> this interface hierarchy would be annoying to use in practice No doubt, but it happens, and sometimes because the refactoring you might prefer to do can't be

> this interface hierarchy would be annoying to use in practice No doubt, but it happens, and sometimes because the refactoring you might prefer to do can't be done or can't be done right away. For example, in Java you might have a subclass of Principal where you store additional metadata and have additional methods, then you might have code like: public static Something princOk(Principal p, Stuff stuff) throws MyPrincipalException { if (p instanceof MyPrincipal) return princOk((MyPrincipal)p, stuff); return princOk(MyPrincipal(p), stuff); } Why would you do this? Well... because many JDK and third-party interfaces take Principal object arguments that you could reuse... if only you could decorate those objects with the additional metadata that you wanted, and... with subclassing you get to. In Java you might say "use Subject", but it's not that easy, and anyways, JAAS is a terrible terrible thing that should have been ripped out when applets were ripped out.