Fill up the cache with mocks of the objects it needs to contain. 10 objects = 10 lines of code.
If you really want to have different cache implementation for testing purpose, use C as just an instance storage, and return reference not to self but to cache interface. Put a switch inside static C.getX() that returns IX. Inside of switch statement see if System.getProperty("isTest")=true and return XSimple (which implements IX), otherwise return XReal (which also implements IX). Few lines of code, transparent and debuggable.
"Fill up the cache with mocks of the objects it needs to contain. 10 objects = 10 lines of code."
But then I'm not testing the code that uses the cache, I'm testing the cache plus the code that uses the cache. Which is a valuable test but a separate one.
Regarding your proposed implementation, that sounds like basically what I proposed here:
Funny, I saw your code, but didn't put your name and code together:) There are many ways to mock a singleton. I am not against mocking it, but against over-complicating. Also, I think that we are taking unit testing a bit too far in trying to decompose the app into smallest pieces. Unit tests have specific goals, like verifying correctness of calculations or performance benchmarks. If test raises a red flag, it takes a few minutes to isolate the piece of code that is at fault.
