Takeaway: Representing dependencies explicitly in code will avoid you lots of trouble. One way to achieve that with an object of type X that requires an object of type Y is to require an y in the constructor for x's.
Maybe I'm being slow, but I didn't get how they were explicitly declaring it except in the test, and I don't see how one test in a whole suite would help a newbie discover this problem.
Are they actually implying the constructor should throw an exception moaning about lack of X dependency?
You can't call the constructor without passing the reference to the object the new object depends on... Thus you can't inadvertantly initialize the objects in the wrong order.
