Why does interfaces allow class definitions within them

Question

Can anybody explain why interfaces can contain class definitions.

Answer 1

Saish wrote:
It does somewhat adulterate the idea that interfaces and implementations should be separate. I would find it more useful if the class that you declare could be something other than public. That way, you could expose an interface, a factory (which would be public) and then have all the implementations private in the same file. A client would only see the factory and the interface (granted, one could achieve the same effect with non public classes in their own source files all in the same package). But to require that member types within the interface be public, that really makes little sense to me.agreed - this limitation is quite annoying. Forces me to use extra level of nesting to work around it:
// see also: http://forums.sun.com/thread.jspa?messageID=10938789#10938789
public interface MyInfo {
  public MyStatus getMyStatus();
  public static interface MyStatus { } // MyStatus
  public static class Factory {
    public static MyStatus newMyInfo() {
      return new MyInfo() {
        public MyStatus getMyStatus() {
          return new Innards.MyStatusImpl();
  } // Factory
  // IMPL NOTE purpose of this class is to keep implementation details inside
  public final /* IMPL NOTE subclassing prohibited */ static class Innards {
    private Innards() {}  /* IMPL NOTE instantiation prohibited */
    private static class MyStatusImpl implements MyStatus { } // MyStatusImpl
  } // Innards
} // MyInfo