The more these tests are tied to the specifics of the implementation, the worse they are at performing that role. The problem is that when you need to refactor the implementation of some aspect of your system, you want the test to protect you. If the tests are too tightly linked to the specifics of the implementation, you will find that making a small change in implementation results in a bigger change in the tests that surround it. Instead of defending the behavior of the system, and so facilitating necessary change, tests that are too tightly coupled to the specifics of the implementation
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