from the button to the listener object The listener object must register itself with the button as an event listener This is done with the statement okButton addActionListener listener This statement tells okButton that when the user clicks the button the ActionEvent that is generated should be sent to listener Without this statement the button 112 has no way of knowing that some other object would like to listen for events from the button This example shows a general technique for programming the behavior of a GUI Write classes that include event handling methods Create objects that belong to these classes and register them as listeners with the objects that will actually detect or generate the events When an event occurs the listener is notified and the c
290. ni 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 186 In 1990 AT amp T s long distance telephone service was disrupted throughout the 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 pro grammers we need to understand why that is true and what can be done about it 9 1 2 Java to the Rescue 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 languages when a variable name is used in a program the variable is created au tomatically 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 programming language the command DO 20 I 1 5 is the first statement of a counting loop Now spaces are insignificant in FORTRAN so this is equivalent to DO20I 1 5 On t
291. ns 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 dif ferent values The values of type int range from 2147483648 to 2147483647 What happens when 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 How ever 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 Val ues 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 while N 1 if N 2 0 If N is even N N 2 else N 3 N41 System out println N But there is a problem here If N is too large then the value of 3 x 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
292. ns were added to the panel using the statements panel add button1 panel add button2 panel add button3 panel add button4 panel add button5 When a container uses a layout manager the layout manager is ordinarily respon sible for computing the preferred size of the container although a different preferred size could be set by calling the container s setPreferredSize method A FlowLayout prefers to put its components in a single row so the preferred width is the total of the preferred widths of all the components plus the horizontal gaps between the compo nents The preferred height is the maximum preferred height of all the components A BorderLayout layout manager is designed to display one large central compo nent with up to four smaller components arranged along the edges of the central com ponent If a container cntr is using a BorderLayout then a component comp should be added to the container using a statement of the form 144 cntr add comp borderLayoutPosition where borderLayoutPosition specifies what position the component should occupy in the layout and is given as one of the constants BorderLayout CENTER BorderLayout NORTH BorderLayout SOUTH BorderLayout EAST or BorderLayout WEST The meaning of the five positions is shown in this diagram a Center Note that a border layout can contain fewer than five compompontnts so that not all five of the possible positions need to be
293. nstance variable size of the display font public void init String value value getParameter message Get message param if any if value null message Hello World Default value if no param is present else message value Value from PARAM tag value getParameter font if value null fontName SansSerif Default value if no param is present else fontName value value getParameter size try fontSize Integer parseInt value Convert string to number catch NumberFormatException e fontSize 20 Default value if no param is present or if the parameter value is not a legal integer Elsewhere in the applet the instance variables message fontName and fontSize would be used to determine the message displayed by the applet and the appear ance of that message Note that the value returned by getParameter is always a String If the param represents a numerical value the string must be converted into a number as is done here for the size parameter 6 4 Graphics and Painting EVERTHING 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 I ll look at some of the most basic of these The physical structure of a GUI is built of components The term component refers to a visual element in a GUI including buttons menus text input boxes scroll bars check
294. nt evt public void mouseReleased MouseEvent evt public void mouseClicked MouseEvent evt public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt 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 methods mousePressed mouseReleased and mouseClicked 131 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 compo nent to change appearance whenever the user moves the mouse over the component you could define these two methods As an example we will look at a small addition to the RandomStringsPanel ex ample from the previous section In the new version the panel will repaint itself when the user clicks on it In order for this to happen a mouse listener should listen for mouse events on the panel and when the listener detects a mousePressed event it should respond by
295. ntWriter that data will go to exactly the same place as data written di rectly 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 if out is a variable of type PrintWriter then the following methods are defined e out print x prints the value of x represented in the form of a string of char acters to the output stream x can be an expression of any type including both primitive types and object types An object is converted to string form using its toString method A null value is represented by the string null out print1n outputs an end of line to the output stream e out println x outputs the value of x followed by an end of line this is equiv alent to out print x followed by out println out printf formatString x1 x2 does formated output of x1 x2 to the output stream The first parameter is a string that specifies the format of the output There can be any number of additional parameters of any type but the types of the parameters must match the formatting directives in the format string 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 catc
296. ntation of an Association in UML Aggregation ear Wheel o i E j e al Figure 3 6 Visual representation of an Aggregation relationship in UML whole is composed has of other classes which take the role of the parts For Agegregations 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 e Composition Compositions are associations that represent very strong aggre gations 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 cre ated of them They can contain operations but no attributes Classes can in herit from 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 lan guage 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
297. ntention 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 9 1 Introduction 9 1 1 Horror Stories MOST COMPUTER USERS HAVE PERSONAL EXPERIENCE with programs that don t work or that crash 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 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 cen timeters 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 sup posed 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
298. od call will correctly convert the string into the int 42 However if the value of str is say fred the method call will fail because fred is not a legal string representation of an int value In this case an exception of type NumberFormatException occurs If nothing is done to handle the exception the program will crash An T1legalArgumentException can occur when an illegal value is passed as a pa rameter to a method For example if a method requires that a parameter be greater than or equal to zero an IllegalArgumentException might occur when a negative value is passed to the method How to respond to the illegal value is up to the person who wrote the method so we can t simply say that every illegal parameter value will result in an IllegalArgumentException However it is a common response One case where an I legalArgumentException can occur is in the valueOf method of an enumerated type Recall that this method tries to convert a string into one of the values of the enumerated type If the string that is passed as a param eter to valueOf is not the name of one of the enumerated type s value then an TllegalArgumentException occurs For example given the enumerated type enum Toss HEADS TAILS Toss valueOf HEADS correctly returns Toss HEADS but Toss valueOf FEET results in an IllegalArgumentException try catch When an exception occurs we say that the exception is thrown For exampl
299. ode that you wrote in one of its event handling methods is executed At first this might seem like a very roundabout and complicated way to get things done but as you gain experience with it you will find that it is very flexible and that it goes together very well with object oriented programming We will return to events and listeners in much more detail in later sections 6 3 Applets and HTML ALTHOUGH STAND ALONE APPLICATIONS are probably more important than applets at this point in the history of JAVA applets are still widely used They can do things on Web pages that can t easily be done with other technologies It is easy to distribute applets to users The user just has to open a Web page and the applet is there with no special installation required although the user must have an appropriate version of JAVA installed on their computer And of course applets are fun now that the Web has become such a common part of life it s nice to be able to see your work running on a web page The good news is that writing applets is not much different from writing stand alone applications The structure of an applet is essentially the same as the structure of the JFrames that were introduced in the previously and events are handled in the same way in both types of program So most of what you learn about applications applies to applets and vice versa Of course one difference is that an applet is dependent on a Web page so to use app
300. of code which defines it and which is executed when the method is called In Java interface is a reserved word with an additional technical meaning An interface in this sense consists of a set of instance method interfaces without any associated implementations Actually a Java interface can contain other things as well but we won t discuss them here A class can implement an interface by providing an implementation for each of the methods 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 draw method 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 public class Line implements Drawable public void draw Graphics g do something presumably draw a line other methods and variables Note that to implement an interface a class must do more than simply pro vide an implementation for each method in the interface it must also state that 94 it implements the interface using the reserved word implements as in this exam ple public class Line implements Drawable Any class that implements the Drawable interface defines a draw instance method Any object created from such a class includes a
301. of the type are the same in each case 2 1 3 Abstraction in Object Oriented Programs There are many important layers of abstraction in object oriented programs 2 At the highest level we view the program as a community of objects that inter act with each other to achieve common goals Each object provides a service that is used by other objects in the community At this level we emphasize the lines of communication and cooperation and the interactions between the objects Another level of abstraction allows the grouping of related objects that work to gether For example JAVA provides units called packages for grouping related objects These units expose certain names to the system outside the unit while hiding certain features For example the java net package provides classes for networking appli cations The JAVA Software Development Kit contains various packages that group different functionality The next levels of abstraction deal with interactions between individual objects A useful way of thinking about objects is to see them as providing a service to other objects Thus we can look at an object oriented application as consisting of service providers and service consumers or clients One level of abstraction looks at this re lationship from the server side and the other looks at it from the client side Clients of the server are interested only in what the server provides its behaviour and not how it provides it its implementati
