How to unit test abstract classes: extend with stubs?

Question

I was wondering how to unit test abstract classes, and classes that extend abstract classes.

Should I test the abstract class by extending it, stubbing out the abstr

Answer 1

What I do for abstract classes and interfaces is the following: I write a test, that uses the object as it is concrete. But the variable of type X (X is the abstract class) is not set in the test. This test-class is not added to the test-suite, but subclasses of it, that have a setup-method that set the variable to a concrete implementation of X. That way I don't duplicate the test-code. The subclasses of the not used test can add more test-methods if needed.