Why is it so bad to mock classes?

Question

I recently discussed with a colleague about mocking. He said that mocking classes is very bad and should not be done, only in few cases.

He says that only interfaces

Answer 1

It depends how often you use (or are forced by bad design) mocks.

If instantiating the object becomes too hard (and it happens more than often), then it is a sign the code may need some serious refactoring or change in design (builder? factory?).

When you mock everything you end up with tests that know everything about your implementation (white box testing). Your tests no longer document how to use the system - they are basically a mirror of its implementation.

And then comes potential code refactoring.. From my experience it's one of the biggest issues related to overmocking. It becomes painful and takes time, lots of it. Some developers become fearful of refactoring their code knowing how long will it take. There is also question of purpose - if everything is mocked, are we really testing the production code?

Mocks of course tend to violate DRY principle by duplicating code in two places: once in the production code and once in the tests. Therefore, as I mentioned before, any change to code has to be made in two places (if tests aren't written well, it can be in more than that..).