302. oid actionPerformed ActionEvent e System exit 0 public static void main String args HelloWorldDisplay displayPanel new HelloWorldDisplay JButton okButton new JButton Ok ButtonHandler listener new ButtonHandler okButton addActionListener listener JPanel content new JPanel content setLayout new BorderLayout content add displayPanel BorderLayout CENTER content add okButton BorderLayout SOUTH JFrame window new JFrame GUI Test window setContentPane content window setSize 250 100 window setLocation 100 100 window setVisible true 6 2 1 JFrame and JPanel In a JAVA GUI program each GUI component in the interface is represented by an object in the program One of the most fundamental types of component is the window Windows have many behaviors They can be opened and closed They can be resized They have titles that are displayed in the title bar above the window And most important they can contain other GUI components such as buttons and menus JAVA of course has a built in class to represent windows There are actually several different types of window but the most common type is represented by the JFrame class which is included in the package javax swing A JFrame is an inde pendent window that can for example act as the main window of an application One of the most important things to understand is that a JFrame object comes with many of the
303. on The client only needs to know the public interface of a class it wants to use what methods it can call their input parameters what they return and what they accomplish The next level of abstraction looks at the relationship from the server side Here we consider the concrete implementation of the abstract behaviour Here we are concerned with how the services are realized 2This discussion is based on Chapter 2 of An Introduction to Object Oriented Programming by Tim othy Budd 39 The last level of abstraction considers a single task in isolation i e a single method Here we deal with the sequence of operations used to perform just this one activity Each level of abstraction is important at some point in the development of soft ware As programmers we will constantly move from one level to another 2 2 Methods as an Abstraction Mechanism IN THIS SECTION we ll discuss an abstraction mechanism you re already familiar with the method variously called subroutines procedures and even functions 2 2 1 Black Boxes A Method is an abstraction mechanism and consists of instructions for performing some task chunked together and given a name Chunking allows you to deal with a potentially very complicated task as a single concept Instead of worrying about the many many steps that the computer might have to go though to perform that task you just need to remember the name of the method Whenever you want your program
304. onceptualising what it means to perform computation and how tasks to be carried out on a computer should be structured and organised We can distinguish between two types of programming languages Imperative languages and declarative languages Imperative knowledge describes how to knowl edge while declarative knowledge is what is knowledge A program is declarative if it describes what something is like rather than how to create it This is a different approach from traditional imperative programming languages such as Fortran and C which require the programmer to specify an al gorithm to be run In short imperative programs make the algorithm explicit and leave the goal implicit while declarative programs make the goal explicit and leave the algorithm implicit Imperative languages require you to write down a step by step recipe specifing how something is to be done For example to calculate the factorial function in an imperative language we would write something like public int factorial int n int ans 1 for int i 2 i lt n i ans ans i return ans Here we give a procedure a set of steps that when followed will produce the answer 12 Functional programming Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions Functional programming emphasizes the def inition of functions in contrast to procedural programming which emphasizes
305. one of B s methods object A sends a message to object B There are three parts of a message The three parts for the message System out printin Hello World are e The object to which the message is addressed System out e The name of the method to perform println e Any parameters needed by the method Hello World Classes In object oriented software it s possible to have many objects of the same kind that share characteristics rectangles employee records video clips and so on A class is a software blueprint for objects A class is used to manufacture or create objects The class declares the instance variables necessary to contain the state of every object The class would also declare and provide implementations for the instance methods necessary to operate on the state of the object Definition A class is a blueprint that defines the variables and the methods common to all objects of a certain kind 17 After you ve created the class you can create any number of objects from that class A class is a kind of factory for constructing objects The non static parts of the class specify or describe what variables and methods 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 struc ture if they are created using the same class Types JAVA like most pro
306. only JLabels are non opaque A non opaque or transparent component ignores its background color and sim ply paints its content over the content of its container This usually means that it inherits the background color from its container comp setToolTipText string sets the specified string as a tool tip for the component The tool tip is displayed if the 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 138 e comp setPreferredSize size sets the size at which the component should be displayed if possible The parameter is of type java awt Dimension where an object of type Dimension has two public integer valued instance variables width and height A call to this method usually looks something like setPreferredSize new Dimension 100 50 The preferred size is used as a hint by layout managers but will not be re spected in all cases Standard components generally compute a correct pre ferred size automatically but it can be useful to set it in some cases For exam ple if you use a JPanel as a drawing surface it might be a good idea to set a preferred size for it Note 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 A
307. onsti tutes the API or Applications Programming Interface associated with the toolbox The Macintosh API is a specification of all the methods 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 methods that programmers can call in order to communicate with and control the device Scientists who write a set of methods for doing some kind of complex compu tation such as solving differential equations say would provide an API to allow others to use those methods 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 methods such as Math sqrt the String data type and its associated methods and the System out print methods The standard JAVA API includes methods for work ing with graphical user interfaces for network communication for reading and writ ing files and more It s tempting to think of these methods as being built into the JAVA language but they are technically methods 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 Linux and others The same JAVA API must work on all these platforms But noti
308. or any other type would be almost identical except for the substitution of one type name for another It seems silly to write essentially the same code over and over Java goes some distance towards solving this problem by providing the ArrayList class An ArrayList is essentially a dynamic array of values of type Object Since every class is a subclass of Object objects of any type can be stored in an ArrayList Java goes even further by providing parameterized types The ArrayList type can be parameterized as in ArrayList lt String gt to limit the val ues that can be stored in the list to objects of a specified type Parameterized types extend Java s basic philosophy of type safe programming to generic programming 167 8 1 Generic Programming in Java JAVA S GENERIC PROGRAMMING FEATURES are represented by group of generic classes and interfaces as a group are known as the Java Collection Framework These classes represents various data structure designed to hold Objects can be used with objects of any type Unfortunately the result is a category of errors that show up only at run time rather than at compile time If a programmer assumes that all the items in a data structure are strings and tries to process those items as strings a run time error will occur if other types of data have inadvertently been added to the data structure In JAVA the error will most likely occur when the program retrieves an Object from the data str
309. or example a constructor for the PairOfDice class 30 could provide the values that are initially showing on the dice Here is what the class would look like in that case 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 expression new PairOfDice 3 4 would create a PairOfDice object in which the values of the instance variables diel and die2 are initially 3 and4 Of course in a program the value returned 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 Now that weve added a constructor to the PairOfDice class we can no longer create an object by saying new PairOfDice The system provides a default con structor for a class only if the class definition does not already include a constructor so there is only one constructor in the class and it requires two actual parameters 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
310. or the actions The message encodes the request for an action and is accompanied by any additional information arguments parameters needed to carry out the request The receiver is the object to whom the message is sent If the receiver accepts 3This discussion is based on Chapter 2 of An Introduction to Object Oriented Programming by Tim othy Budd 14 the message it accepts responsibility to carry out the indicated action In response to a message the receiver will perform some method to satisfy the request There are some important issues to point out here e The client sending the request need not know the means by which the request is carried out In this we see the principle of information hiding e Another principle implicit in message passing is the idea of finding someone else to do the work i e reusing components that may have been written by someone else The interpretation of the message is determined by the receiver and can vary with different receivers For example if you sent the message deliver flowers to a friend she will probably have understood what was required and flowers would still have been delivered but the method she used would have been very different from that used by the florist In object oriented programming behaviour is described in terms of responsibil ities Client s requests for actions only indicates the desired outcome The receivers are free to pursue any technique that achie
311. orrect insertion point and end the loop iter previous break iter add newltem 182 Here stringList may be of type ArrayList lt String gt or of type LinkedList lt String gt The algorithm that is used to insert newItem into the list will be about equally ef ficient for both types of lists and it will even work for other classes that imple ment the interface List lt String gt You would probably find it easier to design an insertion algorithm that uses array like indexing with the methods get index and add index obj However that algorithm would be inefficient for LinkedLists be cause random access is so inefficient for linked lists By the way the insertion algo rithm works when the list is empty It might be useful for you to think about why this is true Sorting Sorting a list is a fairly common operation and there should really be a sorting method in the List interface There is not presumably because it only makes sense to sort lists of certain types of objects but methods for sorting lists are available as static methods in the class java util Collections This class contains a vari ety of static utility methods for working with collections The methods are generic that is they will work for collections of objects of various types Suppose that list is of type List lt T gt The command Collections sort list can be used to sort the list into ascending order The items in the list should implement the
