Polymorphism

Polymorphism

Basic concept: Polymorphism is a mechanism which allows to have many forms of the method having the same name. That means, a method A() may take an instance of an object of one class and may execute its method or the same method A() may take another instance of an object and may execute the method belonging to another class. In the following Java program we have taken the superclass A and build a multilevel inheritance.

Using object of corresponding class corresponding method is invoked by the same function fun.

The implementation is given by following Java program -

Java Program[Polymorphism Demo.java]

class A extends Object

{

public String toString()

{

return "A";

}

}

class B extends A

{

public String toString()

{

return "B";

}

}

class C extends B

{

public String toString()

{

{

return "C";

}

}

public class PolymorphismDemo

{

public static void main(String[] args) {

fun(new C());//invokes the method toString() of class C

fun(new B());//invokes the method toString() of class B

fun(new A());//invokes the method toString() of class A

}

public static void fun(Object x) {

System.out.println(x.toString());

}

}

Output

F:\test>javac PolymorphismDemo.java

F:\test>java PolymorphismDemo

C

B

A

Program Explanation

The same function fun is used to execute the methods of various class. The selection of the method of the class (i.e. toString method) depends upon the instance of the object which is passed to the function fun. Which implementation is to be used is determined dynamically by the Java Virtual Machine. This mechanism is called dynamic binding.

Dynamic Method Dispatch

The dynamic method dispatch is also called as runtime polymorphism. During the run time polymorphism, a call to overridden method is resolved at run time. The overridden method is called using the reference variable of a super class(or base class). The determination of which method to invoke is done using the object being referred by the reference variable.

Following is a simple Java program that illustrates the Run time polymorphism. class Base

{

void display()

{

System.out.println("\n Base Method Called");

}

}

class Derived extends Base

{

void display() //overridden method

{

System.out.println("\n Derived Method Called");

}

}

public class RunPolyDemo

{

public static void main(String args[])

{

Base obj=new Derived();//obj is reference to base class // which is referred by the derived class

obj.display(); //method invocation determined at run time

}

}

Output

D:\test>javac RunPolyDemo.java

D:\test>java RunPolyDemo

Derived Method Called

Program Explanation:

In above program, the display method is an overridden method because with the same signature we are modifying it in its derived class. This method is invoked in the main(). function using the reference obj. This reference variable is of superclass type. It is assigned with the reference of Derived class. At run time it is decided that the display method of derived class should be invoked. Hence it is known as run time polymorphism.

Ex. 2.11.1: Declare a class called book, having author_name as private data member. Extend book class to have two sub classes called book_publication and paper_publication. Each of these classes have private member called title. Write a complete program to show usage of dynamic method dispatch (dynamic polymorphism) to display book or paper publications of given author. Use command line arguments for inputting data.

Sol. :

import java.util.Scanner;

class book

{

private String[] author_name = {"Puntambekar","Godse"}; Meesa & tan

void display()

{

System.out.println("Author names: ");

for(int i=0;i<author_name.length;i++)

{

System.out.println("\t"+author_name[i]);

}

}

}

class book_publication extends book

{

private String[][] title = {{"Java Programming","Compiler Design", "DAA"},{"Digital Electronics","Computer Graphics", "Microprocessor"}};

void display(int j)

{

System.out.println("Books published by Given Author are...");

for(int i=0;i<3;i++)

{

System.out.println(title[j][i]);

}

}

}

class paper_publication extends book

{

private String[][] title = {{"AAA","BBB"},{"XXX", "YYY"}};

void display(int j)

{

System.out.println("Papers published by Given Author are...");

for(int i=0;i<2;i++)

{

System.out.println(title [j][i]);

}

}

}

class MainClass

{

public static void main(String[] args)

{

Scanner s = new Scanner(System.in);

book obj =new book();

book_publication bookpub=new book_publication();

paper_publication paperpub=new paper_publication();

int author_name=0,choice;

if(args[0].equals("Puntambekar"))

author_name=0;

else

author name = 1;

System.out.println("\n 1.Books Published\n 2.Paper Published");

 System.out.println("\n Enter Your Choice: ");

choice=s.nextInt();

switch(choice)

{

case 1:

bookpub.display(author_name);

break;

case 2:

paperpub.display(author_name);

break;

default:

nevi vd bedatidug adfood")mb

System.out.println("wrong choice....");

System.exit(0);

}

}

}

Output