Writing/implementing an API: testability vs information hiding

Question

Many times I am involved in the design/implementation of APIs I am facing this dilemma.

I am a very strong supporter of information hiding and try to use various techniq

Answer 1

SomethingThatExpectsMyInterface can be tested outside Foo, right? You can call its submit() method with your own test class that implements MyInterface. So that unit is taken care of. Now you are testing Foo.someMethod() with that well-tested unit and your untested inner class. That's not ideal - but it's not too bad. As you test-drive someMethod(), you are implicitly test-driving the inner class. I know that's not pure TDD, by some strict standards, but I would consider it sufficient. You're not writing a line of the inner class except to satisfy a failing test; that there's a single level of indirection between your test and the tested code doesn't constitute a big problem.