312. orrect predictions you make before System out println you guess wrong System out println int gamesPlayed 0 Number of games user has played int sumOfScores 0 The sum of all the scores from all the games played double averageScore Average score computed by dividing sumOfScores by gamesPlayed boolean playAgain Record user s response when user is asked whether he wants to play another game do int scoreThisGame Score for one game scoreThisGame play Play the game and get the score sumOfScores scoreThisGame gamesPlayed System out print Play again playAgain keyboard nextBoolean while playAgain averageScore double sumOfScores gamesPlayed System out println System out println You played gamesPlayed games System out printf Your average score was 1 3f n averageScore end main 59 x Let s the user play one game of HighLow and returns the x user s score on that game The score is the number of x correct guesses that the user makes private static int play Deck deck new Deck Get a new deck of cards and store a reference to it in the variable deck Card currentCard The current card which the user sees Card nextCard The next card in the deck The user tries to predict whether this is higher or lower than the current card int correctGuesses The numb
313. ostcondition of a method represents the other side of the contract It is some thing that will be true after the method has run assuming that its preconditions were met and that there are no bugs in the method The postcondition of the method Math sqrt is that the square of the value that is returned by this method is equal to the parameter that is provided when the method 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 method 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 vari ables that are used in the method The postcondition of a method specifies the task that it performs For a method the postcondition should specify the value that the method returns Methods are often described by comments that explicitly specify their preconditions and postconditions When you are given a pre written method a statement of its preconditions and postcondtions tells you how to use it and what it does When you are assigned to write a method the preconditions and postconditions give you an exact specification of what the method is expected to do Its a good idea to write preconditions and postconditions as part of comments are given in the form of
314. owever the crash can be avoided if the exception is caught and handled in some way An exception can be thrown in one part of a program and caught in a different part An exception that is not caught will generally cause the program to crash By the way since Java programs are executed by a Java interpreter having a pro gram 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 termi nating the program In many other programming languages a crashed program will sometimes crash the entire system and freeze the computer until it is restarted With 194 Java such system crashes should be impossible 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 ex ception occurs to the point where it is caught and handled This information always includes the method call stack which is a list of the methods that were being executed when the exception was thrown Since one method can call another several methods can be active at the same time Typically an exception object also includes an error message describing what happened to cause the excep
315. pe value in the list of possible values with counting starting from zero 102 If you find it annoying to use the class name as part of the name of every enu merated type constant you can use static import to make the simple names of the constants directly available but only if you put the enumerated type into a pack age For example if the enumerated type CardValue is defined in a package named cardgames then you could place import static cardgames CardValue x at the beginning of a source code file This would allow you for example to use the name JACK in that file instead of CardValue JACK 103 104 Graphical User Interfaces in JAVA Contents 6 1 Introduction The Modern User Interface 106 6 2 The Basic GUI Application 2 0 0c eee ees 107 6 2 1 JFrameandJPanel 0 00000 e eae 109 6 2 2 Components and Layout 00005 111 6 2 3 Events and Listeners 00 000005 112 6 3 Applets and HTML a6 6 os Re RRS BP SW allel Sieg wii 113 6 34 SAP plete eek ee ede Sk I BE essed da hay yy aE 113 6 3 2 Reusing Your JPanels 0 0002 eee 115 6 3 3 Applets on Web Pages 00002 e eee 117 6 4 Graphics and Painting 2 cee eevee cvens 119 6 4 1 Coordinates soea 4 6 taea aos eh ee a a 121 e E EEE E E ee ee ee eo She BR ee ee E DDD ee 122 6 4 3 HONS eects eee gece SS Boda he AA Me en Mel Yel ee Ga anal eel ed de gee 123 6 44 SH Apes
316. 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 Student std std1 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 std1 std2 std1 Copy the reference value in std1 into the variable std2 std3 null Store a null reference in the variable std3 std name John Smith Set values of some instance variables std1l 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 25 std std std2 std3 Object class Student Object class Student getAverage getAverage Object class String Object class String Mary Jones John Smith This picture shows variables as little boxes labeled with the names of the vari ables Objects are shown as boxes with round corners When a variable contains a reference 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 fro
317. r not it is correct and for finding the errors in an incorrect program After testing 204 and debugging when the program is used in the normal way the assertions in the program will be ignored However if a problem turns up later the assertions are still there in the program to be used to help locate the error If someone writes to you to say that your program doesn t work when he does such and such you can run the program with assertions enabled do such and such and hope that the assertions in the program will help you locate the point in the program where it goes wrong Consider for example the root method that calculates a root of a quadratic equation If you believe that your program will always call this method with legal arguments then it would make sense to write the method using assertions instead of exceptions x Returns the larger of the two roots of the quadratic equation x Axxxx Bex C 0 provided it has any roots x Precondition A 0 and BxB 4xAxC gt 0 static public double root double A double B double C assert A 0 Leading coefficient of quadratic equation cannot be zero double disc BxB 4xAxC assert disc gt 0 Discriminant of quadratic equation cannot be negative return B Math sqrt disc 2A The assertions are not checked when the program is run in the normal way If you are correct in your belief that the method is never called with illegal arg
318. r example Color myColor new Color r g b There are two constructors that you can call in this way In the one that I al most always 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 Recall that a literal of type float is written with an F to distinguish it from a double number Of ten you can avoid constructing new colors altogether since the Color class defines several named constants representing common colors Color WHITE Color BLACK Color RED Color GREEN Color BLUE Color CYAN Color MAGENTA Color YELLOW Color PINK Color ORANGE Color LIGHT_GRAY Color GRAY and Color DARK_GRAY There are older alternative names for these constants that use lower case rather than upper case constants such as Color red instead of Color RED but the upper case versions are preferred because they follow the convention that constant names should be upper case An alternative to RGB is the HSB color system In the HSB system a color is specified by three numbers called the hue the saturation and the brightness The hue is the basic color ranging from red through orange through all the other colors of the rainbow The brightness is pretty much what it sounds like A fully saturated color is a pure color tone Decreasing the saturation is like mixing white or gray paint into the pure color In JAVA the hue saturation and brightness are always specified by val
319. r programming which is called computer sci ence As the term embodying the advocacy of a specific approach to computer pro gramming one that urges that it be treated as an engineering profession rather than an art or a craft and advocates the codification of reeommended practices in the form of software engineering methodologies Software engineering is 1 the application of a systematic disciplined quantifiable approach to the development operation and maintenance of software that is the applica tion of engineering to software and 2 the study of approaches as in 1 IEEE Standard 610 12 3 1 1 Software Engineering Life Cycles A decades long goal has been to find repeatable predictable processes or method ologies that improve productivity and quality of software Some try to systematize or formalize the seemingly unruly task of writing software Others apply project management techniques 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 63 e Requirements Analysis Extracting the requirements of a desired software product is the first task
320. racter and Byte Streams 02040 LO 23 Print writer u ecu 3 845 a SoS kl ee RE AE He Sa Se Fo 10 1 8 Data Streams 2 0 0 00 eee ee ee ee LO 1 4 Reading Text 5 5 5 5 eels eee oe ey ee OR OS a 10 1 5 The Scanner Class 00 00 00 2 eee eee 212 10 2 Wiles in i athe nich Reh io Both e ick Godt Dk en Be este E By Beh Te Macchi eee 213 10 2 1 Reading and Writing Files 02000 214 10 2 2 Files and Directories 0 00 eee ee ee nee 217 10 3 Programming With Files 0 0 0000 a 219 10 3 1 Copying a File 2 2 2 0 0 0 0 0 eee ee eee 219 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 familiar with basic procedural programming and introductory object based pro gramming in Java The student should be familiar with the various control constucts Arrays one and two dimensional the concepts of class and object input output and the concept of 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 5 0 DECEMBER 2006 The online book is available at http math hws edu javanotes We ve refactored and used in whole or parts of Chapters 4 5 6 8 9 10 and 11 Subsections of some these chapters were ommitted minor editing changes were made and a few sub
321. rd so return false return false other methods and constructors 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 prob lem 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 Comparab1le In fact Comparable is defined as a parameterized interface Comparable lt T gt which represents the ability to be compared to an object of type T The interface Comparable lt T gt defines one method public int compareTo T obj The value returned by obj1 compareTo obj2 should be negative if and only if obj1 comes before obj2 when the objects are arranged in ascending order It should be positive if and only if obj1 comes after obj2 A return value of zero means that the objects are considered to be the same for the purposes of this comparison This does not necessarily mean that the objects are equal in the sense that obj1 equals obj2 is true For example if the objects are of type Address representing mailing ad dresses it might be useful to sort the objects by zip code Two Addresses are consid ered the same for the purposes of the sort if they have the same zip code but clearly that would not mean that they are the same address The String class
322. rdVal The blackjack value of the i th card card getCard i cardVal card getValue The normal 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 78 Now val is the value of the hand counting any ace as 17 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 getBlackjack Value end class BlackjackHand Since BlackjackHand is a subclass of Hand an object of type BlackjackHand con tains all the instance variables and instance methods defined in Hand plus the new instance method named getBlackjackValue For example if bjh is a variable of type BlackjackHand then all of the following are legal bjh getCardCount bjh removeCard 0 and bjh getBlackjackValue The first two methods are de fined in Hand but are inherited by BlackjackHand Inherited variables and methods from the Hand class can also be used in the def inition of BlackjackHand except for any that are declared to be private which pre vents access even by subclasses The statement cards getCardCount in the above definition of getBlackjackValue calls the instance method getCardCount which was defined in H
