Polymorphism

[Balaji Bang]

Hi
I am getting very much confused in solving polymorphism questions as shown below...This is from Devaka's Practice Exam1 Q.No:29
Is there any easy way to solve this type of questions??? I am able to answer some questions related to this but not as complicated as this....

  interface Min{}
  class A112 {}
 
  class B112 extends A112 implements Min{}

  class C112 extends B112{}

  public class PolyEx{
      public static void main(String args[]){
           A112 a=new A112();
           B112 b=new B112();
           C112 c=new C112();
           Min m=null;
           b=(C112)m;
           m=(Min)b;
           A112 a2=(A112)m;
           B112 b2=(B112)a2;
           Min m2=(C112)b2;
           C112 c2=(C112)m2;
      }
 }

[Ankit Garg]

Banu I think this is a simple one. There is no method call to confuse you. Just make a hierarchy in your mind. The hierarchy of this question will be like this

Min     A112
  \      /
   \    /
    \  /
     \/
    B112
     |
    C112

[Punit Singh]

This is the trick that depends on interface.

Compiler says you can cast any class to any interface, it's not my work to deal with interface if programmer using explicit casting, JVM will see this matter.

Why?
As any class' subclasses can implement any interface or any number of interface except final class, as final class cannot be subclassed.

So compiler says I can deal with final class as final class can not produce childs, so I am sure final class reference variable will keep only final class Objects.

For non-final class compiler says I can not deal with them as non-final class reference variable could contain any object if object is of its subclasses.

And for null you can remember one thing, null is assignable to all of the class reference as it always come in last in the hierarchy tree of java. And Object comes always top in the hierarchy.

Any doubt than ask which line is confusing you ?

[Balaji Bang]

Compiler says you can cast any class to any interface, it's not my work to deal with interface if programmer using explicit casting, JVM will see this matter.

Thanks Punith ,I dont know this point.....

Now I understood but one question ..

 Min m=null;
b=(C112)m;

when the second line runs ..... b is of type C112 and it contains null . Am I Right??? can we say that : C112 b=null;

[Punit Singh]

No it is of type B112 that is declared there, but C112 is its child type so compiler is ok with it.

Java considers null in lowest in its hierarchy.

They way you can assign any type of object to class Object reference like:

Object o=new String();

Same way you can assign null to any type of reference variables.

String s=null;

ABC a=null;;

WhatEver ref=null;

[EDIT : did not understand question exactly, so typed wrong answer.]

[Sagar Rohankar]

Banu Chowdary wrote:
Now I understood but one question ..

 Min m=null;
b=(C112)m;

when the second line runs ..... b is of type C112 and it contains null . Am I Right??? can we say that : C112 b=null;

No, b is not the type of C112, Its a type of B112. Remember, when you write something like this,

MyClass myClass;

myClass is a reference (type) of the class which declares it, i.e MyClass.

[Balaji Bang]

Then what is the meaning of C112 here which is there in the explicit casting???

If this casting is not converting the reference type from B112 to C112 then next time if we are assigning the b to anything means we are assigning reference type B112 only ...Right??? hmmmmmmmmmmmmmmmmmmmm

[Punit Singh]

You can think it as anonymous reference of C112.

  Min m=null;  
 b=(C112)m;    

it is like:

Min m=null;
C112 temp=null;
b=temp;

[Balaji Bang]

Ok I understood ......... Thank you Punith