abstractclass& interface

7/28/2019 Abstractclass& Interface

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Abstract class

Certain methods in the super class do not contain

any logic and need to be overridden by the subclass.

In such situations, the method in the super class

should be declared by using the keyword abstract.

The subclass provides the implementation details of

such abstract methods. The super class only provides

the name and signatures of the method. Thus, it is

the responsibility of the subclass to override it.

The syntax is:

abstract type methodname(parameter list);

No method body is present.

Any class that contains one or more abstract

methods should also be declared abstract. To

declare a class as abstract, the class keyword will

be preceded by the abstract keyword. Abstract

classes cannot be instantiated. Constructors and

static class methods cannot be declared as

abstract.Any subclass of an abstract class must implements

all the abstract methods declared in the super

class.


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Abstract classes cannot have objects but they

can be used to create object references

because javas runtime polymorphism is

implemented through the use of super class

references. Thus, an object can be used to

create a reference to an abstract class that

can point to a subclass object.

Abstract Class

Use the abstract keyword to declare a class abstract. The keyword appears in the classdeclaration somewhere before the class keyword.

The syntax is:

public abstract class Employee

Abstract Methods

If you want a class to contain a particular method but you want the actual implementation

of that method to be determined by child classes, you can declare the method in the

parent class as abstract.

The abstract keyword is also used to declare a method as abstract.An abstract methodsconsist of a method signature, but no method body.

The syntax is:

abstract type methodname(parameter list);


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Declaring a method as abstract has two results

The class must also be declared abstract. If a class contains

an abstract method, the class must be abstract as well.

Any child class must either override the abstract method or

declare itself abstract.

A child class that inherits an abstract method must override it. If

they do not, they must be abstract, and any of their children must

override it.

Important Point about abstract classes

Abstract class may contain static data.

Any class with an abstract method is automatically

abstract itself, and must be declared as such.

A class may be declared abstract even if it has no abstract

methods. This prevents it from being instantiated.

A compile-time error occurs if a method declaration that

contains the keyword abstract also contains any one of

the keywords private, static ,final, native or synchronized.


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Example1

abstractclass Figure

{

doubledim1;doubledim2;

Figure(double a, double b)

{

dim1 = a;

dim2 = b;

}

abstractdouble area();//abstract method

}

class Rectangle extends Figure

{

Rectangle(double a, double b)

{

super(a, b);

}

// override area for rectangle

double area()

{

System.out.println("Inside Area for Rectangle.");

returndim1 * dim2;

}

}

class Triangle extends Figure

{

Triangle(double a, double b)

{

super(a, b);

}

//override area for right triangle

double area()

{System.out.println("Inside Area for Triangle.");

returndim1 * dim2 / 2;

}

}

class Demo5

{

publicstaticvoidmain(String args[])


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{

// Figure f = new Figure(10, 10); // illegal now

Rectangle r = new Rectangle(9, 5);

Triangle t = new Triangle(10, 8);

Figure figref; // this is OK, no object is createdfigref = r;

System.out.println("Area is " + figref.area());

figref = t;

System.out.println("Area is " + figref.area());

}

}

Example2abstractclass BankInfo

{

final String bankname="Kukreti Reserve Bank Of India";

protectedString cname,address,mobno,email;

protecteddoublebalance;

protectedintcacno;

publicvoidsetCustomerInfo(int cacno,String cname,String

address,String mobno,String email,double balance)

{

this.cacno=cacno;

this.cname=cname;

this.address=address;

this.mobno=mobno;

this.email=email;

this.balance=balance;

}

publicvoidshowCustomerInfo()

{

System.out.println("Bank Name= "+bankname);System.out.println("Name= "+cname);

System.out.println("Acount Number= "+cacno);

System.out.println("Address= "+address);

System.out.println("Mobile No= "+mobno);

System.out.println("EmailId= "+email);

System.out.println("Balance Amount= "+balance);

}

publicabstractvoiddepositAmount(double amt);


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publicabstractvoidwithDrawAmount(double amt);

}

class Transaction extends BankInfo

{

publicvoiddepositAmount(double amt)