323. re 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 202 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 The ability to throw exceptions is particularly useful in writing general purpose methods and classes that are meant to be used in more than one program In this case the person writing the method or class often has no reasonable way of handling the error since that person has no way of knowing exactly how the method 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 error 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 method 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 For example a method may return
324. re specified in the MouseListener inter face Note that four of the five event handling methods have empty defintions We really only want to define a response to mousePressed events but in order to imple ment the MouseListener interface a class must define all five methods We must do one more thing to set up the event handling for this example We must register an event handling object as a listener with the component that will generate the events In this case the mouse events that we are interested in will be generated by an object of type RandomStringsPanel If panel is a variable that refers to the panel object we can create a mouse listener object and register it with the panel with the statements 132 Create MouseListener object RepaintOnClick listener new RepaintOnClick Create MouseListener object panel addMouseListener listener Once this is done the listener object will be notified of mouse events on the panel Whenever a mousePressed event occurs the mousePressed method in the listener will be called The code in this method calls the repaint method in the component that is the source of the event that is in the panel The result is that the RandomStringsPanel is repainted with its strings in new random colors fonts and positions Although the RepaintOnClick class was written for use with the RandomStringsPanel example the event handling class contains no reference at all to the RandomStringsPane
325. rently being processed This vari able should not be declared before the for loop it is essentially being declared in the loop itself To give a concrete example suppose that the following enumerated type has been defined to represent the days of the week enum Day MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SATURDAY SUNDAY Then we could write for Day d Day values System out print d System out print is day number System out println d ordinal Day values represents the list containing the seven constants that make up the enumerated type The first time through this loop the value of d would be the first enumerated type value Day MONDAY which has ordinal number 0 so the output would be MONDAY is day number0 The second time through the loop the value of d would be Day TUESDAY and so on through Day SUNDAY The body of the loop is executed once for each item in the list Day values with d taking on each of those values in turn The full output from this loop would be MONDAY is day number 0 TUESDAY is day number 1 21 WEDNESDAY is day number 2 THURSDAY is day number 3 FRIDAY is day number 4 SATURDAY is day number 5 SUNDAY is day number 6 Since the intent of the enhanced for loop is to do something for each item in a data structure it is often called a for each loop The syntax for this type of loop is unfortunate It would be better if it were written something like foreac
326. ring on the two dice We could make those vari ables private to make it impossible to change their values from outside the class while still allowing read access through getter methods However if we think it pos sible that PairOfDice will be used to create subclasses we might want to make it pos sible for subclasses to change the numbers on the dice For example a GraphicalDice subclass that draws the dice might want to change the numbers at other times be sides when the dice are rolled In that case we could make diel and die2 protected 79 which would allow the subclass to change their values without making them public to the rest of the world An even better idea would be to define protected setter methods for the variables A setter method could for example ensure that the value that is being assigned to the variable is in the legal range 1 through 6 4 2 Inheritance and Class Hierarchy class A superclass class B subclass The term inheritance refers to the fact that one class can inherit part or all of its structure and behavior from another class The class that does the inheriting is said to be a subclass of the class from which it 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 this is the common terminology inC A subclass can add to the structure and behavior that
327. rogram 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 is that after this state ment 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 do Scanner keyboard new Scanner System in System out prinIn 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 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 Postcon
328. rrayLists The class LinkedList lt T gt adds a few additional methods which are not defined for an ArrayList If linkedlist is an object of type LinkedList lt T gt then we have e linkedlist getFirst returns the object of type T that is the first item in the list The list is not modified If the list is empty when the method is called an exception of type NoSuchElementException is thrown the same is true for the next three methods as well e linkedlist getLast returns the object of type T that is the last item in the list The list is not modified e linkedlist removeFirst removes the first item from the list and returns that object of type T as its return value linkedlist removeLast removes the last item from the list and returns that object of type T as its return value linkedlist addFirst obj adds the obj which must be of type T to the be ginning of the list linkedlist addLast obj adds the object obj which must be of type T to the end of the list This is exactly the same as linkedlist add obj and is apparently defined just to keep the naming consistent If list is an object of type List lt T gt then the method list iterator defined in the interface Collection lt T gt 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 ListIterator lt T gt
329. ructures can actually hold any object whatsoever One example is the ArrayList data structure which is defined by the class ArrayList in the package java util An ArrayList is simply a list of Object s This class is very convenient because an ArrayList can hold any number of objects and it will grow when neces sary 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 Shape s that have been drawn on the screen can store those shapes in an ArrayList Suppose that the ArrayList is named listOfShapes A shape oneShape for example can be added to the end of the list by calling the instance method listOfShapes add oneShape and removed from the list with the instance method listOfShapes remove oneShape The number of shapes in the list is given by the method listOfShapes size It is possible to retrieve the i object from the list with the 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
330. s Iterator lt T gt then we have e iter next returns the next item and advances the iterator The return value is of type T This method lets you look at one of the items in the col lection 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 In general 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 Note that this method has no pa rameter It removes the item that was most recently returned by iter next 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 for example that coll is of type Collection lt String gt In that case the value returned by coll iterator is of type Iterator lt String gt and we can say Iterator lt String gt iter Declare the iterater variable iter coll iterator Get an iterator for the collection while iter hasNext String item iter next Get the next item System out println item The same general form will work for other types of processing For example the following co
331. s about some domain We can then query these facts we can ask for example are sally and tom siblings sibling X Y parent Z X parent Z Y parent X Y father X Y parent X Y mother X Y mother trude sally father tom sally father tom erica father mike tom The factorial function is written in prolog as two rules Again notice the declara tive nature of the program fac 0 1 fac N F N gt 0 M is N 1 fac M Fm F is N Fm To summarize e In procedural languages everything is a procedure 2 see http cs wwc edu KU PR Prolog html 13 e In functional languages everything is a function e In logic programming languages everything is a logical expression predicate e In object oriented languages everything is an object 1 1 2 Object Orientation as a New Paradigm The Big Picture It is claimed that the problem solving techniques used in object oriented program ming more closely models the way humans solve day to day problems So lets consider how we solve an everyday problem Suppose you wanted to send flowers to a friend named Robin who lives in another city To solve this problem you simply walk to your nearest florist run by lets say Fred You tell Fred the kinds of flowers to send and the address to which they should be delivered You can be assured that the flowers will be delivered Now lets examine the mechanisms used to solve your problem e You firs
332. s closed in some other way 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 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 6 8 Images and Resources WE HAVE SEEN HOW TO USE THE GRAPHICS class to draw on a GUI component that is visible on the computer s screen Often however it is useful to be able to create a drawing off screen in the computer s memory It is also important to be able to work with images that are stored in files To a computer an image is just a set of numbers The numbers specify the color of each pixel in the image The numbers that represent the image on the computer s screen are stored in a part of memory called a frame buffer Many times each second the computer s video card reads the data in the frame buffer and colors each pixel on the screen according to that data Whenever the computer needs to make some change to the screen it writes some new numbers to the frame buffer and the change appears on the screen a fraction of a second later the next time the screen is redr
333. s constructed so that each student will au tomatically get a different value for its IDvariable 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 student object will have its own permanent unique identification number Which is kind of cool if you think about it 5 3 1 Static Import The import directive makes it possible to refer to a class such as java awt Color using its simple name Color All you have to do is say import java awt Color or import java awt Uou still have to use compound names to refer to static member variables such as System out and to static methods such as Math sqrt Java 5 0 introduced a new form of the import directive that can be used to import static members of a class in the same way that the ordinary import directive im ports classes from a package The new form of the directive is called a static import 100 and it has syntax import static package name class name static member name to import one static member name from a class or import static package name class name x to import all the public static members from a class For example if you preface a class definition with import static java lang System out then you can use the simple name out instead of the compound name System out This means you can use out printlin instead o
334. s pro grammer 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 completely 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 memory or you would have to refer to an array location that lies outside 188 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 re quires tests such as checking whether a pointer is null that take some extra process ing 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 9 1 3 Problems Remain in Java There is one area where the designers of Java chose not to detect errors automati cally numerical computatio
