Are empty abstract classes a bad practice, and why?

Question

We have several empty abstract class in our codebase. I find that ugly. But besides this very stupid reason (ugliness), should I refactor it (into empty interface e.g.) ?

Answer 1

Sounds to me like this is the result of creating an object heirarchy that ended up not having any common functionality at it's top most levels. I suspect that the immediate subclasses are abstract themselves or at least have subclasses of their own. The other likelyhood is that your code has a lot of instanceof functions scattered throughout it.

The empty topmost level isn't a huge deal in and of itself but I would check to verify that no common functioanlity actually exists. Assuming it does exist I would look at pulling the common features up in the parent classes. I would definately keep a look out for instanceof and think seriously about refactoring it (refer Refactoring to Patterns (Kerievsky) for examples).