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Casting the Objects

class A
  {
      // body of class A
  }

class B extends A
   {
      // body of class B
   }


We can cast the subclass object to a super class reference variable. i.e. Super class reference variable can hold the subclass object...

A a = new B();

But Subclass reference variable can not hold the super class object. WHY?

B b = new A();                     // Compile time error

Comments

  1. Anupam21 August 2013 at 09:12

    because Class B is having the data member and methods of class A along with its own data member and methods. class A cannot initialize variables of class B(B b = new A(); ).

    ReplyDelete
    Replies
    1. Havefuninjava24 August 2013 at 21:08

      B b = new A();
      Here, we are calling the constructor of class A. So obviously it will initialize all the instance variables of class A. But the question is why we can't assign the object reference of class A to the reference variable of class B?

      Delete
  2. Unknown24 August 2013 at 01:21

    When a subclass extends a superclass, it can also have additional attributes and methods not existing in the superclass.As the variable type determines what methods you can use "on" it, a variable of a subclass type can hence expose more operations available than the superclass has implemented. That's why a variable of a subclass type can't reference an instance of a
    superclass.

    ReplyDelete
    Replies
    1. Havefuninjava24 August 2013 at 21:13

      Ya, You are right.
      We use the reference variable of a class when we want to access the members of any class. So if we assign the reference of object of class A to reference variable of class A, it will be assumed that it will be having access to all the members of class B. But we are invoking the constructor of class A only, which will not have the members of class B.

      Therefore, the subclass reference variable can't hold the super-class object reference.

      Delete
  3. Unknown1 December 2019 at 20:49

    Good Post! Thank you so much for sharing the nice post, it was so good to read and useful to improve
    Java Training in Electronic City

    ReplyDelete

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