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But Java has a solution for the “good static methods” you’ve described in the Function interface. I’d submit that a FooLib class with these static methods could be replaced with a foo.lib package with the corresponding Function implementations. This allows great flexibility with unit testing because you don’t even need a mocking framework. Instead, you just provide a dummy implementation of the Function the unit under test depends on.

In fact, dependence on mocking frameworks is a huge code smell because it covers over the common practice of not coding to an interface. If you want to expose poor design in a code base, remove the mocking decencies and see how hard it is to rewrite the tests.