I agree. Unit Test is for testing API usage, not implementation. We can use it to figure out the API design good or not. But the problem, is API change is very

I agree. Unit Test is for testing API usage, not implementation. We can use it to figure out the API design good or not. But the problem, is API change is very often, and we don't want to change both implementation and test just for the sake of API change. Integration test is enough in most case. Even API is the implementation details of an abstraction.