Why casting to unrelated interface produce ClassCastException instead of compile time exception?

Question

Why in this snippet while Tree interface is not related to Bug class it does not produce compile time exception?

interface Tree {}
class Bug {
    public static void main(String[] args) {
        Bug bug = new Bug();
        Tree tree = (Tree) bug;
    }
}

Answer 1

Mark Bug as final and you will get your compiler error as expected.

This is because Bug could have a subclass that does implement Tree . If bug actually stores a reference to an instance of that subclass, then the cast will succeed. Since there is a chance that the cast can succeed, the compiler doesn't stop you from casting.

In most cases, you can cast from any non-final class to any interface. According to the JLS §5.5.1 , when you try to cast a variable of reference type S to interface T :

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.

Otherwise, the cast is always legal at compile time ( because even if S does not implement T, a subclass of S might ).