Java casting in interfaces

Question

Can someone please explain to me how the compiler does not complain in the first casting, but does complain in the second?

interface I1 { }
interface I2 { }         


        

Answer 1

According to JLS 5.5.1 - Reference Type casting, the rule(s) apply:

• If T is a class type, then either |S| <: |T|, or |T| <: |S|. Otherwise, a compile-time error occurs.

  I2 y = (I2)o3; //compiler complains here !!

In this case, an Integer and I2 are unrelated in any way, so a compile-time error occurs. Also, because Integer is final, there is no relation between Integer and I2.

I2 and I1 can be related due to both being a marker interface (there are no contract).

As for the compiled code, the rule follows:

• If S is not a final class (§8.1.1), then, if there exists a supertype X of T, and a supertype Y of S, such that both X and Y are provably distinct parameterized types, and that the erasures of X and Y are the same, a compile-time error occurs.

S is o1 and T is I2.

Hope this helps.