Firstly, let me disclose that I’m NOT a Java developer, I currently develop in C#, so any assumptions or statements I make regarding Java may be incorrect or ill informed.
When writing unit tests in .Net with mocked objects, 99% of the time we mock interfaces. It’s possible to mock classes but you’re only going to be able to have control over the virtual (thus mockable) methods.
In Java, as methods are virtual by default then I imagine that mocking a class is going to be just the same as mocking an interface because everything is going to be overridden with mocking behaviour.
So, is it still best practice to have dependencies on interfaces (and consequently mock interfaces in unit tests) in Java, or do most Java developers use classes to reduce the number of types (no need for an interface)?
I’d mock interfaces in Java as well. I think that’s the whole idea: clients only know about the interface reference type. You can inject in a mock implementation behind it and the client is none the wiser.