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Mocks, stubs and fakes

April 25, 2006 by Artem

I am rather new to the Test-Driven-Development (TDD) and continuous testing in general. Therefore I quite often experience the terminology difficulties: how to call fully functional alternative object, how to call almost empty stub objects, etc. It looks like I am not alone.

Martin Fowler highlights the Gerard Meszaros's proposal:

The generic term he uses is a Test Double (think stunt double). Test Double is a generic term for any case where you replace a production object for testing purposes. There are various kinds of double that Gerard lists:

  • Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
  • Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an InMemoryDatabase is a good example).
  • Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
  • Mocks are pre-programmed with expectations which form a specification of the calls they are expected to receive. They can throw an exception if they receive a call they don't expect and are checked during verification to ensure they got all the calls they were expecting.

Nice dictionary. I am going to try using it for some time. How do you like these term definitions? Worth trying? Worth advertising to colleagues?


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