335. se 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 type casts are valid So for example if myVehicle refers to an object of type Truck then the type cast Car myVehicle would be an error When this happes an exception of type ClassCastException is thrown 4 4 Polymorphism 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 A roundrect is just a rectangle with rounded corners Rectangles Ovals RoundRects 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 and it could include instance methods for changing the color position and size 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 import
336. sections were added A notable change has been the use of the Scanner class and the printf method for input and output Some sections were also 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 definitions of various software engineering terms and concepts were adapted from wikipedia http wikipedia org This work is licensed under a Creative Commons Attribution ShareAlike 2 5 Li cense This license allows you to redistribute this book in unmodified form It allows you to make and distribute modified versions as long as you include an attribu tion to the original author clearly describe the modifications that you have made and distribute the modified work under the same license as the original See the http creativecommons org licenses by sa 2 5 for full details The TFX source for these notes are available on request 10 Introduction to Objects Contents 1 1 What is Object Oriented Programming 11 1 1 1 Programming Paradigms 00000 4 12 1 1 2 Object Orientation as a New Paradigm The Big Picture 14 1 22 Fundamentals of Objects and Classes 2 2e06 16 1 2 1 Objects and Classes 0 2 00 00 eee eee 16 1 2 2 Class Members and Instance Members 22 1 2 3 Access Controkes 3 2 64a ei lee gk ek Bee oo tes
337. sed to read one byte This method 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 of 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 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 command line arguments are stored in the array of strings args which is a parameter to the main method The program can retrieve the command line argu ments from this array For example if the program is named CopyFile and if the user 219 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 argu ments It prints an error message and exits if the file names are not specified To
338. serve to be mentioned Integer contains constants Integer MIN_VALUE and Integer MAX_VALUE which are equal to the largest and smallest possible values of type int that is to 2147483648 and 2147483647 respectively It s certainly easier to remember the names than the nu merical values There are similar named constants in Long Short and Byte Double and Float also have constants named MIN_VALUE and MAX_VALUE MAX_VALUE still gives the largest number that can be represented in the given type but MIN_VALUE represents the smallest possible positive value For type double Double MIN_VALUE is 4 9 x 10 824 Since double values have only a finite accuracy they can t get arbi trarily close to zero This is the closest they can get without actually being equal to zero The class Double deserves special mention since doubles are so much more com plicated than integers The encoding of real numbers into values of type double has room for a few special values that are not real numbers at all in the mathemati cal sense These values named constants in the class Double POSITIVE_INFINITY Double NEGATIVE_INFINITY and Double NaN The infinite values can occur as val ues of certain mathematical expressions For example dividing a positive number by zero will give Double POSITIVE_INFINITY It s even more complicated than this ac tually because the double type includes a value called negative zero written 0 0 Dividing a positive numb
339. 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 getXx 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 7 3 3 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 The DiscardPile Class This maintains a pile of cards that do not go into any of the other piles Override the addCard method to check first if the card is faceUp and flip it if its not Then add the card to the pi
340. size and style of text The same character will appear different 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 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 123 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 an exact rule The size of the default font is 12 JAVA uses the class named java awt Font for representing fonts You can con struct a new font by specifying its font name style and size in a constructor Font plainFont new Font Serif Font PLAIN 12 Font bigBoldFont new Font SansSerif Font BOLD 24 Every graphics context has a curr
341. so prints the number x of terms in the sequence The value of the parameter startingValue x must be a positive integer 49 static void print3NSequence int startingValue You can have Javadoc comments for methods member variables and for classes The Javadoc comment always immediately precedes the thing it is commenting on Like any comment a Javadoc comment is ignored by the computer when the file is compiled But there is a tool called javadoc that reads Java source code files extracts any Javadoc comments that it finds and creates a set of Web pages containing the comments in a nicely formatted interlinked form By default javadoc will only col lect information about public classes methods and member variables but it allows the option of creating documentation for non public things as well If javadoc doesn t find any Javadoc comment for something it will construct one but the comment will contain only basic information such as the name and type of a member variable or the name return type and parameter list of a method This is syntactic information To add information about semantics and pragmatics you have to write a Javadoc comment In addition to normal text the comment can contain certain special codes For one thing the comment can contain HTML mark up commands HTML is the language that is used to create web pages and Javadoc comments are meant to be shown on web pages The javadoc tool will copy any HT
342. space for a new item or to fill in the hole left by the removal of an item On the other hand the ArrayList class is more efficient when random access to items is required Random access means accessing the k th item in the list for any integer k Random access is used when you get or change the value stored at a specified position in the list This is trivial for an array But for a linked list it means starting at the beginning of the list and moving from node to node along the list for k 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 methods from interface Collection lt T gt that were dis cussed in previously These methods include size isEmpty remove Object add T and clear The add T 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 interface List lt T gt adds some methods for accessing list items according to their numerical positions in the list Suppose that list is an object of type List lt T gt Then we have the methods list get index returns the object of type T that is 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 ra
343. ssary 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 extend ing systems to do new things which in many ways can be considered new work In comparison about 2 3 of all civil engineering architecture and construction work is maintenance in a similar way 3 1 2 Object oriented Analysis and Design A large programming project goes through a number of stages starting with specifi cation 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 fix ing 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
344. sume that 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 The statement super 3 4 calls the constructor from the superclass This call must be the first line of the constructor in the subclass Note that if you don t explic itly call a constructor from the superclass in this way then the default constructor from the superclass the one with no parameters will be called automatically 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 91 92 Interfaces Nested Classes and Other Details Contents 5 1 Interfaces 0 2 2 ce cee ee ee ew ewe re eee ew wees 93 5 2 Nested Classes 2 ee ewe ee wwe ew wee ween 96 5 2 1 Anonymous Inner Classes 0 0000005 98 5 3 Mixing Static and Non static 22 00 cee vees 99 5 3 1 Static Import iea d aea i i e ah eh eh dS A oe eee 100 5
345. t the standard input and output streams System in and System out are byte streams rather than character streams However 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 either importing individual classes or putting the directive import java io at the beginning of your 208 source file Streams are necessary for working with files and for doing communication over a network They can be also used for communication 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 as 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 exceptio
346. t directly with the computer at the same time On a timesharing system users sit at terminals where they type commands to the computer and the computer types back its response Early personal computers also used typed commands and responses except that there was only one person involved at a time This type of interaction between a user and a computer is called a command line interface Today most people interact with computers in a completely different way They use a Graphical User Interface or GUI The computer draws interface components on the screen The components include things like windows scroll bars menus buttons and icons Usually a mouse is used to manipulate such components A lot of GUI interface components have become fairly standard That is they have similar appearance and behavior on many different computer platforms including MACINTOSH WINDOWS and LINUX JAVA programs which are supposed to run on many different platforms without modification to the program can use all the standard GUI components They might vary a little in appearance from platform to platform but their functionality should be identical on any computer on which the program runs Below is a very simple JAVA program actually an applet that shows a few stan dard GUI interface components There are four components that the user can interact with a button a checkbox a text field and a pop up menu These components are labeled There are
347. t found an appropriate agent Fred in this case and you passed to this agent a message containing a request e It is the responsibility of Fred to satisfy the request e There is some method an algorithm or set of operations used by Fred to do this e You do not need to know the particular methods used to satisfy the request such information is hidden from view Off course you do not want to know the details but on investigation you may find that Fred delivered a slightly different message to another florist in the city where your friend Robin lives That florist then passes another message to a subordinate who makes the floral arrangement The flowers along with yet another message is passed onto a delivery person and so on The florists also has interactions with whole salers who in turn had interactions with flower growers and so on This leads to our first conceptual picture of object oriented programming An object oriented program is structured as community of interacting agents called objects Each object has a role to play Each object provides a ser vice or performs an action that is used by other members of the community Messages and Responsibilities Members of an object oriented community make requests of each other The next important principle explains the use of messages to initiate action Action is initiated in object oriented programming by the transmission of a message to an agent an object responsible f
348. t you should not do this and I will avoid it for the most part But I have found it convenient to use getGraphics ina few cases The paintComponent method in the JPanel class simply fills the panel with the panel s background color When defining a subclass of JPanel for use as a drawing surface you will almost always want to fill the panel with the background color be fore drawing other content onto the panel although it is not necessary to do this if the drawing commands in the method cover the background of the component com pletely This is traditionally done with a call to super paintComponent g so most paintComponent methods that you write will have the form public void paintComponent g super paintComponent g Draw the content of the component 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 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 component s repaint method The method public void repaint is de fined in the Component class and so can be used with any component You should call repaint to inform the
