test double troubles

Notes from Uncle Bob

Dummy

• Returns null

Stub

• Returns stubbed values

Spy

• Spies on the module being tested
• Records what the module was doing
• The more you spy, the tighter the implementation coupling

Mock

• Mocks test behavior
• Not interested in return values of functions
• Interested in what functions were called, with what arguments, when, and how often
• Mock spies on behavior of the module being tested
• Mocks are spies
• Moves the expectation into the mock results in coupling
• Mock libraries are easy and fast to use that’s why it’s used a lot

Fake

• Has real business behavior
• Fake out the data
• Fakes are extremely complicated
• Sometimes need tests of their own
• Uncle bob doesn’t use fakes

“Mock is a kind of spy, a spy is a kind of stub, and a stub is a kind of dummy. But a fake isn’t a kind of any of them. It’s a completely different kind of test double.”

Uncle Bob almost always uses stubs and spies since they are very easy to write!