Applying Interfaces

Applying Interfaces

The interface is a powerful tool. The same interface can be used by different classes for some method. Then this method can be implemented by each class in its own way. Thus same interface can provide variety of implementation. The selection of different implementations is done at the run time. Following is a simple Java program which illustrates this idea.

Step 1: Write a simple interface as follows

Java Program [interface1.java]

interface interface1

{

void MyMsg(String s);

}

Step 2: Write a simple Java Program having two classes having their own implementation of MyMsg method.

Java Program[Test.java]

import java.io.*;

import java.util.*;

class Class1 implements interface1

{

private String s;

public void MyMsg(String s)

{

System.out.println("Hello "+s);

}

}

class Class2 implements interface1

{

private String s;

public void MyMsg(String s)

{

System.out.println("Hello "+s);

}

}

class Test

{

public static void main(String[] args)

{

interface1 inter;

Class1 obj1=new Class1();

Class2 obj2=new Class2();

inter=obj1;

inter.MyMsg("User1");

inter=obj2;

inter.MyMsg("User2");

}

}

Step 3: Execute the program.

Program Explanation:

• The selection of method MyMsg is done at the runtime and it depends upon the object which is assigned to the reference of the interface.

• We have created reference inter for the interface in which the method MyMsg is declared. Then the objects obj1 and obj2 are created for the classes class1 and class2 respectively. When the obj1 is assigned to the reference inter then the message "Hello User1" will be displayed because the string passed to the method MyMsg is "User1". Similarly, when the obj2 is assigned to the reference inter then the message "Hello User2" will be displayed because the string passed to the method MyMsg is "User2".

• Thus using the same interface different implementations can be selected.

Ex. 2.18.1: Write a program to create interface method named customer. In this keep the methods called information(), show() and also maintain the tax rate. Implement this interface in employee class and calculate the tax of the employee based on their income.

Sol.:

Step 1: Create an interface file named Customer.java. It is as follows -

interface Customer

{

double rate1=0.10;

double rate2=0.20;

double rate3=0.25;

double income=0.0;

void information();

void show();

}

Step 2: Create a Java program named Employee.java. It is as follows -

Employee.java

import java.lang.System;

import java.util.Scanner;

class TestClass implements Customer

{

private char sex;

private double tax=0;

private double netpay;

private double income=0.0;

public void information()

{

Scanner in=new Scanner(System.in);

System.out.print("\n Enter Sex(M or F)");

sex = in.next().charAt(0);

System.out.println("\n Enter your income");

income in.nextDouble();

if(sex == 'M')

{

if((income>=190000)&&(income<200000))

{

netpay income;

}

else if((income>=200000)&&(income<500000))

{

netpay income-(income *rate1);

}

else if((income>=500000)&&(income <1000000))

{

netpay income-(income*rate2);

}

else if(income>1000000)

netpay income-(income*rate3);

}

else

{

if((income>=190000)&&(income<200000))

{

netpay income;

}

else if((income>=200000)&&(income<500000))

{

netpay income;

}

else if((income>=500000)&&(income<1000000))

{

netpay income-(income*rate1);

}

else if(income>=1000000)

netpay-income-(income*rate2);

}

}

public void show()

{

System.out.print("\n The net Income of the person is\n" +netpay);

}

}

class Employee

{

public static void main(String[] args)

{

Customer cust;

TestClass obj = new TestClass();

cust=obj;

cust.information();

cust.show();

}

}

Step 3: Open the command prompt and issue the following commands to execute the above program

D:\>javac Employee.java

D:\>java Employee

Enter Sex(M or F)F

Enter your income

500000

The net Income of the person is 450000.0

Ex. 2.18.2: Define an interface using JAVA that contains a method to calculate the perimeter of object. Define two classes - circle and rectangle with suitable fields and methods. Implement the interface "perimeter" in these classes. Write the appropriate main() method to create object of each class and test all methods.

Sol.:

Step 1: We will write the simple interface. The name of this interface is perimeter. In this interface the method calculate is declared.

perimeter.java

interface perimeter

{

void calculate();

}

Step 2: Following is a java program in which two classes are defined- circle and rectangle. The perimeter for each of this class is calculated using the function declared in the interface.

