This is true for totally isolated unit tests but false for tests that require stubbing. You can theoretically use the same tests for a map implemented with one

This is true for totally isolated unit tests but false for tests that require stubbing. You can theoretically use the same tests for a map implemented with one version of a hash table or the other. But once you start testing classes with dependencies, your tests inevitably touch implementation details. For example, unit testing a hash map that is backed by a memcached server would require some form of stubbing. The metatesting reason for this confusion is that in the example you used, the scope of unit testing is the same as the scope of integration testing. If a class has 0 dependencies, a unit test is also an integration test, because it tests all the dependencies of the class, of which there are none. So going back to your original point, your affirmation is true for full integration tests, i.e. tests that do not stub any dependency. If you do not stub your network connection to the memcached server, the same test - albeit with a different setup - can be used for a local hash table implementation.