Unit testing several implementation of the same trait/interface

Question

I program mostly in scala and java, using scalatest in scala and junit for unit testing. I would like to apply the very same tests to several implementations of the same interface/trait. The idea is to verify that the interface contract is enforced and to check Liskov substitution principle.

For instance, when testing implementations of lists, tests could include:

• An instance should be empty, if and only if and only if it has zero size.
• After calling clear, the size should be zero.
• Adding an element in the middle of a list, will increment by one the index of rhs elements.
• etc.

What are the best practices ?

Answer 1

In Java/JUnit, I generally handle this by having an abstract testcase from which tests for the specific test class inherit all the tests and have a setup method instantiating the implementation. I can't watch the video abyx posted right now, but I suspect it's this general idea.

Another interesting possibility if you don't mind introducing yet another testing framework would be to use JDave Specification classes.

I haven't tried using either of these with Scalatest or with Scala traits and implementations, but it should be possible to do something similar.