Design For Testing, But Don't Code For Testing
You use private methods to reduce code. Less code is always more stable. Extracting a private method to another class so it can be tested makes more code, and makes something that was in the domain of the class now an external thing.
If a class is used only by one other class. That the two classes are partial representations of a whole class, then you have a problem. While your unit tests are going to pass, they aren't going to pass for long. They are going to break. It's like working on fixing a table thinking it stands on 3 legs when infact there are 4. The 4'th leg is just not within view when you're working on it.
Design for testing means you've created a collection of classes, interfaces and other things that will remain stable as your modify it. There shouldn't be a huge ripple effect when you break something (there can be a ripple cause by dependency, but failure should be horizontal. Not vertical).
A user of a class wants reliabilityfirst. Your unit tests should show that the class is stable under all possible states. You could make every method public and get 100% code coverage. Does this really make it more reliable? No it doesn't.
Think first, what unit tests I need to have to verify this stuff actually works as intended.
Having a private method
double_price() doesn't mean it needs to be tested. It's likely called a few dozen times from different parts of the class. It doubles the price, and that happens in lots of places so it's
Does this method risk stability? If it does then ask yourself "Does the risk justify a different set of unit tests?" If so, then encapsulate and write unit tests for that.
Think Of It Like This.
To build a system you want to use small pieces that you know work. If you take pieces
B that you know pass unit tests, then creating
C more stable.
Don't Think Like This
I have a system called
A. Is it stable? I don't know. Let's open
A and rip out
C. Now I can test
Why is this bad? Because this can happen.
C pass unit tests but
B fails. How can
C be stable? Because you didn't rip
A apart correctly.
A is actually unstable because part of it is pretending to be
oh man, I think I just gave myself a headache.