Mocking Java collections is rarely a good idea and can lead to brittle tests, unrealistic behavior, and unnecessary coupling to implementation details.

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Mocking Java collections like List, Set, or Map with Mockito is an anti-pattern that leads to brittle tests and unrealistic behavior. Collections are deterministic data structures, not external dependencies requiring isolation. On Java 21+, mocking collections may fail due to stricter JVM instrumentation rules. Instead of mocking, use real collection instances in tests to create clearer, more maintainable tests that focus on observable behavior rather than implementation details. This approach exposes design issues and encourages better separation of concerns.

Why We Should Not Mock Collections With Mockito

5. Tests Become Over-Specified and Fragile

Yeah, we should only mock behaviour we are not guranteed is bug free. Example: we have a another service class we wrote as dependency and we want to isolate it so we know which exact class fails. However I trust Java 99,99% percent that Collections will work as expected.
One thing that is mentioned on the article but I don’t see a use case: what is the scenario your logic is so bad you need to mock collections? Can you provide an example?

Got to learn new thing today about not mocking the collections. Thanks!