Understand Polymorphism to solve A a =new B() problem

Hi friends today I come across that many people stumble on a common question asked in JAVA/Android interview many time.

Problem states that if there is class as A and B and C. B extends A and in class C we have something like

A a =new B();

          B b= new A();

Ok So question is whether above statements are valid or not.

To validate it remember Polymorphism in Java - like biology term species (object) can have many forms - stages.

Means SuperClasses can be initialized with SubClasses. but reverse is not true.

If class B is extending class A, so here class A is superclass and class B is subclass.

Now by this statement

          // is valid 

A a= new B();

But reverse is not true.

B b= new A(); // will not compile and gives error - incompatible types

now call method on

  a.someMethod();

if both class A and B have same method like someMethod(); which method will be called.

No doubt class B's method will be called as B's instance is assigned to a.

A a =new B();

       a.someMethod();  ///calls class B 's method.


Now complete picture is here.

***************************************************************************

import java.util.*;

class Test{

      public static void main(String args[]){

             Test test=new Test ();

              test.run();

       }

       public void run(){

               // Valid class A's object assign to  A

              A a =new A();

             // Valid as class B's object assign to B

              B b=new B();

            // Call display method of class A

            a.display();

           // Call display method of class B

           b.display();

           // Valid as object of B class is assign to a 

           a=new B();

          // Here call display method of class B???

          a.display();

         //??Why because now A a holds object of class B

         // Now call to this method?  What will Happen

         b=new A();                

         // throws runtime exception "incompatible types -                 // class A can not be converted to  B"          

        // So this method won't work.

        b.display();

        // Then why A a =new B(); is valid.

       // By the property of polymorphism 

}

             class A{

                      public void display(){

                                System.out.println("Method of A");

                      }

              }

          class B extends A{

                   public void display(){

                               System.out.println("Method of B");

                   }

          }

}

*****************************OUTPUT**************************************

Method of A
Method of B
Method of B



 That't it copy above program to save as Test.java  modify it according your doubt and clear them.