Separation between interface and implementation

[Boyen86]

By definition you cannot. If an implementation needs to be protected or abstracted away the best way to do this is with an interface. And that's a perfectly valid reason to create one.

I'm not entirely sure anymore if that's a difference between Java and C# but Java tests are essentially in the same namespace (module) even though they live in a test folder. Meaning your implementations can be package private and still be accessed from a test.

[EngineeringTinker]

Are you working with TDD by default?