interface question

[Vad Fogel]

Hi all, here's a question. Who can explain what's going on there?

interface I{
    class A{    //1
        static void doStuff(){  //2
            System.out.println("Doing stuff...");
        }
    }
}
class Test implements I{
    public static void main (String[] args) {
        A a=new Test().new B(); //3
        a.doStuff();
    }
    
    class B extends A{} //4
}
/** What is the output of trying to compile and run the following?
 *  A. Compiles and runs with output of "Doing stuff..."
 *  B. Compiles but throws a runtime error.
 *  C. Doesn't compile because of the error at line 1.
 *  D. Doesn't compile because of the error at line 2.
 *  E. Doesn't compile because of the error at line 3.
 *  F. Doesn't compile because of the error at line 4.
 *  G. Doesn't compile because of the errors at lines 3 and 4.
 */

[Barkat Mardhani]

Hi there,
As you might have already run the program. The answer is A.
Here is how I understand it:
1. class A is nested in interface.
2. it is treated like any interface var i.e. static and final
3. Test implements I
4. so Test inherits class A as an static nested class.
5. inner class B is extending A but not changing any functionality
6. B inherits doStuff() from A.
7. Object type is B, so the inherited version of doStuff() is called.
Only question in my mind is that if class A is treated as a static and final member from interface, why was it allowed to be extended?
[ October 02, 2003: Message edited by: Barkat Mardhani ]

[Vad Fogel]

This is exactly the point I have a question about. If an interface member variable is implicitly final, then it must remain final in the class implementing that interface. class A is apparently not final because it can be subclassed. The only solution I can offer is that class members of an interface are static, public, but not final, so interface classes kind of combine declarative features of both method and variable members. :roll:

[Steve Lovelace]

I believe that you are not declaring a variable, you are declaring a type. It's implicitly static, but not final. Look at JLS 9.5.

[Vad Fogel]

Thanks Steve, I believe it's true as per JLS.

[Asha Sat]

Interfaces may contain member type declarations .A member type declaration in an interface is implicitly static and public. A member method is abstract ,public. Every field declaration in the body of an interface is implicitly public, static, and final
member type declarations could be A member class or a member interface