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

The obvious questions are, Why was it put there? and How is it used?

My guess is that, either (a) This is the vestige of some false start. An early draft had some data or functions in the abstract class, but as the programmer worked these were all moved elsewhere, until there was nothing left but an empty shell. But more likely (b) At some point there is code that says "if (x instanceof ...". I think this is bad design, basically using a class membership as a flag. If you need a flag, create a flag, don't pretend you're being "more object-oriented" by creating a class or an interface and then using it as a flag. That may fit some textbook definition of better coding but in reality just makes code more confusing.

+1 to Monachus for pointing out that an empty interface is no better than an empty abstract class. Yes, yes, I know Sun did that themselves with Cloneable, but I think that was a lame solution to the problem.