Imho, organizing code in a braindead way (like single-implementation-interfaces) just to be able to test it is one of the main problems of bad code and bad lang

Imho, organizing code in a braindead way (like single-implementation-interfaces) just to be able to test it is one of the main problems of bad code and bad languages. You make your code harder to understand, more convoluted and less refactorable. The far better approach is to seek out side-effects and state and organize your code like you would in functional languages like Haskell. Then testing is just adding a) testing all the stateless, functional and stateful stuff by pushing through prepared test objects and b) testing the side-effectful stuff through mocked side-effects or a test-mode (which you need anyways for integration tests). Cleaner, faster, easier to understand, easier to reason about. Only problem is that Java is relatively bad to use in a fuctional style.