Endo-Testing: Unit Testing with Mock Objects

Unit testing is a fundamental practice in Extreme Programming, but most non-trivial code is difficult to test in isolation. It is hard to avoid writing test suites that are complex, incomplete, and difficult to maintain and interpret. Using Mock Objects for unit testing improves both domain code and test suites. They allow unit tests to be written for everything, simplify test structure, and avoid polluting domain code with testing infrastructure.