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©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter Four Defining Your Own Classes.

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1 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter Four Defining Your Own Classes

2 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 4 Objectives After you have read and studied this chapter, you should be able to Define an instantiable class with multiple methods and constructors. Differentiate the local and instance variables. Define and use value-returning methods. Distinguish private and public methods. Distinguish private and public data members.

3 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 4 Objectives, cont. After you have read and studied this chapter, you should be able to Describe how the arguments are passed to the parameters in method definitions. Describe how the result is returned from a method. Define a reusable class for handling input routines. Define an instantiable main class.

4 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes Learning how to define instantiable classes is the first step toward mastering the skills necessary in building large programs. A class is instantiable if we can create instances of the class. The DecimalFormat, GregorianCalendar, and String classes are all instantiable classes, while the Math class is not.

5 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes Currency converter example: We need two methods for conversion: fromDollar and toDollar. CurrencyConverteryenConverter; double amountInYen, amountInDollar; yenConverter = new CurrencyConverter( );... amountInYen = yenConverter.fromDollar(200); //from dollar to yen amountInDollar = yenConverter.toDollar(15000); //from yen to dollar

6 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes Since the exchange rate fluctuates, we need a method to set the exchange rate. CurrencyConverteryenConverter; yenConverter = new CurrencyConverter( ); yenConverter.setExchangeRate(130.77);

7 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes Class diagram for a CurrencyConverter object.

8 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.1 A program template for a class definition.

9 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes Once the CurrencyConverter class is defined, we can use its multiple instances. CurrencyConverter yenConverter, markConverter; double amountInYen, amountInMark, amountInDollar; yenConverter = new CurrencyConverter(); yenConverter.setExchangeRate(130.77); markConverter = new CurrencyConverter( ); markConverter.setExchangeRate(1.792); amountInYen = yenConverter.fromDollar( 200 ); amountInMark = markConverter.fromDollar( 200 ); amountInDollar= yenConverter.toDollar( ); amountInMark= markConverter.fromDollar(amountInDollar);

10 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.2 Every object of a class has its own copy of instance variables.

11 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes Syntax for defining a method.

12 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes If the method declaration includes the static modifier, it is a class method. Class methods can access only class variables and constants.

13 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes If the method declaration does not include the static modifier, it is an instance method. Instance methods can access class variables and constants, as well as instance variables.

14 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.1 Defining Instantiable Classes We call a method that returns a value a value-returning method, or non-void method. A value-returning method must include a return statement in the following format: return ; public double toDollar( double foreignMoney ) { return (foreignMoney / exchangeRate); }

15 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.2 Instantiable Classes and Constructors A constructor is a special method that is executed when a new instance of the class is created. The purpose of the constructor is to initialize an object to a valid state. Whenever an object is created, we should ensure that it is created in a valid state by properly initializing all data members in a constructor.

16 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.2 Instantiable Classes and Constructors The name of a constructor must be the same as the name of the class. If no constructor is defined for a class, then the Java compiler will include a default constructor.

17 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.2 Instantiable Classes and Constructors Syntax of a constructor.

18 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.2 Instantiable Classes and Constructors The default constructor will have the following form: public ( ) { } public CurrencyConverter( ) { }

19 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.2 Instantiable Classes and Constructors A constructor does not have a return type. It is possible to create multiple constructors for a class, as long as the constructors have either A different number of parameters, or Different data types for the parameters if the number of parameters is the same.

20 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.2 Instantiable Classes and Constructors Examples of multiple constructors public MyClass( int value ) { … } public MyClass( ) { … } public MyClass( float value ) { … }

21 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.3 Information Hiding and Visibility Modifiers Public methods of a class determine the behavior of its instances. Internal details are implemented by private methods and private data members. Declaring the data members private ensures the integrity of the class.

22 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.3 Information Hiding and Visibility Modifiers The modifiers public and private designate the accessibility of data members and methods. If a class component (data member or method) is declared private, no outside methods can access it. If a class component is declared public, any outside method can access it.

23 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.3 Information Hiding and Visibility Modifiers Class constants may be declared public because: A constant is “read only” by nature A constant is a clean way to make characteristics of the instances known to client programmers. Public class data members are accessed by the syntax.

24 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.3 Information Hiding and Visibility Modifiers Plus and minus signs designate public and private components, respectively.

25 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing A local variable is a variable that is declared within a method declaration. Local variables are accessible only from the method in which they are declared.

26 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing Memory space for local variables is allocated only during the execution of the method. When the method execution completes, memory space will be cleared. The parameters of a method are local to the method.

