cast null to interface and class

[Wei Lien Chun]

interface I1{}
class A{}
class B{}
class Null
{
    public static void main(String[]args)
    {
        A a1 = null;
        I1 i1 = (I1)a1;     //1: OK
        A a2 = null;
        I1 i2 = a2;         //2:incompatible types
        A a3 = null;
        B b1 = (B)a3;       //3:incompatible types
        A a4 = null;       
        B b2 = a3;        //4: incompatible types 
    }
}

Why only line1 compile ok? Is interface different from class when handling casting? how?
I was told that null can be cast to any classes, but it seems wrong.
[ April 12, 2004: Message edited by: Wei Lien Chun ]
Edited by Corey McGlone: Added CODE Tags
[ April 12, 2004: Message edited by: Corey McGlone ]

[Corey McGlone]

Actually, the fact that these variables refer to null doesn't make any difference whatsoever. Let's go through the various lines and determine why you're getting errors:

I1 i1 = (I1)a1;

This line compiles fine. The reason for that is that, even though A does not implement I1, some subclass of A might, which might make this a legal cast. Therefore, this line will compile correctly. Check out my blog entry for more details on this case.

I1 i2 = a2;

This case is not allowed because a2, which is of type A does not implement I1. Therefore, the compiler knows that this assignment makes no sense and flags it with an error.

B b1 = (B)a3;

This is also illegal because a3, which is of type A, can't possibly refer to an object of type B as A and B are not related. You can also look at my blog entry for more information, if you'd like.

B b2 = a3;

Again, this doesn't work because A and B are not related. This is essentially the same case as the one above.
I hope that helps,
Corey

[Wei Lien Chun]

Wow!!Corey
Thanks a lot to all of you!