{

balance=balance+amt;

}

publicvoidwithDrawAmount(double amt)

{

if(balance>amt)

{

balance=balance-amt;

}

}

double getBalance()

{

returnbalance;

}

}

publicclass Demo99 {

publicstaticvoidmain(String[] args)

{

Transaction ob=new Transaction();

ob.setCustomerInfo(11207,"Vedik

Kukreti","DehraDun","222272221","[email protected]",5000);

ob.showCustomerInfo();

System.out.println("========After Deposit Transaction==========");

ob.depositAmount(7000);

System.out.println("Balance will be ="+ob.getBalance());

System.out.println("========After WithDraw Transaction==========");

ob.withDrawAmount(2000);


}

}

Output will be

Bank Name= Kukreti Reserve Bank Of India


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Name= Vedik Kukreti

Acount Number= 11207

Address= DehraDun

Mobile No= 222272221

EmailId= [email protected]

Balance Amount= 5000.0

========After Deposit Transaction==========

Balance will be =12000.0

========After WithDraw Transaction==========


Java - Interfaces

An interface is a collection of abstract methods. A

class implements an interface, thereby inheritingthe abstract methods of the interface.

An interface is not a class. Writing an interface is

similar to writing a class, but they are two different

concepts. A class describes the attributes and

behaviors of an object. An interface contains

behaviors that a class implements.

Unless the class that implements the interface is

abstract, all the methods of the interface need to be

defined in the class.


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An interface is similar to a class in the following

ways:

An interface can contain any number ofmethods.

An interface is written in a file with a .java

extension, with the name of the interface

matching the name of the file.

The bytecode of an interface appears in a

.class file.

Interfaces appear in packages, and their

corresponding bytecode file must be in a

directory structure that matches the package

name.

However, an interface is different from a class inseveral ways, including:

You cannot instantiate an interface.

An interface does not contain any constructors.

All of the methods in an interface are abstract.

An interface cannot contain instance fields.The only fields that can appear in an interface

must be declared both static and final.


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An interface is not extended by a class; it is

implemented by a class.

An interface can extend multiple interfaces.Declaring Interfaces:

The interface keyword is used to declare an interface. Here

is a simple example to declare an interface:

Example:

Let us look at an example that depicts encapsulation:

public interface NameOfInterface

{

//Any number of final, static fields

//Any number of abstract method declarations

}

Interfaces have the following properties:

An interface is implicitly abstract. You do not

need to use the abstract keyword when

declaring an interface.

Each method in an interface is also implicitly

abstract, so the abstract keyword is not

needed.

Methods in an interface are implicitly public.


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Implementing Interfaces:

When a class implements an interface, you can think of the class as

signing a contract, agreeing to perform the specific behaviors of the

interface. If a class does not perform all the behaviors of the interface,the class must declare itself as abstract.

A class uses the implements keyword to implement an interface. The

implements keyword appears in the class declaration following the

extends portion of the declaration.

When overriding methods defined in interfaces there areseveral rules to be followed:

Checked exceptions should not be declared on

implementation methods other than the ones declared

by the interface method or subclasses of those

declared by the interface method.

The signature of the interface method and the samereturn type or subtype should be maintained when

overriding the methods.

An implementation class itself can be abstract and if

so interface methods need not be implemented.

When implementation interfaces there are several rules:

A class can implement more than one interface at a

time.


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A class can extend only one class, but implement

many interface.

An interface can extend another interface, similarly tothe way that a class can extend another class.

Extending Interfaces:

An interface can extend another interface, similarly to the

way that a class can extend another class. The extendskeyword is used to extend an interface, and the child

interface inherits the methods of the parent interface.

