Why can't my class implement an interface declared inside it?

Question

I just encountered a behavior I first thought it was a bug in Eclipse. Consider this simple class:

public class Foo {
    public static interface Callback {
        public void onAction();
    }
}

This is perfectly valid. However, this isn't:

public class Foo implements Callback {
    public static interface Callback {
        public void onAction();
    }

    public void onAction() { /*some implementation*/ }
}

But this is valid, too:

public class Foo {
    public static interface Callback {
        public void onAction();
    }

    private final Callback mCallback = new Callback() {
        public void onAction() { /*some implementation*/ }
    };
}

Why does Java force me to kind of 'waste' a member for it, if it could simply save it by letting me implement this itself? I'm well aware of the 'workaround' to put this interface in its own file, but out of curiosity: Is there a reason why this won't work?

Answer 1

Using this code

public class Foo implements Callback{

public static interface Callback{
    public void onAction()
}

public void onAction(){//some implementation}
}

what is Callback ? The compiler (btw which is different from Eclipse, ) doesn't know what is Callback.

You defined the Callback interface after you used it.

Answer 2

In your 2nd case, since the signatures are checked before the bodies of the classes, when the compiler tries to compile the Foo class, the Callback interface isn't defined yet.