27 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing Sample method:

28 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.3 Memory space for local variables and parameters is allocated and erased.

29 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.3 cont. Memory space for local variables and parameters is allocated and erased.

30 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing When a method is called, the value of the argument is passed to the matching parameter, and separate memory space is allocated to store this value. This way of passing the value of arguments is called a pass-by-value, or call-by-value, scheme.

31 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing The data type of the argument must be assignment-compatible with the data type of the matching parameter.

32 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.4 Memory space for the parameters is allocated and erased.

33 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.4 cont. Memory space for the parameters is allocated and erased.

34 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing Parameters and return types are designated in the program diagram:

35 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.4 Local Variables, Return Values, and Parameter Passing The same designation applies to data members:

36 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.5 Accessors, Mutators, and Overloaded Methods A set method is called a mutator because it changes the property of an object. An accessor is a method that returns a property of an object.

37 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.5 Accessors, Mutators, and Overloaded Methods Like constructors, methods may have the same name as long as the methods have either A different number of parameters, or Different data types for the parameters if the number of parameters is the same. The methods with the same name are called overloaded methods.

38 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.5 Accessors, Mutators, and Overloaded Methods Dot notation is optional when you call a method from another method if the two methods belong to the same object. If dot notation is used, use the reserved word this to refer to the same object.

39 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.5 Calling a method belonging to the same object vs. calling a method belonging to a different object.

40 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.6 Passing and Returning Objects Passing and returning objects follow the same process as passing and returning primitive data types. The only difference is that with objects, the value being passed is the reference (or address) to an object.

41 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.6 Passing and Returning Objects When a variable is an object name, the value of the variable is the address in memory where the object is stored. The effect of passing this value, or reference, is to have two variables (object names) referring to the same object.

42 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.6 How an object is passed to a method.

43 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.6 cont. How an object is passed to a method.

44 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.7 How an object is passed to a method (cont.).

45 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.7 cont. How an object is passed to a method (cont.).

46 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.8 How an object is returned from a method.

47 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.8 cont. How an object is returned from a method.

48 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.9 How an object is returned from a method.

49 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 4.9 cont. How an object is returned from a method.

50 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.7 Modularizing the Input Routine Functionality When a common task is repeated over and over, it is best to capture the common task into a class and use an instance of the class to perform the task.

51 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.8 Organizing Classes into a Package 1. Include the statement package as the first statement of the source file for the class you are packaging. 2.The class declaration must include the visibility modifier public. 3. Create a folder with the same name as the package name.

52 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.8 Organizing Classes into a Package 4. Place the class into the folder and compile it. 5. Modify the CLASSPATH environment variable to include the folder that contains the package. Note that the steps to change the CLASSPATH environment variable are different for each platform and IDE.

53 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.9 Sample Development: Defining and Using Instantiable Classes Example: Loan and LoanCalculator classes. 1.Consider problem statement. Write a loan calculator program that computes both monthly and total payments for a given loan amount, annual interest rate, and loan period.

54 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.9 Sample Development: Defining and Using Instantiable Classes Develop overall plan: 1.Get three input values: loanAmount, interestRate, and loanPeriod. 2.Compute the monthly and total payments. 3.Output the results.

55 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.9 Sample Development: Defining and Using Instantiable Classes Five steps of implementation: 1.Start with the main class and a skeleton of the LoanCalculator class. The skeleton class will include only an object/variable declaration and a constructor to create objects. Define a temporary placeholder for the Loan class.

56 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.9 Sample Development: Defining and Using Instantiable Classes Five steps of implementation: 2. Implement the input routine to accept three input values. 3. Implement the output routine to display the results.

57 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. 4.9 Sample Development: Defining and Using Instantiable Classes Five steps of implementation: 4. Implement the computation routine to compute the monthly and total payments. 5. Finalize the program, implementing any remaining temporary methods and adding necessary methods as appropriate.

58 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig The program diagram for design alternative 1.

59 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig The program diagram for alternative design 2.

60 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display Making an Instantiable Class the Main Class LoanCalculator example: Copy the main method of LoanCalculatorMain and paste it to the LoanCalculator class: //Instantiable Main Class class LoanCalculator { //exactly the same code as before comes here public static void main (String [ ] args) { LoanCalculator loanCalculator; loanCalculator = new LoanCalculator( ); loanCalculator.start( ); }

61 ©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Javadoc comments Javadoc comments begin with /** and end with */. Javadoc tags are special markers that begin @return


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