Why is it hard to unit test a system that depends on singletons?

Question

I\'ve read cases for and against using the singleton pattern. One common case against describes the difficulties in unit testing with singletons, but I\'m unclear as to why

Answer 1

I don't recall ever reading that, but I suspect the problem is the fact that you can only create one. In some cases, that may not be a problem, just test it normally.

But what if you want to create and test a different one, perhaps with different constructor/factory method parameters? Do you restart the JVM? Or create your singleton so that it's not really a singleton and can be reset? Not good.