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

I don't see how information hiding, in the abstract, is reducing your testability.

If you were injecting the SomethingThatExpectsMyInterface used in this method rather than constructing it directly:

public void someMethod() {
   // calculate x, y and z
   SomethingThatExpectsMyInterface something = ...;
   something.submit(new InnerFoo(x, y, z));
}

Then in a unit test you could inject this class with a mock version of SomethingThatExpectsMyInterface and easily assert what happens when you call someMethod() with different inputs - that the mockSomething receives arguments of certain values.

I think you may have over-simplified this example anyway as InnerFoo cannot be a private class if SomethingThatExpectsMyInterface receives arguments of its type.

"Information Hiding" doesn't necessarily mean that the objects you pass between your classes need to be a secret - just that you aren't requiring external code using this class to be aware of the details of InnerFoo or the other details of how this class communicates with others.