Singleton and unit testing

Question

The Effective Java has the following statement on unit testing singletons

Making a class a singleton can make it difficult to test its clients, as it’s im

Answer 1

Mocks require interfaces, because what you're doing is replacing the real underlying behavior with an imposter that mimics what you need for the test. Since the client only deals with an interface reference type, it doesn't need to know what the implementation is.

You can't mock a concrete class without an interface, because you can't replace the behavior without the test client knowing about it. It's a completely new class in that case.

It's true for all classes, Singleton or not.