1. Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley John Lewis, Peter DePasquale, and Joseph Chase Chapter 5: Writing Classes (Part 2)

2. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.1 – Classes and Object Revisited The programs we’ve written in previous examples have used classes defined in the Java standard class library Now we will begin to design programs that rely on classes that we write ourselves The class that contains the main method is just the starting point of a program True object-oriented programming is based on defining classes that represent objects with well-defined characteristics and functionality

3. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.1 – Identifying Classes and Objects Remember that a class represents a group (classification) of objects with the same behaviors Generally, classes that represent objects should be given names that are singular nouns Examples: Coin, Student, Message A class represents the concept of one such object We are free to instantiate as many of each object as needed

4. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.1 – Identifying Classes and Objects Recall from our overview of objects in Chapter 1 that an object has state and behavior Consider a university student object –it’s state can be defined as the student’s name, address, major, and GPA –it’s primary behavior may be to update the address or to recalculate the GPA We can represent a student in software by designing a class called Student that models this state and behavior –the class serves as the blueprint for a student object We can then instantiate as many student objects as we need for any particular program

5. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.1 – Classes A class can contain data declarations and method declarations int size, weight; char category; Data declarations Method declarations

6. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.1 – Classes The values of the data define the state of an object created from the class The functionality of the methods define the behaviors of the object For our Student class, we might declare a String that represents the student’s major One of the methods would update the student’s major by setting that value to the value of a new string (the new major).

7. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – Anatomy of a Class Consider a six-sided die (singular of dice) –It’s state can be defined as which face is showing –It’s primary behavior is that it can be rolled We can represent a die in software by designing a class called Die that models this state and behavior We’ll want to design the Die class with other data and methods to make it a versatile and reusable resource Any given program will not necessarily use all aspects of a given class See Die.java and SnakeEyes.java

8. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – The Die Class The Die class contains two data values –a constant MAX that represents the maximum face value –an integer faceValue that represents the current face value The roll method uses the random method of the Math class to determine a new face value There are also methods to explicitly set and retrieve the current face value at any time

9. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – The toString Method All classes that represent objects should define a toString method The toString method returns a character string that represents the object in some way It is called automatically when an object is concatenated to a string or when it is passed to the println method

10. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – Constructors As mentioned previously, a constructor is a special method that is used to set up an object when it is initially created A constructor has the same name as the class The Die constructor is used to set the initial face value of each new die object to one We examine constructors in more detail later in this chapter

11. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – Data Scope The scope of data is the area in a program in which that data can be referenced (used) Data declared at the class level can be referenced by all methods in that class Data declared within a method can be used only in that method Data declared within a method is called local data In the Die class, the variable result is declared inside the toString method -- it is local to that method and cannot be referenced anywhere else

12. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – Instance Data The faceValue variable in the Die class is called instance data because each instance (object) that is created has its own version of it A class declares the type of the data, but it does not reserve any memory space for it Every time a Die object is created, a new faceValue variable is created as well The objects of a class share the method definitions, but each object has its own data space That's the only way two objects can have different states

13. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.2 – Instance Data We can depict the two Die objects from the SnakeEyes program as follows die1 5 faceValue die2 2 faceValue Each object maintains its own faceValue variable, and thus its own state

14. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-14 5.3 – Encapsulation We can take one of two views of an object –internal - the details of the variables and methods of the class that defines it –external - the services that an object provides and how the object interacts with the rest of the system From the external view, an object is an encapsulated entity, providing a set of specific services These services define the interface to the object

15. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-15 5.3 – Encapsulation One object (called the client) may use another object for the services it provides The client of an object may request its services (call its methods), but it should not have to be aware of how those services are accomplished Any changes to the object's state (its variables) should be made by that object's methods We should make it difficult, if not impossible, for a client to access an object’s variables directly That is, an object should be self-governing

16. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-16 5.3 – Encapsulation An encapsulated object can be thought of as a black box – its inner workings are hidden from the client The client invokes the interface methods of the object, which manages the instance data Methods Data Client

17. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-17 5.3 – Visibility Modifiers In Java, we accomplish encapsulation through the appropriate use of visibility modifiers A modifier is a Java reserved word that specifies particular characteristics of a method or data We've used the final modifier to define constants Java has three visibility modifiers: public, protected, and private The protected modifier involves inheritance, which we will discuss later

18. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-18 5.3 – Visibility Modifiers Members of a class that are declared with public visibility can be referenced anywhere Members of a class that are declared with private visibility can be referenced only within that class Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package An overview of all Java modifiers is presented in Appendix E

19. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-19 5.3 – Visibility Modifiers Public variables violate encapsulation because they allow the client to “reach in” and modify the values directly Therefore instance variables should not be declared with public visibility It is acceptable to give a constant public visibility, which allows it to be used outside of the class Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed

20. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-20 5.3 – Visibility Modifiers Methods that provide the object's services are declared with public visibility so that they can be invoked by clients Public methods are also called service methods A method created simply to assist a service method is called a support method Since a support method is not intended to be called by a client, it should not be declared with public visibility

21. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.3 – Visibility Modifiers publicprivate Variables Methods Provide services to clients Support other methods in the class Enforce encapsulation Violate encapsulation

22. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.3 – Accessors and Mutators Because instance data is private, a class usually provides services to access and modify data values An accessor method returns the current value of a variable A mutator method changes the value of a variable The names of accessor and mutator methods take the form getX and setX, respectively, where X is the name of the value

23. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.3 – Accessors and Mutators They are sometimes called “getters” and “setters” See Coin.java, CountFlips.java and FlipRace.java –The isHeads method in Coin.java is an accessor –The flip method in Coin.java is a mutator

24. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.3 – Mutator Restrictions The use of mutators gives the class designer the ability to restrict a client’s options to modify an object’s state A mutator is often designed so that the values of variables can be set only within particular limits For example, the setFaceValue mutator of the Die class should have restricted the value to the valid range (1 to MAX ) Such restrictions can be implemented through the use of an if statement in the body of the constructor.

25. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Method Declarations Let’s now examine method declarations in more detail A method declaration specifies the code that will be executed when the method is invoked (called) When a method is invoked, the flow of control jumps to the method and executes its code When complete, the flow returns to the place where the method was called and continues The invocation may or may not return a value, depending on how the method is defined

26. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley myMethod(); myMethodcompute 5.4 – Method Control Flow If the called method is in the same class, only the method name is needed

27. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley doIt helpMe helpMe(); obj.doIt(); main 5.4 – Method Control Flow The called method is often part of another class or object

28. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-28 5.4 – The return Statement The return type of a method indicates the type of value that the method sends back to the calling location A method that does not return a value has a void return type A return statement specifies the value that will be returned return expression; Its expression must conform to the return type

29. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Parameters When a method is called, the actual parameters in the invocation are copied into the formal parameters in the method header char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } ch = obj.calc (25, count, "Hello");

30. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Local Data As we’ve seen, local variables can be declared inside a method The formal parameters of a method create automatic local variables when the method is invoked When the method finishes, all local variables are destroyed (including the formal parameters) Keep in mind that instance variables, declared at the class level, exists as long as the object exists

31. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Bank Account Example Let’s look at another example that demonstrates the implementation details of classes and methods We’ll represent a bank account by a class named Account It’s state can include the account number, the current balance, and the name of the owner An account’s behaviors (or services) include deposits and withdrawals, and adding interest

32. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Driver Programs A driver program drives the use of other, more interesting parts of a program Driver programs are often used to test other parts of the software See Account.java and Transactions.java The Transactions class contains a main method that drives the use of the Account class, exercising its services

33. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Bank Account Example acct1 72354 acctNumber 102.56 balance name “Ted Murphy” acct2 69713 acctNumber 40.00 balance name “Jane Smith”

34. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.4 – Bank Account Example There are some improvements that can be made to the Account class Formal getters and setters could have been defined for all data The design of some methods could also be more robust, such as verifying that the amount parameter to the withdraw method is positive

35. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-35 5.4 – Constructors Revisited Note that a constructor has no return type specified in the method header, not even void A common error is to put a return type on a constructor, which makes it a “regular” method that happens to have the same name as the class The programmer does not have to define a constructor for a class Each class has a default constructor that accepts no parameters

36. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-36 5.5 – Static Class Members Recall that a static method is one that can be invoked through its class name For example, the methods of the Math class are static: result = Math.sqrt(25) Variables can be static as well Determining if a method or variable should be static is an important design decision

37. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.5 – The static Modifier We declare static methods and variables using the static modifier It associates the method or variable with the class rather than with an object of that class Static methods are sometimes called class methods and static variables are sometimes called class variables Let's carefully consider the implications of each

38. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-38 5.5 – Static Variables Normally, each object has its own data space, but if a variable is declared as static, only one copy of the variable exists private static float price; Memory space for a static variable is created when the class is first referenced All objects instantiated from the class share its static variables Changing the value of a static variable in one object changes it for all others

39. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-39 5.5 – Static Methods public class Helper { public static int cube (int num) { return num * num * num; } Because it is declared as static, the method can be invoked as value = Helper.cube(5);

40. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-40 5.5 – Static Class Members The order of the modifiers can be interchanged, but by convention visibility modifiers come first Recall that the main method is static – it is invoked by the Java interpreter without creating an object Static methods cannot reference instance variables because instance variables don't exist until an object exists However, a static method can reference static variables or local variables

41. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.5 – Static Class Members Static methods and static variables often work together The following example keeps track of how many Slogan objects have been created using a static variable, and makes that information available using a static method See Slogan.java and SloganCounter.java

42. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.6 – The this Reference The this reference allows an object to refer to itself That is, the this reference, used inside a method, refers to the object through which the method is being executed Suppose the this reference is used in a method called tryMe, which is invoked as follows: obj1.tryMe(); obj2.tryMe(); In the first invocation, the this reference refers to obj1 ; in the second it refers to obj2

43. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.6 – The this reference The this reference can be used to distinguish the instance variables of a class from corresponding method parameters with the same names The constructor of the Account class could have been written as follows: public Account (String name, long acctNumber, double balance) { this.name = name; this.acctNumber = acctNumber; this.balance = balance; }

44. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-44 5.8 – Method Overloading Method overloading is the process of giving a single method name multiple definitions If a method is overloaded, the method name is not sufficient to determine which method is being called The signature of each overloaded method must be unique The signature includes the number, type, and order of the parameters

45. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5.8 – Method Overloading The compiler determines which method is being invoked by analyzing the parameters float tryMe(int x) { return x +.375; } float tryMe(int x, float y) { return x * y; } result = tryMe(25, 4.32) Invocation

46. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-46 5.8 – Method Overloading The println method is overloaded println (String s) println (int i) println (double d) and so on... The following lines invoke different versions of the println method System.out.println ("The total is:"); System.out.println (total);

47. Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1-47 5.8 – Overloading Methods The return type of the method is not part of the signature That is, overloaded methods cannot differ only by their return type Constructors can be overloaded Overloaded constructors provide multiple ways to initialize a new object