Comment by shagie

With Mockito, I can mock the returned result of someCall().

However, it also means mocking list.stream() and mocking the Stream for stream.filter() and mock the call stream.toList() to return a new mocked object that has those mocks on it again.

I could catch the object passed in to printEven(...) but that has no history on it to see if filter was called on it before.

Trying to do the filter(...) call would be especially hard since you'd be parameterize it with a code block.

And all this returns to "is this a useful test?"

Testing should only be done on the observable parts of the function. Does printEven only print even numbers?

The tests that you are proposing are testing the implementation of those calls to work in a specific way. "It must call filter" - but if it's changed to a different filter or if it's changed to not use a filter but has the same functionality the code breaks.

Inefficient? Yes. Bad? Yes. Wrong - no. And not being wrong it isn't something that a unit test could validate without going unnecessarily into the implementation of the internals for the method. Internals changing while the contract remains the same is perfectly acceptable and shouldn't be breaking a unit test.