Week 4Inheritance and PolymorphismCS 180Sunil PrabhakarDepartment of Computer Science Purdue UniversityObjectivesUnderstand Inheritanceexpressing inheritance: extends visibility and inheritance: protectedoverriding, final constructors and inheritance: superPolymorphismpolymorphic messagesinstanceof operatorabstract classes & methods: abstract23IntroductionInheritance and polymorphism are key concepts of Object Oriented Programming.Inheritance facilitates the reuse of code.A subclass inherits members (data and methods) from all its ancestor classes.The subclass can add more functionality to the class or replace some functionality that it inherits.Polymorphism simplifies code by automatically using the appropriate method for a given object.Polymorphism also makes it easy to extend code.Inheritance45Sample applicationBanking Example:There are two types of accounts: checking and savings.All accounts have a number, and an owner (with name, and a Social Security number), and balance.There are different rules for interest and minimum balance for checking accounts and savings accounts.How should we model this application?Two classes, one for each type of account? Have to repeat code for common parts.can lead to inconsistencies, harder to maintain.Create three classes: Account; SavingsAccount, and CheckingAccount6InheritanceA superclass corresponds to a general class, and a subclass is a specialization of the superclass.E.g. Account, Checking, Savings.Behavior and data common to the subclasses is often available in the superclass.E.g. Account number, owner name, data opened.Each subclass provides behavior and data that is relevant only to the subclass.E.g. Minimum balance for checking a/c, interest rate and computation for savings account.The common behavior is implemented once in the superclass and automatically inherited by the subclasses.InheritanceIn order to inherit the data and code from a class , we have to create a subclass of that class using the extends keyword.public class SavingsAccount extends Account {SavingsAccount will inherit the data members and methods of Account.SavingsAccount is a sub (child, or derived) class; Account is a super (parent or base) class.A parent (of a parent ...) is an ancestor class. A child (of a child ...) is a descendant class.78The Account classclass Account { protected String ownerName; protected int socialSecNum; protected float balance; public Account() { this(“Unknown”, 0, 0.0); } public Account(String name, int ssn) { this(name, ssn, 0.0); } public Account(String name, int ssn, float bal) { ownerName = name; socialSecNum = ssn; balance = bal; } public String getName( ) { return ownerName; } public String getSsn( ) { return socialSecNum; } public float getBalance() { return balance; } public void setName(String newName) { ownerName = newName; } public void accrueInterest() { System.out.println(“No interest”); }public void deposit(float amount) { balance += amount; }}9Savings Accountclass SavingsAccount extends Account{ protected static final float MIN_BALANCE=100.0; protected static final float OVERDRAW_LIMIT=-1000.0; protected static final float INT_RATE=5.0; public void accrueInterest() { balance *= 1 + INT_RATE/100.0; } public void withdraw(float amount) { float temp; temp = balance - amount;if (temp >= OVERDRAW_LIMIT) balance = temp; else System.out.println(“Insufficient funds”); }}10Checking Accountclass CheckingAccount extends Account{ protected static final float MIN_INT_BALANCE=100.0; protected static final float INT_RATE=1.0; public void accrueInterest() {if (balance > MIN_INT_BALANCE) balance *= 1 + INT_RATE/100.0; } public void withdraw(float amount) { float temp; temp = balance - amount;if (temp >= 0) balance = temp; else System.out.println(“Insufficient funds”); }}Visibility1112The visibility modifierspublic data members and methods are accessible to everyone.private data members and methods are accessible only to instances of the class.protected data members and methods are accessible only to instances of the class and descendant classesprotected is similar to:public for descendant classesprivate for any other class13Visibility (unrelated class)class Test {Sup sup = new Sup();Sub sub = new Sub(); sup.a = 5;sup.b = 5;sup.c = 5;sub.a = 5;sub.b = 5;sub.c = 5;sub.d = 5;sub.e = 5;sub.f = 5; } class Sup {public int a;protected int b;private int c;}class Sub extends Sup {public int d;protected int e;private int f;}From an unrelated class, only public members are visible.14Visibility (related class)class Sup {public int a;protected int b;private int c;}class Sub extends Sup {public int d;protected int e;private int f;public void methodA(){a=5;b=5;c=5;d=5;e=5;f=5;}}From a descendant class, only private members of ancestors are hidden.15Visibility (static members)class Test {Sup sup = new Sup();Sub sub = new Sub(); sup.a = 5;sup.b = 5;sup.c = 5;sub.a = 5;sub.b = 5;sub.c = 5;sub.d = 5;sub.e = 5;sub.f = 5; } class Sup {public static int a;protected static int b;private static int c;}class Sub extends Sup {public static int d;protected static int e;private static int f;}Same rules for class (static) members.16Visibility (static members)class Sup {public static int a;protected static int b;private static int c;}class Sub extends Sup {public int d;protected int e;private int f;public void methodA(){a=5;b=5;c=5;d=5;e=5;f=5;}}Same rules for class (static) members.17Visibility (across instances)class Sup {public int a;protected int b;private int c;}class Sub extends Sup {public int d;protected int e;private int f;public void methodA(Sub s){s.a=5;s.b=5;s.c=5;s.d=5;s.e=5;s.f=5;}}An instance method has the same access to data members of any object of that class.Overriding1819OverridingAll non-private members of a class are inherited by derived classesThis includes instance and class membersA derived class may however, override an inherited methodData members can also be overridden but should be avoided since it only creates confusion.To override a method, the derived class simply defines a method with the same signature (same name, number and types of parameters)An overridden method cannot change the return type!A subclass may also