Loosely speaking, it says, “convert the stream of albums to a map, mapping each artist to the album that has the best album by sales.” This is surprisingly close to the problem statement.
Much better than an imperative method where one would have to deduce that sentence from inspecting the code. With a more functional method using streams (as done here) the code and the actual problem statement are almost the same and it reads nicely (it's also easier to develop and less prone to bugs IMO)