Example1 Class Rectangle implements FigureData interface

interface FigureData

{

double area();//abstract method

}class Rectangle implements FigureData

{doubledim1,dim2;


{

dim1=a;

dim2=b;


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}

publicdouble area()

{returndim1 * dim2;

}

}

class Demo5

{


{


double a=r.area();

System.out.println("Area= "+a);

}

}

Output will Be

Area= 45.0

Example2 Class Rectangle extends Figure class and

implements FigureData interface


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{


}

class Figure

{

doubledim1;

doubledim2;


{

dim1 = a;

dim2 = b;}

}

class Rectangle extends Figure implements FigureData

{


{

super(a,b);

}

publicdouble area()

{

returndim1 * dim2;

}

}

class Demo5

{


{



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double a=r.area();


}

}

Example3 interface can be used as a Reference

class Demo5

{


{

FigureData ob;


ob=r;

double a=ob.area();


}

}

Example4 implements more than one interfaces

Class Rectangle extends Figure class and implements

FiguareData interface and CalculateData interface


{



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}

interface CalculateData

{

voidsetValue(int a ,int b);

int cal();}

class Figure

{

doubledim1;

doubledim2;


{

dim1 = a;

dim2 = b;}

}

class Rectangle extends Figure implements FigureData,CalculateData

{

intn1,n2;


{

super(a,b);

}

publicvoidsetValue(int a,int b)

{

n1=a;

n2=b;

}

publicint cal()

{

returnn1+n2;}

publicdouble area()

{

returndim1 * dim2;

}

}


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class Demo5

{


{

Rectangle ob = new Rectangle(9, 5);double a=ob.area();


ob.setValue(50,60);

int res=ob.cal();

System.out.println("sum= "+res);

}

}

Example5 interface extends another

interface

interface Exampleone

{

voidshowResult();

}

interface Exampletwo extends Exampleone

{

intcalculate();

}

class InterDemo implements Exampletwo

{

intn1,n2,res;

InterDemo(int a,int b)


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{

n1=a;

n2=b;

}

publicvoidshowResult(){

System.out.println("N1= "+n1);

System.out.println("N2= "+n2);

System.out.println("Output= "+calculate());

}

publicint calculate()

{

returnn1+n2;}

}

publicclass Demo99 {

publicstaticvoidmain(String[] args)

{

InterDemo ob=new InterDemo(50,60);ob.showResult();

}

}

Output will be

N1= 50

N2= 60Output= 110


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Example6 Class Transaction extends Bankinfo class and

implements Bank interface

interface Bank

{String bankname="Kukreti Reserve Bank Of India";

voiddepositAmount(double amt);

voidwithDrawAmount(double amt);

}

class BankInfo

{protectedString cname,address,mobno,email;

protecteddoublebalance;protectedintcacno;

publicvoidsetCustomerInfo(int cacno,String cname,String

address,String mobno,String email,double balance)

{

this.cacno=cacno;

this.cname=cname;

this.address=address;

this.mobno=mobno;

this.email=email;

this.balance=balance;

}

publicvoidshowCustomerInfo(){

System.out.println("Name "+cname);

System.out.println("Acount Number "+cacno);

System.out.println("Address "+address);

System.out.println("Mobile No "+mobno);

System.out.println("EmailId "+email);

System.out.println("Balance Amount "+balance);

}

}

class Transaction extends BankInfo implements Bank

{

publicvoiddepositAmount(double amt)

{

balance=balance+amt;

}

publicvoidwithDrawAmount(double amt)


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{

if(balance>amt)

{

balance=balance-amt;

}

}

double getBalance()

{

returnbalance;

}

voidshowBankName()

{

System.out.println("Bank Name= "+bankname);

}

}

publicclass Demo99

{

publicstaticvoidmain(String[] args){

Transaction ob=new Transaction();

ob.setCustomerInfo(11207,"Vedik

Kukreti","DehraDun","222272221","[email protected]",5000);

ob.showBankName();

ob.showCustomerInfo();

System.out.println("========After Deposit Transaction==========");

ob.depositAmount(7000);System.out.println("Balance will be ="+ob.getBalance());

System.out.println("========After WithDraw Transaction==========");

ob.withDrawAmount(2000);


}

}

Output Will Be

Bank Name= Kukreti Reserve Bank Of India

Name Vedik Kukreti

Acount Number 11207

Address DehraDun

Mobile No 222272221

EmailId [email protected]


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Balance Amount 5000.0

========After Deposit Transaction==========


========After WithDraw Transaction==========


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