Doing x=C.get(x) is bad because there's no way of mocking C, thereby making unit testing a component that uses C impossible (the unit test will need to use the concrete implementation of C). Using dependency injection as described in the accepted answer, and particularly separating the concerns of C by using several different interfaces allows you to not only mock C, but actually create mock classes each of which is mocking one particular logical subset of C's functionalities.
It's easy to dismiss all this as "bloated" and "enterprisey" and people that use this as "architecture astronauts" but in reality, this pattern really does help. Well, at least if you want to be able to unit test your code properly. Otherwise, you might as well just use a singleton object.
I realized recently that the reason that there is no way of mocking C is because of a weakness in the language implementation/design. In Python, it's easy to mock and unit test globals, because in the test you can just overwrite them in the test's setup, and restore them in the test's teardown.
You could argue that it's a static vs dynamic typing thing, and I don't know enough to really dispute that, but it seems to me that there's no inherent reason that a static type system can't handle using a different concrete implementation. I think you can do this with Java/C# by doing some library loading trickery, but it'd be inconvenient.
And that's the whole point of all this, isn't it? Automated testing is a convenience that saves you from doing manual testing. Unit testing is a convenience that catches test failures before commit rather than during the automated testing phase. Creating the unit test, well, is it more convenient to create 8 interfaces and completely change the way the code is structured, or is it more convenient to mock out the singleton? It depends on the circumstance, I've done both in various languages. It's good to have the option.
It's a scoping thing that has no direct relation to static/dynamic typing, though probably has plenty relation through the underlying philosophies of many lanaguages which motivate them to choose particular typing and scoping rules.
I might be speaking blasphemy here, but is the advantages of unit testing worth the increased code complexity?
In quite a few codebases I've seen, the amount of tests was entirely unrelated to how stable and maintainable the system was. Some times developers just fixed the tests, rather than the bugs. And in many cases, there were a slew of failing tests even though the system ran fine.
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? In most cases singleton is used as a global state that can be easily accessed from all parts of the app without explicitly passing the state obj. We are dismissing a useful technique (singleton) based on requirement, that is not applicable to this technique.
> 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.
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.
a) Now your singleton is also responsible for providing a mock implementation of itself (which shouldn't really be one of its concerns).
b) You didn't really do anything here: the class under test is still going to use the concrete implementation of C.
You need a way of providing a mock implementation to your class while testing. The obvious answer to how to do it is dependency injection. But if you're going down that route, you can just ditch the whole singleton thing, instantiate a class using "new" somewhere else and pass it to your class. And in your unit test, you simply pass the mock implementation. This is what DI containers like Spring do. We'd be doing the exact same thing here, only manually.
a) True, but it can probably be pushed into the parameterized Singleton class. I didn't try, since that might get more language dependent and since I wanted to keep things explicit-but-short for demonstration purposes anyway.
b) You're touching a tiny part of the implementation of the real C, yes. If you're able to push the Instance and Mock functions into Singleton, you might be touching literally no code of C other than relying on the fact that it in-fact inherits from Singleton.
I nonetheless agree dependency injection is a better solution. The biggest objection I have to this implementation is that there's nothing that tells me I should be mocking C, or catches it when I forget to mock C.
Not only can you mock it for testing but it encapsulates the system better, there is no reason for straight access to C.get with this pattern you can easily drop in a brand new system for caching and not break your original code. Some might find that a bit too much like coding for the future, but in my case that is quite a likely situation to occur especially if you ever run into scaling issues.
It's easy to dismiss all this as "bloated" and "enterprisey" and people that use this as "architecture astronauts" but in reality, this pattern really does help. Well, at least if you want to be able to unit test your code properly. Otherwise, you might as well just use a singleton object.