Sometimes when writing tests or defining a testing strategy, we need to replace part of a system at run time with a modified version, in order to set up, simplify, isolate or verify a particular behaviour.

In his book xUnit Test Patterns: Refactoring Test Code, Gerard Meszaros came up with the term test doubles to describe the various types of objects or procedures used to facilitate and support testing:

• Dummy objects are passed around but never used. They are commonly used to fill parameter lists, fill in required properties in other objects, etc.
• Fake objects have working implementations, but are not suitable for production, given they might have some implementation shortcuts. In-memory database is a common example.
• Stubs provide predictible responses to the calls made during the test, usually programmed to only respond to calls that happen in the context of the test.
• Spies are stubs that record some information based on how they were called. They are generally used when we need to verify if/how many times a function call has been made.
• Mocks are used to verify behaviour by throwing exceptions or failing when they receive calls that differ from their expectations.

More on this topic:

• Mocks Aren’t Stubs
• Test double