349. tem out println Your prediction was incorrect break End the game else nextCard is lower if guess L System out println Your prediction was correct correctGuesses else System out println Your prediction was incorrect break End the game x To set up for the next iteration of the loop the nextCard becomes the currentCard since the currentCard has to be the card that the user sees and the nextCard will be set to the next card in the deck after the user makes his prediction x currentCard nextCard System out println System out println The card is currentCard end of while loop System out println System out println The game is over System out println You made correctGuesses correct predictions System out println return correctGuesses end play end class 61 62 Tools for Working with Abstractions 3 1 Introduction to Software Engineering THE DIFFICULTIES INHERENT with the development of software has led many computer scientists to suggest that software development should be treated as an engineering 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 opposed to the theory of compute
350. ter gt lt applet code HelloWorldApplet class height 100 width 250 gt lt applet gt lt p gt This assumes that the file Hel loWorldApplet class is located in the same direc tory 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 class not a URL 117 If the applet uses other classes in addition to the applet class itself then those class files must be in the same directory as the applet class always assuming that your classes are all in the default package see Subection2 6 4 Ifan applet requires more than one or two class files it s a good idea to collect all the class files into a single jar file Jar files are archive files which hold a number of smaller files If your class files are in a jar archive then you have to specify the name of the jar file in an archive modifier in the lt applet gt tag as in lt applet code HelloWorldApplet class archive HelloWorld jar height 50 Applets can use applet parameters to customize their behavior Applet parame ters 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
351. that the method 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 The static and the non static portions of a class are very different things and serve very different purposes Many classes contain only static members or only non static However it is possible to mix static and non static members in a single class The static definitions in the source code specify the things that are part of the class itself whereas the non static definitions in the source code specify things that will become part of every instance object that is created from the class Static member variables and static member methods 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 So far we ve been talking mostly in generalities Let s now look at a specific example to see how classes and objects work Consider this extremely simplified version of a Student class which could be used to store information about students taking a course public class Student public String name Student s name public double testi test2 test3 Grades on three tests public double getAverage compute average test grade return testl test2 test3 3 end of class Student None of the members of this class are declared to
352. 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 precondition namely that A is not null but let s leave that aside for the moment Consider the following code which searches for the number z in the array A and sets the value of i to be the index of the array element that contains z 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 A i x 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 con ditions caused the loop to end i 0 while i lt A length amp amp A i x i if i A length System out println x is not in the array else System out println x is in position i 9 2 2 Robust Handling of Input One place wher
353. the temptation of saying Well I ll 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 best just to let the program crash 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 together many common exceptions including all those that have been covered in pre vious sections For example Il1legalArgumentException and NullPointerException are subclasses of RuntimeException A RuntimeException generally indicates a bug in the program which the programmer should fix RuntimeExceptions and Errors share the property 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 example a program does this every time it uses an array refer ence like A i without making arrangements to catch a possible ArrayIndexOutOfBoundsException For all other exception classes besides Error RuntimeException and their subclasses exception handling is mandatory in a sense that Pll discuss below The following diagram is a class hierarchy showing the class Throwable and just a few of its subclasses Class
354. thod gets a chance to handle the exception That is if the method was called inside a try state ment that has an appropriate catch clause then that catch clause will be executed and the program will continue on normally from there Again if the second method does not handle the exception then it also is terminated and the method that called it Gf any gets the next shot at the exception The exception will crash the program only if it passes up through the entire chain of method calls without being handled A method that might generate an exception can announce this fact by adding a clause throws exception class name to the header of the method For example x Returns the larger of the two roots of the quadratic equation 199 x Axxxx Bex C 0 provided it has any roots If A 0 or x if the discriminant BxB 4xAxC is negative then an exception x Of type IllegalArgumentException is thrown static public double root double A double B double C throws IllegalArgumentException if A 0 throw new IllegalArgumentException A can t be zero else double disc B xB 4xAxC if disc lt 0 throw new IllegalArgumentException Discriminant lt zero return B Math sqrt disc 2A As discussed in the previous section the computation in this method has the pre conditions that A 0 and Bx B 4xAxC gt 0 The method throws an exception of type IllegalArgumentExceptio
355. throw an ArithmeticException when an attempt is made to divide by zero So why should a programmer bother to throw the exception Re calls that if the numbers that are being divided are of type int then division by zero will indeed 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 In some situations you might prefer to throw an ArithmeticException when a real number is divided by zero An exception can be thrown either by the system or by a throw statement The exception is processed in exactly the same way in either case Suppose that the ex ception 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 exe cutes it The exception has been handled After handling the exception the computer executes the finally clause of the try statement if there is one It then continues nor mally 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 method and the exception is not handled in the same method then that method is terminated after the execu tion of any pending finally clauses Then the method that called that me
356. tial Fortunately in Java I can ease the embarrassment be cause 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 par ticularly 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 is any class whose definition is inside the definition of another class Nested classes can be either named or anonymous I will come back to the topic of anonymous classes later in this section A named nested class like most other things that occur in classes can be either static or non static The definition of a static nested looks just like the definition of any other class except that it is nested inside another class and it has the modifier static as part of its declaration 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
357. tion and it can contain other data as well All exception objects must belong to a subclass of the standard class java lang Throwable In general each different type of exception is represented by its own subclass of Throwable and these subclasses are arranged in a fairly com plex class hierarchy that shows the relationship among various types of exceptions Throwable has two direct subclasses Error and Exception These two subclasses in turn have many other predefined subclasses In addition a programmer 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 In general you should not try to catch and handle such errors An example is a ClassFormatError 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 anticipate and possibly respond to in some reasonable way However you should avoid
358. tion of an in stance 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 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 lets rewrite the Student class ll add a constructor and I ll also take the opportunity to make the instance variable name private public class Student private String name Student s name public double test1 test2 test3 Grades on three tests Constructor for Student objects provides a name for the Student Student String theName name theName Getter method for the private instance variable name public String getName return name Compute average test grade public double getAverage return testl test2 test3 3 end of class Student 33 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 th
359. to store an int than it is to store an object Sometimes however it is necessary to manipulate the primitive types as if they were objects To make this possible you can define wrapper classes whose sole aim is to contain one of the primitive types They are used for creating objects that represent primitive type values For example the JAVA API contains the classes Double that wraps a single double and Integer that wraps a single integer These classes contain various static meth ods including Double parseDouble and Integer parseInteger that are used to con vert strings to numerical values The Character class wraps a single char type There is a similar class for each of the other primitive types Long Short Byte Float and Boolean Remember that the primitive types are not classes and values of primitive type are not objects However sometimes it s useful to treat a primitive value as if it were an object You can t do that literally but you can wrap the primitive type value in an object belonging to one of the wrapper classes For example an object of type Double contains a single instance variable of type double The object is a wrapper for the double value For example you can create an object that wraps the double value 6 0221415e23 with Double d new Double 6 0221415e23 The value of d contains the same information as the value of type double but it is an object If you want to retrieve the double value that
360. to perform the task you just call the method Methods are a major tool for dealing with complexity A method is sometimes said to be a black box because you can t see what s in side it or to be more precise you usually don t want to see inside it because then you would have to deal with all the complexity that the method is meant to hide Of course a black box that has no way of interacting with the rest of the world would be pretty useless A black box needs some kind of interface with the rest of the world which allows some interaction between what s inside the box and what s outside A physical black box might have buttons on the outside that you can push dials that you can set and slots that can be used for passing information back and forth Since we are trying to hide complexity not create it we have the first rule of black boxes The interface of a black box should be fairly straightforward well defined and easy to understand Your television your car your VCR your refrigerator are all examples of black boxes in the real world You can turn your television on and off change channels and set the volume by using elements of the television s interface dials remote control don t forget to plug in the power without understanding anything about how the thing actually works The same goes for a VCR although if stories about how hard people find it to set the time on a VCR are true maybe the VCR
