Here are qualities of good unit tests: 1) They run fast. 2) They help us localize problems. In the industry, people often go back and forth about whether par

Here are qualities of good unit tests: 1) They run fast. 2) They help us localize problems. In the industry, people often go back and forth about whether particular tests are unit tests. Is a test really a unit test if it uses another production class? I go back to the two qualities: Does the test run fast? Can it help us localize errors quickly? Naturally, there is a continuum. Some tests are larger, and they use several classes together. In fact, they may seem to be little integration tests. By themselves, they might seem to run fast, but what happens when you run them all together? When you have a test that exercises a class along with several of its collaborators, it tends to grow. If you haven't taken the time to make a class separately instantiable in a test harness, how easy will it be when you add more code? It never gets easier. - From "Working Effectively With Legacy Code" by Michael Feathers. (A really good book that actually does a better job of teaching design than many a book because like all good teaching it starts with the examples and abstracts from there, not the other way around.)