Programming With Java ICS201 University Of Ha’il1 Chapter 7 Inheritance.

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University Of Ha’il 1

Chapter 7

Inheritance

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Introduction

Inheritance is the process by which a new class is created

from another class

The new class is called a derived class

The original class is called the base class

A derived class automatically has all the instance

variables and methods that the base class has, and it can

have additional methods and/or instance variables as well.

Inheritance is especially advantageous because it allows

code to be reused, without having to copy it into the

definitions of the derived classes.

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Introduction

The extends clause in a class declaration establishes an

inheritance relationship between two classes. It has the

following syntax:

class ClassName2 extends ClassName1

{

// body of the class

}

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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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Example (Inheritance)//This example shows how to declare a simple class inheritance hierarchyclass Grandparents{ }class Parents extends Grandparents{ }class Uncles extends Grandparents{ }class Sisters extends Parents{ }class Brothers extends Parents{ }class Cousins extends Uncles{ }class InheritanceHierarchy{ public static void main(String[ ] a) {

Grandparents Grandfather = new Grandparents(); System.out.println( "Instantiated Grandparents” ); Grandfather = new Parents(); System.out.println( "Instantiated Parents" );

Grandfather = new Sisters(); System.out.println( "Instantiated Sisters" );Grandfather = new Brothers(); System.out.println( "Instantiated Brothers" ); Grandfather = new Uncles(); System.out.println( "Instantiated Uncles" ); Grandfather = new Cousins(); System.out.println( "Instantiated Cousins" );}

}

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Example (Inheritance)

Output:

Instantiated Grandparents

Instantiated Parents

Instantiated Sisters

Instantiated Brothers

Instantiated Uncles

Instantiated Cousins

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Inherited Members

A derived class automatically has all the instance

variables, all the static variables, and all the public

methods of the base class

Members from the base class are said to be inherited.

Definitions for the inherited variables and methods do not

appear in the derived class

The code is reused without having to explicitly copy it,

unless the creator of the derived class redefines one or

more of the base class methods.

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Inheritance and Variables

Inherit Variable and Methods

A class inherits the state (determined by variables) and

behavior (determined by methods) defined by all of its

super classes. Therefore an object has one copy of every

instance variable of its own class and its super classes.

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 keyword

super.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 {

int x = 200;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=200super.x=100

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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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Method Overriding Method overriding occurs when a class declares a

method that has the same type signature as a method declared by one of its super classes.

When a method in a subclass overrides a method in a superclass, the method in the super class is hidden relative to the subclass object.

Method overriding is a very important capability because it forms the basis for run-time polymorphism (means one interface, multiple implementation).

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Method Overriding

The access permission of an overridden method can be

changed from private in the base class to public (or

some other more permissive access) 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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Note Do not confuse overriding a method in a derived class

with overloading a method name

When a method is overridden, the new method definition given in the derived class has the exact same number and types of parameters as in the base class.

When a method in a derived class has a different signature from the method in the base class, that is overloading.

Note that when the derived class overloads the original method, it still inherits the original method from the base class as well.

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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

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The final Modifier

If the modifier final is placed before the definition of a

method, then that method may not be redefined in a

derived class.

If the modifier final is placed before the definition of a

class, then that class may not be used as a base class to

derive other classes.

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Example (Inheritance and Methods)class I1{

void hello( String s ){

System.out.println( "I1:"+ s ); }

}class J1 extends I1{

void hello( String s ){

super.hello(s); System.out.println( "J1:"+ s );

}}class K1 extends J1{

void hello( String s ) {

super.hello(s); System.out.println( "K1:"+ s );

}}

class SuperForMethod{ public static void main( String[] ar ) {

I1 obj;System.out.println( "Initiating I1" ); obj = new I1(); obj.hello( "morning" );System.out.println( "Initiating J1" ); obj = new J1(); obj.hello( "noon" );System.out.println( "Initiating K1" );obj = new K1(); obj.hello( "evening" );

}}

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Example (Inheritance and Methods)

Output:

Initiating I1

I1:morning

Initiating J1

I1:noon

J1:noon

Initiating K1

I1:evening

J1:evening

K1:evening

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Exercise (Inheritance and Methods)

Write an application that illustrates how a method

can invoke a superclass method. Class I2 is extended

by J2. Class J2 is extended by K2. Each of these

classes defines a getDescription() method that

returns a string. That string includes a description of

the class plus descriptions of each superclass.

