In testing, which statement accurately describes the role of mocks?

• A. A mock only provides canned responses
• B. A stub provides canned responses, while a mock verifies interactions
• C. A mock is used to verify interactions and expectations
• D. A mock has no use in unit testing

Answer: (C)

In unit testing, mocks are test doubles used to both supply responses and observe how the system under test interacts with its dependencies. They let you set expectations about which methods should be called, with what arguments, in what order, and how many times. After the test runs, you verify those expectations to confirm the unit’s behavior matches the intended interaction pattern. This focus on validating interactions and expectations is what makes mocks the best description. By contrast, a stub simply returns predefined values without asserting how it's used, and saying mocks have no role in unit testing is not accurate.