If you’ve barely passed the title and you’re already seething with violent rage: This post is for you. Like medicine that smells of battery acid, this’ll taste bad and potentially kill you but you’ll feel better for it in a few days. Let it settle in for a bit, mull it over.
In a language with real privates, such as C++, you can not physically test your private methods. Either you make them so simple that they do not need testing, make them public or extract them into another class.
These approaches are not always valid or possible in the Real WorldTM, so the methods in question can sometimes go untested (may the lords of TDD have mercy upon you).
Not testing a private because it doesn’t make sense to expose it in other ways is ridiculous. Especially in a language where you can actually access it programmatically.
Testing every method
I’m a firm believer in TDD and I personally think writing even one test for your method before writing the implementation helps you to think from a consumer’s perspective. This consumer usually ends up being you.
You may write a test for a boolean returning private that a public method depends on. Half way through writing it you could realise it’s bat guano insane and there are far better solutions. TDD ends up being JIT planning (I’m coining that).
So testing every method allows you to: Get far more coverage (even for trivial methods), imagine how your function will be used before you write it and force you to only write testable functions. What happens when your untestable private gets made public and needs tests? A mess, that’s what.
Tests are friends
Literally. You should treat your test file for a class as if it were a friend class. Friend classes have access to the other class’ protected and private values. You can still keep the idea of privates when one class is talking to another, but the test file is a best bro 4 lyfe. It can check out its class’ privates any time it wants to.
This way, you can still change private methods on a whim without breaking other classes, you just need to update your tests first. This is how it should be.
Potentially: 100% test coverage. Most likely: Easily testable and well thought out methods. This leads to a generally cleaner codebase that doesn’t have a plethora of private methods that were taped onto the side and not tested “because private”.