Chapter 1 Inheritance

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University Of Ha’il 1

Chapter 1

Inheritance

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Introduction to Inheritance

Inheritance is one of the main techniques of object-oriented programming (OOP)

Using this technique, further classes can be created from existing ones; those classes are said to inherit the methods and instance variables of the class they inherited The original class is called the base class The new class is called a derived class

Advantage: Reusing existing code

7-2

Base Class

Derived Class

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Inheritance

Animal

Dog FoxCat

3

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Inheritance

Land Vehicle

Truck CarBus

4

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Inheritance

Land Vehicle

Car

Toyota

TruckBus

Yaris Corolla Camry5

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6

An Example Inheritance Hierarchy

Data and behavior associated with super-classes are accessible to those subclasses

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7

The is-a Rule

Always check generalizations to ensure they obey the is-a rule

“A checking account is an account” “A village is a municipality” “A cat is an animal”

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Derived Class (subclass) A derived class, also called a subclass (or child class), is

defined by starting with another already defined class, called a base class or superclass or parent class, and adding (and/or changing) methods, instance variables, and static variables The derived class inherits:

all the public methods, all the public and private instance variables, and all the public and private static variables from the base class.

The derived class can add more instance variables, static variables, and/or methods.

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Derived Classes

The extends clause in a class declaration establishes an

inheritance relationship between two classes. It has the

following syntax:class DerivedClass extends BaseClass

{// body of the class

}Base Class

Derived Class

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Parent and Child Classes

A base class is often called the parent class A derived class is then called a child class

These relationships are often extended such that a class that is a parent of a parent . . . of another class is called an ancestor class If class A is an ancestor of class B, then class B can be

called a descendent of class A

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Inheritance and Variables

Variable Hiding If a variable of a class has a same name (type maybe

different) as a superclass variable, in that case class variable hides the superclass variable.

You can access a hidden variable by using super keywordsuper.variable

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Example (Inheritance and variables)class C1 {

static int x;}class C2 extends C1 {

static String x;}class Cc {

public static void main(String[] args){

C1 p = new C1();p.x = 55;System.out.println("p.x=" +

p.x);C2 q = new C2();q.x = "This is a String";System.out.println( "q.x=" +

q.x);}

}

Output:p.x=55q.x=This is a String

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Example (Inheritance and variables)class M100 {

int x = 100;}class M200 extends M100 {

String x = " Welcome “ ;void display(){

System.out.println( "x=" + x);System.out.println( "super.x="

+super.x);}

}class SuperKeyword {

public static void main(String[] args){

M200 m200 = new M200();m200.display();

}}

Output:x= Welcome super.x=100

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Example

Inherit1.java

Inherit2.java


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Dr.Mwaffaq Abu Alhija University Of Ha’il 15

Homework

Write an application that illustrates how to access a hidden

variable. Class G declares a static variable x. Class H

extends G and declares an instance variable x. A display()

method in H displays both of these variables.

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Inheriting Methods Override method:

Supply a different implementation of a method that exists in the superclass

Must have same signature (same name and same parameter types)

Inherit method: Don't supply a new implementation of a method that

exists in superclass Superclass method can be applied to the subclass

objects

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Inheriting Methods (Cont’d) Add method:

Supply a new method that doesn't exist in the superclass

New method can be applied only to subclass objects

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18

Overloading vs Overriding

Comparison between overloading and overriding

Overriding deals with two methods, one in a parent

class and one in a child class with the same signature

Overriding lets you define a similar operation in different

ways for different object types

Overloading deals with multiple methods in the same class with the same name but

different signatures.

Overloading lets you define a similar operation in different

ways for different data

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Method Overriding

The access permission of an overridden method can be changed from private in the base class to public in the derived class.

However, the access permission of an overridden method can not be changed from public in the base class to a more restricted access permission in the derived class.

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Method Overriding Given the following method header in a base case:

private void doSomething()

The following method header is valid in a derived class:public void doSomething()

However, the opposite is not valid Given the following method header in a base case:

public void doSomething()

The following method header is not valid in a derived class: private void doSomething()

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Example (Method Overriding)class A1{

void hello(){

System.out.println( "Hello from A1" );}

}class B1 extends A1{

void hello(){

System.out.println( "Hello from B1" );}

}class C1 extends B1{ void hello() { System.out.println( "Hello from C1"); }}

class MethodOverriding{ public static void main(String[] arg) { C1 obj = new C1(); obj.hello(); A1 a = new C1() ; a.hello(); }}

Output:Hello from C1Hello from C1

Chapter 1 Inheritance

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