> Who says that you have to be able to mock C? Not every class should be mockable. In case described by OP, why would we want to mock cache anyways?
Well it isn't really a unit test if you don't, is it? If you don't mock it then you're not only testing classes under unit tests, but also C's methods. And you're doing that not just in one place but in several different places in your testbase. The same methods. Over and over again.
C.getX() is going to be called by many different methods in many classes. This means that each time you unit test any of these methods, you're also testing C.getX() when you should only be testing what the method does.
+ sign and - sign are also called by unit tests many times, so you also repeatedly testing arithmetic operations "each time you unit test any of these methods, ... when you should only be testing what the method does".
Edit: I see what you say, but I think that effort we spend on decomposing app into perfectly isolated unit tests is far greater then the effort of identifying and troubleshooting where the less perfect test failed.
In my case a cache could be something remote, having any calls directly to that will slow down unit tests a lot, having one or two calls to it unit testing the cache implementation it self is OK, but having every method which makes use of it make those calls isn't going to work.
In reply to your original question, its not extremely bad if you can't unit test that class alone , but in this case you wouldn't be able to unit test anything which uses it either.
Well it isn't really a unit test if you don't, is it? If you don't mock it then you're not only testing classes under unit tests, but also C's methods. And you're doing that not just in one place but in several different places in your testbase. The same methods. Over and over again.