Is it at all acceptable to have nested public interfaces?

Question

I have several interfaces all with only a single method and they are all very similar. I know it works, but should I or should I not group them like this:

public class InterfaceGroup {
    public interface Type1 {
        public void method(int a);
    }
    public interface Type2 {
        public void method(String s);
    }
    public interface Type3 {
        public void method();
    }
}

And then reference them externally as InterfaceGroup.Type1.

Answer 1

Yes, it is sometimes helpful to have such a design. For example, it is the choice for Map.Entry in Java standard library.

The blemish on such a design, though, is that you define a type that doesn't play the role of a type, and it is actually never appropriate to implement it. If I found out that in a library I use such a type exists for no other reason than to reduce the number of source code files, I'd be at least slightly annoyed :-) On the other hand, when I'm writing code whose scope of visibility is just within the implementation, I use such tricks remorselessly.