361. 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 methods getWidth and getHeight which return the number of pixels in the horizontal and vertical directions respectively In general it s not a good idea to assume that you know the size of a component since the size is often set by a layout manager and can even change if the component is in a window and that window is resized by the user 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 super paintComponent g int width getWidth Find out the width of this component int height getHeight Find out its height Draw the content 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 component 6 4 2 Colors You will probably want to use some color when you draw JAVA is designed to work with the RGB color system An RGB color is specified by three numbers that give the level of red green and blue respectively in the 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 Fo
362. type These conversions are called autoboxing and un boxing This means that once you have created a generic data structure to hold ob jects belonging to one of the wrapper classes you can use the data structure pretty much as if it actually contained primitive type values For example if numbers is a variable of type Collection lt Integer gt it is legal to call numbers add 17 or numbers remove 42 You can t literally add the primitive type value 17 to num bers but Java will automatically convert the 17 to the corresponding wrapper object new Integer 17 and the wrapper object will be added to the collection The cre ation of the object does add some time and memory overhead to the operation and you should keep that in mind in situations where efficiency is important An array of int is more efficient than an ArrayList lt Integer gt 8 8 Lists IN THE PREVIOUS SECTION we looked at the general properties of collection classes in Java In this section we look at a few specific collection classes lists in particular 179 and how to use them 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 ArrayList and LinkedList There are two obvious ways to represent a list as a dynamic array and as a linked list Both of these options are available in generic form as the collection classes java util ArrayList and java util LinkedList These
363. type double In the case of a scanner tokens must be separated by delimiters By default the delimiters are whitespace characters such as spaces and end of line markers In normal processing whitespace characters serve simply to separate tokens and are discarded by the scan ner A scanner has instance methods for reading tokens of various types Suppose that scanner is an object of type Scanner Then we have 212 e scanner next reads the next token from the input source and returns it as a String e scanner nextInt scanner nextDouble and so on reads the next token from the input source and tries to convert it to a value of type int double and so on There are methods for reading values of any of the primitive types scanner nextLine reads an entire line from the input source up to the next end of line and returns the line as a value of type String The end of line marker is read but is not part of the return value Note that this method is not based on tokens An entire line is read and returned including any whites pace characters in the line All of these methods can generate exceptions If an attempt is made to read past the end of input an exception of type NoSuchElementException is thrown Methods such as scanner getInt will throw an exception of type InputMismatchException if the next token in the input does not represent a value of the requested type The exceptions that can be generated do not require
364. ucture and tries to type cast it to type String If the object is not actually of type String the illegal type cast will throw an error of type ClassCastException JAVA 5 0 introduced parameterized types such as ArrayList lt String gt This made it possible to create generic data structures that can be type checked at compile time rather than at run time With these data structures type casting is not necessary so ClassCastExceptions are avoided The compiler will detect any attempt to add an object of the wrong type to the data structure it will report a syntax error and will refuse to compile the program In Java 5 0 all of the classes and interfaces in the Collection Framework and even some classes that are not part of that framework have been parameterized In this chapter I will use the parameterized types almost exclusively but you should remember that their use is not mandatory It is still legal to use a parameterized class as a non parameterized type such as a plain ArrayList With a Java parameterized class there is only one compiled class file For exam ple there is only one compiled class file ArrayList class for the parameterized class ArrayList The parameterized types ArrayList lt String gt and ArrayList lt Integer gt both use the some compiled class file as does the plain ArrayList type The type parameter String or Integer just tells the compiler to limit the type of object that can be stored in the data structure
365. ude zero or more catch clauses and optionally a finally clause The try statement must include one or the other That is a try statement can have either a finally clause or one or more catch clauses or both 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 state ment 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 One example of this type of cleanup is closing a network connection Although you don t yet know enough about networking to look at the actual programming in this case we can consider some pseudocode try open a network connection catch IOException e report the error return Don t continue if connection can t be opened At this point we KNOW that the connection is open try communicate over the connection catch IOException e handle the error finally close the connection The finally clause in the second try statement ensures that the network con nection will definitely be closed whether or not an error occurs during the commu 19
366. ues of type float in the range from 0 0F to 1 0F The Color class has a static 122 member method named getHSBColor for creating HSB colors To create the color with HSB values given by h s and b you can say Color myColor Color getHSBColor h s b For example to make a color with a random hue that is as bright and as saturated as possible you could use Color randomColor Color getHSBColor float Math random 1 0F 1 0F The type cast is necessary because the value returned by Math random is of type double and Color getHSBColor requires values of type float By the way you might ask why RGB colors are created using a constructor while HSB colors are cre ated using a static member method The problem is that we would need two different constructors both of them with three parameters of type float Unfortunately this is impossible You can have two constructors only if the number of parameters or the parameter types differ The RGB system and the HSB system are just different ways of describing the same set of colors It is possible to translate between one system and the other The best way to understand the color systems is to experiment with them In the following applet you can use the scroll bars to control the RGB and HSB values of a color A sample of the color is shown on the right side of the applet One of the properties of a Graphics object is the current drawing color which is used for all drawing of shap
367. uments then checking the conditions in the assertions would be unnecessary If your belief is not correct the problem should turn up during testing or debugging when the program is run with the assertions enabled If the root method is part of a software library that you expect other people to use then the situation is less clear Sun s Java documentation advises that assertions should not be used for checking the contract of public methods If the caller of a method violates the contract by passing illegal parameters then an exception should be thrown This will enforce the contract whether or not assertions are enabled However while it s true that Java programmers expect the contract of a method to be enforced with exceptions there are reasonable arguments for using assertions instead in some cases On the other hand it never hurts to use an assertion to check a postcondition of a method A postcondition is something that is supposed to be true after the method has executed and it can be tested with an assert statement at the end of the method If the postcodition is false there is a bug in the method itself and that is something that needs to be found during the development of the method To have any effect assertions must be enabled when the program is run How to do this depends on what programming environment you are using In the usual com mand line environment assertions are enabled by adding the enableassertions optio
368. use a class named Vehicle to represent all types of vehicles Since cars trucks and motorcycles are types of vehicles they would be represented by subclasses of the Vehicle class as shown in this class hierarchy diagram The Vehicle class would include instance variables such as registrationNumber and owner and instance methods such as transferOwnership These are variables and methods common to all vehicles The three subclasses of Vehicle Car Truck and Motorcycle could then be used to hold variables and methods specific to partic ular 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 boolean variable hasSidecar Well it could in theory at 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 although in practice they would probably be public classes defined in separate files class Vehicle int registrationNumber Person owner Assuming that a Person class has been defined void transferOwnership Person newOwner ek class Car extends Vehicle int numberOfDoors class Truck extends Vehicle int numberOfAxels class Motorcycle extends Vehicle boolean hasSidecar 81 Suppose that myCar is a variable of type Car that has been declared and initial ized with the state
369. useClicked MouseEvent evt public void mouseReleased MouseEvent evt public void mouseEntered MouseEvent evt public void mouseExited MouseEvent evt Often when a mouse event occurs you want to know the location of the mouse cursor This information is available from the MouseEvent parameter to the event handling method which contains instance methods that return information about the event If evt is the parameter then you can find out the coordinates of the mouse cursor by calling evt getX and evt getY These methods return integers which give the x and y coordinates where the mouse cursor was positioned at the time when the event occurred 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 6 5 3 Anonymous Event Handlers 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 134 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
370. ut println std test1 System out println std test2 System out println std test3 Your test grades are This would output the name and test grades from the object to which std refers 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 24 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 pass it as a parameter to method You can even use it to call methods 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 anull 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 st is null then it would be illegal to refer to std test1 If your
371. utput can be done using streams Human readable character 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 machine format the appropriate I O classes are FileInputStream and FileOutputStream In this section I will only discuss character oriented file I O using the FileReader and FileWriter classes However 213 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 net work connection are not allowed to access files unless you have made a very foolish change to your web browser s configuration This is a security consideration You can download and run an applet just by visiting a Web page with your browser If down loaded 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 al lowed 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
