chapter 13 · 2 introduction inheritance and polymorphism are key concepts of object oriented...

Chapter 13

Inheritance and Polymorphism

CS 180Sunil PrabhakarDepartment of Computer Science Purdue University

2

Introduction

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.

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Inheritance 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.

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Defining Classes with Inheritance

Case Study: Suppose we want implement a class roster that

contains both undergraduate and graduate students. Each student’s record will contain his or her name,

three test scores, and the final course grade. The formula for determining the course grade is

different for graduate students than for undergraduate students.

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Modeling Two Types of Students

There are two ways to design the classes to model undergraduate and graduate students. We can define two unrelated classes, one for

undergraduates and one for graduates. We can model the two kinds of students by using

classes that are related in an inheritance hierarchy. Two classes are unrelated if they are not

connected in an inheritance relationship. NOTE: all classes inherit from Object

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Classes for the Class Roster

For the Class Roster sample, we design three classes: Student UndergraduateStudent GraduateStudent

The Student class will incorporate behavior and data common to both UndergraduateStudent and GraduateStudent objects.

The UndergraduateStudent class and the GraduateStudent class will each contain behaviors and data specific to their respective objects.

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Creating a subclass

We use the keyword extends in the class header to declare that it is a subclass:public class GraduateStudent extends Student {

Student is the superclass, parent, or base class. A parent (of a parent …) is an ancestor class.

GraduateStudent is the subclass, child, or derived class. A child (of a child ...) is a descendent class.

If a class does have the extend keyword, then it derives from the Object class.

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Example: Studentclass Student { protected final static int NUM_OF_TESTS = 3; protected String name; protected int[] test; protected String courseGrade;

public Student( ) { this("No Name"); } public Student(String studentName) { name = studentName; test = new int[NUM_OF_TESTS]; courseGrade = "****"; } public String getCourseGrade( ) { return courseGrade; } public String getName( ) { return name; } public int getTestScore(int testNumber) { return test[testNumber-1]; } public void setName(String newName) { name = newName; } public void setTestScore(int testNumber, int testScore) { test[testNumber-1] = testScore; } public void computeCourseGrade();

}

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Example: GraduateStudent

class GraduateStudent extends Student{

public void computeCourseGrade() { int total = 0;

for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; }

if (total / NUM_OF_TESTS >= 80) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } }}

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Example: UndergraduateStudent

class UndergraduateStudent extends Student{

public void computeCourseGrade() { int total = 0;

for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; }

if (total / NUM_OF_TESTS >= 70) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } }}

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Inheritance Hierarchy

+ NUM_OF_TESTS# name# test# courseGrade

Student

+ Student(): void+ Student(String): void+ getCourseGrade(): String+ getName(): String+ getTestScore(int): int+ setName(String): void+ setTestScore(int, int): void

UndergraduateStudent

+ computeCourseGrade(): void

GraduateStudent

+ computeCourseGrade(): void

New visibility modifier: #protected

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Overriding 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 name, number and types of parameters (same signature) An overridden method cannot change the return type!

A subclass may overload any method (inherited or otherwise) by using the same name, but different signature.

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Overriding and overloadingSup

Sub

methodA(){}methodA(int i){}

methodA(){}methodA(char c){}

Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c';

sup.methodA();

sup.methodA(i);

sub.methodA();

sub.methodA(i);

sub.methodA(c);

sup.methodA(c);

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Sub




sup.methodA();

sup.methodA(i);

sub.methodA();

sub.methodA(i);

sub.methodA(c);

sup.methodA(c);

overloaded