Trying to write the easiest code that I could test... I don't think I can without writing an excessively brittle test that would break at the slightest implementation change.

So you've got this Java:

    public List<Integer> someCall() {
        return IntStream.range(1,10).boxed().toList();
    }

    public List<Integer> filterEvens(List<Integer> ints) {
        return ints.stream()
                .filter(i -> i % 2 == 0)
                .toList();
    }

    int aMethod() {
        List<Integer> data = someCall();
        return filterEvens(data.stream().filter(i -> i % 2 == 0).toList()).size();
    }

And I can mock the class and return a spied'ed List. But now I've got to have that spied List return a spied stream that checks to see if .filter(i -> i % 2 == 0) was called. But then someone comes and writes it later as .filter(i -> i % 2 != 1) and the test breaks. Or someone adds another call to sort them first, and the test breaks.

To that end, I'd be very curious to see the test code that verifies that when aMethod() is called that the List returned by SomeCall is not filtered twice.

What's more, it's not a useful test - "not filtered twice" isn't something that is observable. It's an implementation detail that could change with a refactoring.

Writing a test that verifies that filterEvens returns a list that only contains even numbers? That's a useful test.

Writing a test that verifies that aMethod returns back the size of the even numbers that someCall produced? That's a useful test.

Writing a test that tries to enforce a particular implementation between the {} of aMethod? That's not useful and incredibly brittle (assuming that it can be written).