casting from examlab

[Shardul Kumar]

  1.  interface Min{}
       2.
       3.  class A{}
       4. 
       5.  class B extends A implements Min{}
       6.
       7.  class C extends B{}
       8.
       9.  public class CCExp{
     10.      public static void main(String args[]){
     11.           A a=new A();
     12.           B b=new B();
     13.           C c=new C();
     14.           Min m=null;
     15.           b=(C)m;
     16.           m=(Min)b;
     17.           A a2=(A)m;
     18.           B b2=(B)a2;
     19.           Min m2=(C)b2;
     20.           C c2=(C)m2;
     21.      }
     22. }

  

can anyone xplain this casting problem from line 15 to 20.....
thanks.

[Vivek Singh]

works fine for me!!!

Use CODE TAGS!!!

[Vivek Singh]

Vivek Singh wrote:works fine for me!!! No compilation problem

Use CODE TAGS!!!

[Ankit Garg]

please Use Code Tags when you post a source code. You can edit your message using button and then add code tags to it.

15. b=(C)m;
16. m=(Min)b;
17. A a2=(A)m;
18. B b2=(B)a2;
19. Min m2=(C)b2;
20. C c2=(C)m2; 

Line 15 is just a normal downcast. m is downcasted to C and automatically upcasted to B (Since class B implements Min and C extends B, thus C IS-A Min). Line 17 is tricky. But suppose the reference m points to an object of type B, then the object pointed by m will be assignable to a reference of type A (as B IS-A A). That's why the cast is allowed. So the fact is, a cast between an interface and a non-final class is always allowed. That's because there might be a class that implements that interface and extends that class. Again line 18 is just a normal downcast. Line 19 downcasts b2 to type C and then it is automatically upcasted to Min. Line 20 is also a normal downcast...

[Shardul Kumar]

Ankit Garg wrote:please Use Code Tags when you post a source code. You can edit your message using button and then add code tags to it.

15. b=(C)m;
16. m=(Min)b;
17. A a2=(A)m;
18. B b2=(B)a2;
19. Min m2=(C)b2;
20. C c2=(C)m2; 

Line 15 is just a normal downcast. m is downcasted to C and automatically upcasted to B (Since class B implements Min and C extends B, thus C IS-A Min). Line 17 is tricky. But suppose the reference m points to an object of type B, then the object pointed by m will be assignable to a reference of type A (as B IS-A A). That's why the cast is allowed. So the fact is, a cast between an interface and a non-final class is always allowed. That's because there might be a class that implements that interface and extends that class. Again line 18 is just a normal downcast. Line 19 downcasts b2 to type C and then it is automatically upcasted to Min. Line 20 is also a normal downcast...

but m is null

[Shardul Kumar]

please let me clear it that when we do casting is it reference variable casting i mean does object type created play any role???

[Ankit Garg]

Shardul I know that m is null, but that doesn't matter at compile time. The compiler only sees the type of the reference and not the actual object it is pointing to. And null can be type casted to any type so there won't be any exception at runtime...