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Object-Oriented Programming

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. 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
6. 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
7. 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
8. 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
9. 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
10. 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
11. 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
12. 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
13. 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
14. 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
15. 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
16. 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
17. 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
18. 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
19. 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
20. 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
21. 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
22. 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
23. 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
24. 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
25. 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
26. 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
27. 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
28. 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
29. 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
30. 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
31. 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
32. 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
33. 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
34. 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
35. 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
36. 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
37. 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
38. 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
39. 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
40. 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
41. 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
42. 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
43. 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
44. 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
45. 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
46. 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
47. 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
48. 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
49. 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
50. 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
51. 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
52. 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
53. 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
54. 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
55. 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
56. 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
57. 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
58. 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
59. 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
60. 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
61. 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
62. 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
63. 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
64. 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
65. 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
66. 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
67. 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
68. 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
69. 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
70. 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
71. 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
72. 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
73. 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
74. 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
75. 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
76. 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
77. 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
78. 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
79. 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
80. 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
81. 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
82. 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
83. 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
84. 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
85. 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
86. 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
87. 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
88. 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
89. 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
90. 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
91. 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
92. 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
93. 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
94. 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
95. 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
96. 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
97. 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
98. 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
99. 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
100. 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
101. 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
102. 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
103. 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
104. 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
105. 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
106. 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
107. 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
108. 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
109. 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
110. 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
111. 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
112. 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
113. 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
114. 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
115. 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
116. 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
117. 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
118. 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
119. 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
120. 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
121. 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
122. 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
123. 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
124. 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
125. 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
126. 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
127. 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
128. 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
129. 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
130. 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
131. 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
132. 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
133. 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
134. 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
135. 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
136. 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
137. 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
138. 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
139. 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
140. 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
141. 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
142. 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
143. 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
144. 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
145. 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
146. 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
147. 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
148. 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
149. 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
150. 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
151. 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
152. 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
153. 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
154. 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
155. 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
156. 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
157. 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
158. 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
159. 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
160. 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
161. 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
162. 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
163. 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
164. 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
165. 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
166. 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
167. 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
168. 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

Object-Oriented Programming

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