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1 Home

Chapter 10

Object-Oriented Programming

Part 3:

Inheritance, Polymorphism, and

Interfaces

Date Chapter

11/6/2006 Chapter 10, start Chapter 11

11/13/2006 Chapter 11, start Chapter 12

11/20/2006 Chapter 12

11/27/2006 Chapter 13

12/4/2006 Final Exam

12/11/2006 Project Due

Home Home

Topics

• Inheritance Concepts • Inheritance Design - Inherited Members of a Class - Subclass Constructors - Adding Specialization to the Subclass - Overriding Inherited Methods • The protected Access Modifier •AbstractClasses and Methods • Polymorphism • Interfaces Home

Inheritance Concepts

• A common form of reuse of classes is inheritance. • We can organize classes intohierarchiesof functionality. • The class at the top of the hierarchy (superclass) defines instance variables and methods common to all classes in the hierarchy. • We derive a subclass,which inheritsbehavior and fields from the superclass. Home

A Sample Vehicle Hierarchy

• This hierarchy is depicted using a Unified

Modeling Language (UML) diagram.

• In UML diagrams, arrows point from the subclass to the superclass. 2 Home

Superclasses and Subclasses

• A superclass can have multiple subclasses. • Subclasses can be superclasses of other subclasses. • A subclass can inherit directly from only one superclass. • All classes inherit from the

Objectclass.

Home

Superclasses and Subclasses

• A big advantage of inheritance is that we can write common code once and reuse it in subclasses. - Generalization • A subclass can define new methods and instance variables, some of which may override(hide) those of a superclass. - Specialization Home

Specifying Inheritance

• The syntax for defining a subclass is to use the extends keyword in the class header, as in accessModifier class SubclassName extendsSuperclassName // class definition • The superclass name specified after the extendskeyword is called the direct superclass. • As mentioned, a subclass can have many superclasses, but only one direct superclass. Home

An Applet Hierarchy

• When we wrote an applet, we defined a subclass. • We say that inheritance implements an " is a" relationship, in that a subclass object " is a" superclass object as well. • Thus,RollABall"is a"JApplet(its direct superclass), Applet, Panel,

Container, Component, and

Object.

•RollABallbegins with more than

275 methods and 15 fields inherited

from its 6 superclasses. 3 Home

The Bank Account Hierarchy

• TheBankAccountclass is the superclass. - Instance variables: •balance(double) •MONEY (final DecimalFormat) - Methods: • Default and overloaded constructors •depositand withdrawmethods •balanceaccessor •toString •See Example 10.1 BankAccount.java (next slide) Home

BankAccount.java 1/3

import java.text.DecimalFormat; public class BankAccount { public final DecimalFormat MONEY = new DecimalFormat( "$#,##0.00" ); private double balance; public

BankAccount( ) {

balance = 0.0; public

BankAccount( double startBalance ) {

deposit( startBalance ); Home

BankAccount.java 2/3

public doublegetBalance( ) { return balance; public void deposit( double amount ) { if ( amount >= 0.0 ) balance += amount; else

System.err.println( "Deposit amount must be

positive." ); Home

BankAccount.java 3/3

public void withdraw( double amount ) { if ( amount >= 0.0 && amount <= balance ) balance -= amount; else System.err.println( "Withdrawal amount must be positive " + "and cannot be greater than balance" ); public String toString( ) return ( "balance is " + MONEY.format( balance ) ); 4 Home

TheCheckingAccountClass

• We derive theCheckingAccountsubclass from

BankAccount:

public class CheckingAccount extends BankAccount • A subclass inherits all the publicmembers of a superclass. Thus, theCheckingAccountclass inherits - the MONEY instance variable - ThegetBalance, deposit, withdraw, andtoString methods •See Example 10.3 CheckingAccountClient.java (next slide) Home

CheckingAccountClient.java

public class CheckingAccountClient { public static void main( String [] args ) {

CheckingAccount c1 = new CheckingAccount( );

System.out.println( "New checking account: " + c1 ); c1.deposit( 350.75 ); System.out.println( "\nAfter depositing $350.75: " + c1 ); c1.withdraw( 200.25 ); System.out.println( "\nAfter withdrawing $200.25: " + c1 ); Home privateMembers • Superclass members declared as privateare NOT inherited, although they are part of the subclass. • Thus, the balanceinstance variable is allocated to all

CheckingAccountobjects, but methods of the

CheckingAccountclass cannot directly access balance. • To set or get the value of balance, theCheckingAccount methods must call the withdraw, deposit,orgetBalance methods of the Superclass BankAccount. • This simplifies maintenance because theBankAccount class enforces the data validation rules for balance. Home protectedMembers •protectedmembers are inherited by subclasses (like publicmembers), while still being hidden from client classes (like privatemembers). • Also, any class in the same package as the superclass can directly access a protectedfield, even if that class is not a subclass. • Disadvantage: - Because more than one class can directly access a protectedfield, protectedaccess compromises encapsulation and complicates maintenance of a program. - For that reason, we prefer to use private, rather than protected,for our instance variables. 5 Home

Inheritance Rules

nononoprivatemethodsno, must call accessorsand mutators no, must call accessors

and mutatorsnoprivate fieldsnoyes, by calling method from subclass methods yesprotected methodsno, must call accessorsand mutators

yes, by using field

nameyesprotected fieldsyesyes, by calling method from subclass methodsyespublic methodsyesyes, by using field nameyespublicfields

Directly Accessible by

Client of Subclass?Directly Accessible by Subclass?Inherited by subclass?Superclass Members Home

Subclass Constructors

• Constructors are not inherited. • However, the subclass can call the constructors of the superclass to initialize inherited fields. •Implicitinvocation - The default constructor of the subclass automatically calls the default constructor of the superclassquotesdbs_dbs19.pdfusesText_25