Main.java

import java.io.*;

import java.util.*;

import java.util.Scanner;

class circle implements perimeter

{

private double r;

public void calculate()

{

System.out.print("Enter radius:");

Scanner input=new Scanner(System.in);

double r=input.nextDouble();

double p=2*3.14*r;

System.out.println("Perimeter of Circle: "+p);

}

}

class rectangle implements perimeter

{

private double length, breadth;

public void calculate()

{

System.out.print("Enter length:");

Scanner input=new Scanner(System.in);

double length=input.nextDouble();

System.out.print("Enter breadth:");

double breadth=input.nextDouble();

double p=2*(length+breadth);

System.out.println("Perimeter of rectangle: "+p);

}

}

class Main

{

public static void main(String[] args)

{

perimeter obj;

circle obj1=new circle();

rectangle obj2=new rectangle();

System.out.println("\n\tCalculating Perimeter of Circle....\n");

obj=obj1;

obj.calculate();

System.out.println("\n\t Calculating Perimeter of Rectangle....\n");

obj=obj2;

obj.calculate();

}

}

Step 3: The output for this program will be as follows –

Ex. 2.18.3 Write an interface called shape with methods draw() and getArea(). Further design two classes called Circle and Rectangle that implements Shape to compute area of respective shapes. Use appropriate getter and setter methods. Write a Java program for the same.

Sol. :

interface Shape

{

void draw();

double getArea();

}

class Circle implements Shape

{

private double radius;

public void setRadius(double r)

{

radius=r;

}

public double getRadius()

{

return radius;

}

public double getArea()

{

return (3.14* radius*radius);

}

public void draw()

{

System.out.println("Area of Circle is :"+getArea());

}

}

class Rectangle implements Shape

{

private double length, breadth;

public void setLength(double 1)

{

length=1;

}

public double getLength()

{

return length;

}

public void setBreadth (double b)

{

breadth=b;

}

public double getBreadth()

{

return breadth;

}

public double getArea()

{

return (length*breadth);

}

public void draw()

{

System.out.println("Area of Rectangle is :"+getArea());

}

}

class Test

{

public static void main(String[] args)

{

Circle c=new Circle();

c.setRadius(10);

System.out.println("The Radius of Circle is "+c.getRadius());

c.draw();

Rectangle r=new Rectangle();

r.setLength(10);

r.setBreadth(20);

System.out.println("Length= "+r.getLength()+", Breadth = "+r.getBreadth());

 r.draw();

}

Output

The Radius of Circle is 10.0

Area of Circle is :314.0

Length = 10.0, Breadth = 20.0

Area of Rectangle is :200.0

Nested Interface

Nested interface is an interface which is declared within another interface. For example -

interface MyInterface1 {

interface MyInterface2 {

void show();

}

}

class NestedInterfaceDemo implements MyInterface1.MyInterface2 {

public void show() {

System.out.println("Inside Nested interface method");

}

public static void main(String args[]) {

MyInterface1.MyInterface2 obj = new NestedInterfaceDemo();

obj.show();

}

}

Output

Inside Nested interface method

Ex. 2.18.4 The transport interface declares a deliver() method. The abstract class Animal is the super class of Tiger, Camel, Deer and Donkey classes. The transport interface is implemented by the Camel and Donkey classes. Write a test program to initialize an array of four animal objects. If the object implements the transport interface, the deliver() method is invoked.

Sol. :

interface Transport

{

public void deliver();

}

abstract class Animal

{

String item;

abstract void deliver();

}

class Tiger extends Animal

{

public void deliver(){

}

}

class Camel extends Animal implements Transport

{

Camel(String s)

{

item=s;

}

public void deliver(){

System.out.println("\tCamels are used in deserts to carry "+item+".");

}

}

class Deer extends Animal

{

public void deliver(){

}

}

class Donkey extends Animal implements Transport

{

Donkey(String s) {

item=s;

}

public void deliver(){

System.out.println("\tDonkeys can carry "+item+".");

}

}

class testclass

{

public static void main(String args[]) throws NullPointerException

{

Transport[] t=new Transport[4];

t[1]=new Camel("goods and people");

t[2]=new Donkey("heavy load");

System.out.println("Transport interface is implemented by following animals...");

t[1].deliver();

t[2].deliver();

}

}

Output

Transport interface is implemented by following animal...

Camels are used in deserts to carry goods and people.

Donkeys can carry heavy load.

Review Question

1. Explain simple interfaces and nested interfaces with examples.