interface question

[Vad Fogel]

Another small implementation question:

class InterTest extends Base implements I{
    public static void main(String []s){
        I base=(I)(new Base());
    }
}
class Base{
    public void doStuff(){}
}
interface I{
    void doStuff();
}
[B]
/*  What's the result of trying to compile and run the code above?
 *  A. Compiles and runs with no output.
 *  B. Doesn't compile: incompatible types found.
 *  C. Doesn't compile: class InterTest should be declared abstract.
 *  D. Compiles but throws a runtime exception.
 *  E. Doesn't compile: ambiguous method declaration detected in InterTest.
 **/[/B]

[Jose Botella]

"I base=(I)(new Base());"
This compiles ok because though Base does not implement I, it's not final; thus a subclass of it could.
But it throws a ClassCastException at runtime because the class Base does not implement I. After all, Base is not really a (kind of) I.
Notice that InterTest implements I via inheriting "doStuff()" from Base.

[Vad Fogel]

Thanks Jose. I think, this question goes slightly beyond the scope of the real exam covering more intricate rules of casting. Or do you think differently?

[Dan Gun]

wow, that's quite tricky

[Jose Botella]

That is the goog thing about JavaRanch. My colleagues answer the difficult questions here to make you find the exam easy.
[ November 04, 2003: Message edited by: Jose Botella ]

[Cathy Song]

Good question !!
[ November 04, 2003: Message edited by: Cathy Song ]

[Vinod Sinha]

This implies that if base had implemented i then the code would have compiled !

[Harwinder Bhatia]

Nice question, Vad.
More than the
I base=(I)(new Base());
part, I found it more interesting that the implementation of the doStuff method is not coming from the class which "implements" the interface but from another class which was inherited.
Btw, what is the source of this question? Did you make it up or just ran across it in some mock? If former, then we have another "Dan" in the making
Cheers
Harwinder

[Vad Fogel]

Thanks all for appreciation . Sometimes, I make up questions that seem to promote better understanding of tricky concepts, especially when discussed on the forum!