Instantiate each of these classes and invoke the

getDescription() mehod.

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Inheritance and Methods

When we define a method with the same signature as a

base class methods, we effectively override that base

method. That means when we send the same message

to base or subclass, the methods defined for that class

will be executed. This is called overriding. This is what

we do in C++ when we define a method as virtual.

To access the parent method we can use modifier super

followed by the dot and the parent method name.

Super.parentMethod()

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The super Constructor

A derived class uses a constructor from the base class to initialize all the data inherited from the base class

In order to invoke a constructor from the base class, it uses a special syntax:

public derivedClass(int p1, int p2, double p3) { super(p1, p2); instanceVariable = p3; }

In the above example, super(p1, p2); is a call to the base class constructor

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The super Constructor

A call to the base class constructor can never use the

name of the base class, but uses the keyword super

instead.

A call to super must always be the first action taken in a

constructor definition.

An instance variable cannot be used as an argument to

super.

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Example (Inheritance & Constructor)class S1 {

int s1;S1( ) { System.out.println( "print S1" ) ;

s1=1;}

}class T1 extends S1 {

int t1;T1() { System.out.println( "print T1" );

t1=2;}

}class U1 extends T1 { int u1; U1( ) {

System.out.println( "print U1" ); u1=3; }}

class InhAndConstructor{ public static void main(String[ ] ar) {

U1 u1 = new U1();System.out.println( "u1.s1=" + u1.s1);System.out.println( "u1.t1=" + u1.t1);System.out.println( "u1.u1=" + u1.u1);

}}

Output:print S1print T1print U1u1.s1=1u1.t1=2u1.u1=3

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Example (Inheritance & Constructor)class S2 {

int s2; S2( int s2) {

this.s2 = s2;}

}class T2 extends S2 {

int t2; T2( int s2, int t2) { super(s2);

this.t2 = t2; }

}class U2 extends T2 {

int u2;U2(int s2, int t2, int u2) { super(s2,t2);

this.u2 = u2; }

}

class SuperAndConstructor{ public static void main(String[] arg) {

U2 u2 = new U2(1,2,3);System.out.println( "u2.s2 =" + u2.s2);System.out.println( "u2.t2 =" + u2.t2);System.out.println( "u2.u2 =" + u2.u2);

}}

Output:u2.s2 =1u2.t2 =2u2.t2 =3

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Inheritance & Final classes

final classes – A class can be defined as ‘final’ when we do not allow any further sub classing.

This can be done for security reasons, or to allow Java to perform some optimization in accessing methods.

final class Polygon extends Shape{

}

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Abstract Class

There are 3 possible modifiers that may precede the class

keyword:

abstract Cannot be instantiated

final Cannot be extended

public Can be accessed by any other class. If this

keyword is missing, access to the class is

limited to the current package.

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Example (Abstract Class)abstract class Shape {

void display() {}

}class Circle extends Shape {

void display(){

System.out.println( "Circle" );}

}class Rectangle extends Shape{

void display() {

System.out.println( "Rectangle" ); }

}

class Triangle extends Shape { void display() {

System.out.println( "Triangle" ); }}class AbstractClass { public static void main(String[] arg) {

Shape s = new Circle(); s.display();s = new Rectangle();s.display();s = new Triangle(); s.display();

}}

OUTPUT:CircleRectangleTriangle

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Variable Modifiers

The possible modifiers that may precede the declaration of

a variable are:

final Is a constant

private Can be accessed only by code in the same class

protected Can be accessed only by code in a subclass or

the same package

public Can be accessed by any other class

static Is not an instance variable

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1Access Modifiers

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Polymorphism

Polymorphism (from the Greek, meaning "many forms", or something similar) is the quality that allows one name to be used for more than one (hopefully related) but technically different purpose.

Method overriding occurs when a class declares a method having same types of signature as a method by one of its super class.

Note: Super class reference can refer to objects of its own or its subclass.

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Polymorphism Methods can be overloaded. A method has the same

name as another member methods, but the type and/or the count of parameters differs.

class Integer {

float val;void add( int amount ){

val = val + amount; }void add( int num, int den){

val = float(num) / den; }

}

Programming With Java ICS201 University Of Ha’il1 Chapter 7 Inheritance.

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