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Test double
A test double is a stand-in object used in a test to replace a real dependency, such as a database or external service, so the test stays fast and isolated.
Prerequisite: Unit test
What matters
Using a test double lets a unit test avoid depending on slow or unreliable external systems, but relying on one also means the test isn't checking the real dependency's actual behavior.
Example
A test might use a fake payment service that always reports success, rather than calling a real payment provider during every test run.
Common misconception
A test relying entirely on test doubles does not prove the real integration actually works; that gap is why integration tests against real dependencies still matter.
Recall question
Why doesn't a test using only test doubles prove the real integration works?
Keywords and related concepts
test doublemockfake dependencyunit test