I very often hear the following: "If you want to test private methods, you'd better put that in another class and expose it."
While sometimes that's the case and we have a hiding concept inside our class, other times you end up with classes that have the same attributes (or, worst, every attribute of one class become a argument on a method in the other class) and exposes functionality that is, in fact, implementation detail.
Specially on TDD, when you refactor a class with public methods out of a previous tested class, that class is now part of your interface, but has no tests to it (since you refactored it, and is a implementation detail).
Now, I may be not finding an obvious better answer, but if my answer is the "correct", that means that sometimes writting unit tests can break encapsulation, and divide the same responsibility into different classes.
A simple example would be testing a setter method when a getter is not actually needed for anything in the real code.
Please when aswering don't provide simple answers to specific cases I may have written. Rather, try to explain more of the generic case and theoretical approach. And this is neither language specific.
Thanks in advance.
EDIT: The answer given by Matthew Flynn was really insightful, but didn't quite answer the question. Altough he made the fair point that you either don't test private methods or extract them because they really are other concern and responsibility (or at least that was what I could understand from his answer), I think there are situations where unit testing private methods is useful. My primary example is when you have a class that has one responsibility but the output (or input) that it gives (takes) is just to complex. For example, a hashing function. There's no good way to break a hashing function apart and mantain cohesion and encapsulation. However, testing a hashing function can be really tough, since you would need to calculate by hand (you can't use code calculation to test code calculation!) the hashing, and test multiple cases where the hash changes. In that way (and this may be a question worth of its own topic) I think private method testing is the best way to handle it. Now, I'm not sure if I should ask another question, or ask it here, but are there any better way to test such complex output (input)?
OBS: Please, if you think I should ask another question on that topic, leave a comment. :)
EDIT2: I accepted a answer as my correct because it made me think and decide my course of action, altough it didn't answered my queston completely. But for those who face the same problem as I do (one cohesive class that will change together, but is still too hard to test by itsfelf), I'll tell what I did and why. I decided that the output of that class is simply too hard to a computer to test correctly, so I didn't test it. I could have used a framework to test it's private methods (which would be the best idea, I think), but I didn't want to get to that point. If you are wondering what it is that is cohesive and respects SRP, and still is too hard to a computer to test, I'll give some examples: heightmap generation, hashing functions, procedural music generation (you can test some units, but the highest level unit is simply too subjective).