A test double replaces a real dependency. A stub supplies predetermined answers, a fake provides a lightweight working implementation, and a mock is commonly used to verify interactions.

Over-mocking makes tests brittle when they verify implementation details.

UserRepository repository = new InMemoryUserRepository();
UserService service = new UserService(repository);

assertTrue(service.exists("u-1"));

Prefer real lightweight implementations where practical. Mock external systems when setup is expensive or unreliable.