This is going to invalidate solutions that return the array in getLastWeek, like this one.

If our objective is to just capture the incrementTodaysCount, I think we could just change itIncrementTodaysCount.

I see what you’re getting at, but this solution is fundamentally incorrect:

 public int[] getLastWeek() {
 return birdsPerDay;
 }

This method returns the current week, not the previous week.
According to the exercise instructions: "For comparison purposes, you always keep a copy of last week's counts nearby, which were: 0, 2, 5, 3, 7, 8 and 4. "

If birdsPerDay were truly representing the previous week, then the last index of the array could not correspond to today. That alone makes this implementation inconsistent with the intended model.

These solutions were effectively getting a free pass because the test data happened to use the same values for both the current week and the "previous week" mentioned in the introduction. As a result, logically incorrect implementations still passed the tests.

Curious to hear your thoughts on this.

I can see where you are coming from. The instructions for getLastWeek seem to imply students should always be defining the returned array. This would be within scope of this exercise (design.md mentions one of the objects is to be able to define an array).

But I think the setup with the two arrays in the test class could be confusing as their usage or intent might not be as clear. I prefer the C# implementation, where each test creates the BirdWatcher with its own input and getLastWeek is like a static method (so that we don't have to make an instance of BirdWatcher to call getLastWeek).

Alternatively, if we were to stick with a common array that gets reused, I'd preferring sticking to just one array in the test class that is used by the test method instead of having two arrays. In this case, we could change the array contents in this PR so that it is no longer the matches the expected lastWeek.

Ah, this is a clever way (using getCountForFirstDays). Just couple of minor comments for this one:

• We could simplify this by doing calculating the expected value directly, like we currently do in the other test for getCountForFirstDays (i.e. assertThat(birdWatcher.getCountForFirstDays(6)).isEqualTo(DAY1 + DAY2 + DAY3 + DAY4 + DAY5 + DAY6);).
• I think it can be confusing why we're asserting getCountForFirstDays in this test. I'd suggest moving to a separate test so that the test can be described in the @DisplayName and test name. For example:

public void itIncrementDoesNotChangeCountForOtherDays() {
 birdWatcher.incrementTodaysCount();
 assertThat(birdWatcher.getCountForFirstDays(6)).isEqualTo(DAY1 + DAY2 + DAY3 + DAY4 + DAY5 + DAY6);
}