Test double

In computer programming and computer science, especially in object-oriented programming, programmers and developers employ a technique called automated unit testing to enhance the quality of the software. Frequently, the final release software consists of a complex set of objects or procedures interacting together to create the final result. In automated unit testing, it may be necessary to use objects or procedures that look and behave like their release-intended counterparts, but are actually simplified versions that reduce the complexity and facilitate testing. A test double is a generic (meta) term used for these objects or procedures.

Gerard Meszaros identified several different terms for what he calls, "Test Doubles." Using his vocabulary, there are at least five types of Test Doubles:

Another form of test double is the Verified Fake, a Fake object whose behavior has been verified to match that of the real object using a set of tests that run against both the Verified Fake and the real implementation.

While there is no open standard for test double and the various types, there is momentum for continued use of these terms in this manner. Martin Fowler used these terms in his article, Mocks Aren't Stubs referring to Meszaros' book. Microsoft also used the same terms and definitions in an article titled, Exploring The Continuum Of Test Doubles.

Gerard Meszaros:

Martin Fowler:

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