Test Doubles Taxonomy¶

Dependencies can be replaced with stand-ins that vary in behavioral fidelity: from completely inert (dummy) to partially functional (stub, spy, mock) to fully functional but simplified (fake) - each trade-off serves different verification needs.

Tension Addressed¶

Isolation vs. Integration

Recognizable As¶

Objects named *Mock, *Stub, *Fake; constructor injection of interfaces; "test mode" configurations; in-memory implementations of storage interfaces; recorded interaction assertions

Canonical Sources¶

Meszaros, "xUnit Test Patterns" (2007); Fowler, "Mocks Aren't Stubs" (2007)

Transfers To¶

Simulation in engineering (model fidelity trade-offs); training environments (simplified versions of production systems); prototyping (stand-ins for unbuilt components)

Sub-concepts: - Dummy: Passed to satisfy signatures, never actually used - Stub: Provides canned responses to specific queries - Spy: Records interactions for later verification - Mock: Pre-programmed with expectations, verifies interactions occurred - Fake: Working implementation unsuitable for production (in-memory database)