372. ves the desired outcomes Thinking in this way allows greater independence between objects e Thus objects have responsibilities that they are willing to fulfill on request The collection of reponsibilities associated with an object is often called a protocol Classes and Instances The next important principle of object oriented programming is All objects are instances of a class The method invoked by an object in response to a message is determined by the class of the receiver All objects of a given class use the same method in response to similar messages Fred is an instance of a category or class of people i e Fred is an instance of a class of florists The term florist represents a class or category of all florists Fred is an object or instance of a class We interact with instances of a class but the class determines the behaviour of in stances We can tell a lot about how Fred will behave by understanding how Florists behave We know for example that Fred like all florists can arrange and deliver flowers In the real world there is this distinction between classes and objects Real world objects share two characteristics They all have state and behavior For example dogs have state name color breed hungry and behavior barking fetching wagging tail Students have state name student number courses they are registered for gender and behavior take tests attend courses write tests party 15 A F Variables
373. violates the simple interface rule Now a black box does have an inside the code in a method that actually performs the task all the electronics inside your television set The inside of a black box is called its implementation The second rule of black boxes is that To use a black box you shouldn t need to know anything about its imple mentation all you need to know is its interface In fact it should be possible to change the implementation as long as the behavior of the box as seen from the outside remains unchanged For example when the insides of TV sets went from using vacuum tubes to using transistors the users of the sets didn t even need to know about it or even know what it means Similarly it should 40 be possible to rewrite the inside of a method to use more efficient code for example without affecting the programs that use that method Of course to have a black box someone must have designed and built the im plementation in the first place The black box idea works to the advantage of the implementor as well as of the user of the black box After all the black box might be used in an unlimited number of different situations The implementor of the black box doesn t need to know about any of that The implementor just needs to make sure that the box performs its assigned task and interfaces correctly with the rest of the world This is the third rule of black boxes The implementor of a black box s
374. w to write the class This has been an exercise in design not in programming With this information you can use the class in your programs without understanding the implementation The description above is a contract between the users of the class and implementors of the class it is the public interface of the class The Hand Class 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 re move 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 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
375. ways check the return value For example boolean hasFormat ImageI0O write OSC format selectedFile if hasFormat throw new Exception format format is not available If the image format is recognized it is still possible that that an IOExcption might be thrown when the attempt is made to send the data to the file The ImageI0 class also has a static read method for reading an image from a file into a program The method ImageIO read inputFile takes a variable of type File as a parameter and returns a BufferedImage The return value is null if the file does not contain an image that is stored in a supported format Again no exception is thrown in this case so you should always be careful to check the return value It is also possible for an IOException to occur when the attempt is made to read the file There is another version of the read method that takes an InputStream instead of a file as its parameter and a third version that takes a URL Earlier in this section we encountered another method for reading an image from a URL the createImage method from the Toolkit class The difference is that ImageIO read reads the image data completely and stores the result in a BufferedImage On the other hand createImage does not actually read the data it really just stores the image location and the data won t be read until later when the image is used This has the advantage that the createImage method itself can co
376. when the user clicks one of the buttons This method must retrieve the user s numbers from the text field perform the appropriate arithmetic operation on them depending on which button was clicked and set the text of the label to represent the result However the contents of the text fields can only be retrieved as strings and these strings must be converted into numbers If the conversion fails the label is set to display an error message public void actionPerformed ActionEvent evt double x y The numbers from the input boxes try String xStr xInput getText x Double parseDouble xStr catch NumberFormatException e The string xStr is not a legal number answer setText Illegal data for x xInput requestFocus return try String yStr yInput getText y Double parseDouble yStr 147 catch NumberFormatException e The string xStr is not a legal number answer setText IIlegal data for y yinput requestFocus return x Perfrom the operation based on the action command from the button The action command is the text displayed on 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
377. 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 methods 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 format javadoc can separate out the documentation and make it into a set of web pages The web pages are automatically formatted and linked into an easily browseable Web site Sun Microsystem s documentation for the standard Java API was produced using javadoc Javadoc documentation is prepared from special comments that are placed in the Java source code file Recall that one type of Java comment begins with and ends with A Javadoc comment takes the same form but it begins with rather than simply x This method prints a 3N 1 sequence to standard output using x StartingValue as the initial value of N It al
378. x coord of the upper left corner of the card x param y the y coord of the upper left corner of the card public void drawCard Graphics g Card card int x int y int cx x coord of upper left corner of the card inside cardsimage int cy y coord of upper left corner of the card inside cardslmage if card null cy 4x123 coords for a face down card cx 279 else cx card getValue 1 79 switch card getSuit case Card CLUBS cy 0 break case Card DIAMONDS cy 123 break case Card HEARTS cy 2 123 break default spades cy 3123 break g drawlmage cardImages x y xX 79 y 123 cx cy Cx 79 cy 123 this I will tell you later in this section how the image file cards png can be loaded into the program 6 8 2 Image File I O The class javax imageio mageIO makes it easy to save images from a program into files and to read images from files into a program This would be useful in a program such as PaintWithOffScreenCanvas so that the users would be able to save their work and to open and edit existing images See Exercise12 1 There are many ways that the data for an image could be stored in a file Many standard formats have been created for doing this Java supports at least three standard image formats PNG JPEG and GIF Individual implementations of Java might support more The JPEG format is lossy which means that the picture that you get when you r
379. y a eal are ais Sh JA Tels Tals bi Ad Tris Tals e e ee Ale Ae elle ASe D t ES Ne Kea bad o gt e This image is from the Gnome desktop project http www gnome org and is shown here much smaller than its actual size Now only one Image object is needed Drawing one card means drawing a rectangular region from the image This tech nique is used in a variation of the sample program HighLowGUI java In the original version the cards are represented by textual descriptions such as King of Hearts In the new version HighLowWithImages java the cards are shown as images Here is an applet version of the program In the program the cards are drawn using the following method The instance variable cardImages is a variable of type Image that represents the image that is shown above containing 52 cards plus two Jokers and a face down card Each card is 79 by 123 pixels These numbers are used together with the suit and value of the card to compute the corners of the source rectangle for the drawImage command 154 x Draws a card in a 79x123 pixel rectangle with its x upper left corner at a specified point x y Drawing the card x requires the image file cards png x param g The graphics context used for drawing the card x param card The card that is to be drawn If the value is null then a x face down card is drawn x param x the
380. y we are able to recognise an object as a chair even if it is different from any other chair we ve seen previously We form concepts of everyday objects and events by a process of abstraction where we remove unimportant details and concentrate on the essential attributes of the thing Abstraction in mathematics is the process of extracting the underlying essence of a mathematical concept removing any dependence on real world objects with which it might originally have been connected and generalising it so that it has wider applica tions Many areas of mathematics began with the study of real world problems before the underlying rules and concepts were identified and defined as abstract structures For example geometry has its origins in the calculation of distances and areas in the real world statistics has its origins in the calculation of probabilities in gambling Roughly speaking abstraction can be either that of control or data Control ab straction is the abstraction of actions while data abstraction is that of data For exam ple control abstraction in structured programming is the use of methods and format ted control flows Data abstraction allows handling of data in meaningful ways For example it is the basic motivation behind datatype Object oriented programming can be seen as an attempt to abstract both data and control 2 1 1 Control Abstraction Control abstraction is one of the main purposes of using programming languages
381. ype Object that contains all the items in the collection The return value can be type cast to another array type if appropriate Note that the return type is Object not T However you can type cast the return value to a more specific type For example 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 173 Since these methods are part of the Collection lt T gt interface they must be de fined 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 af ter 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 the exception thrown is UnsupportedOperationException Furthermore since Collection lt T gt is only an in terface not a concrete class the actual implementation of the method is left to the classes that implement the interface This means that the semantics of the methods as described above are not guaranteed to be valid for all collection objects they are valid however for classes in the Java Collection Framework There is also the question of efficiency Even when an operation is defined for sev eral types of collections it might not be equally eff
382. ything about how the GUI behaves That is what can the user do to the GUI and how will it respond GUIs are largely event driven that is the program waits for events that are generated by the user s actions or by some other cause When an event occurs the program responds by executing an event handling method In order to program the behavior of a GUI you have to write event handling methods to respond to the events that you are interested in Event listeners are the most common technique for handling events in JAVA A listener is an object that includes one or more event handling methods When an event is detected by another object such as a button or menu the listener object is notified and it responds by running the appropriate event handling method An event is detected or generated by an object Another object the listener has the responsibility of responding to the event The event itself is actually represented by a third object which carries information about the type of event when it occurred and so on This division of responsibilities makes it easier to organize large programs As an example consider the OK button in the sample program When the user clicks the button an event is generated This event is represented by an object be longing to the class ActionEvent The event that is generated is associated with the button we say that the button is the source of the event The listener object in this case is an object belo

Object-Oriented Programming - HWS Department of Mathematics

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