All examples and tasks in the waves-chapter have one thing in common: They have no background-clearing in the draw-loop. When i was working with mixed waves i slowly recognized a problem: It is simply impossible to access the drawn image. Not clearing the background for drawing is something like an ugly hack, because it creates many problems afterwards, if we want to edit the drawn image: It is simplay impossible, because clearing the background would mean to delete the whole image.
Luckily there's a good solution for that, that points to the way i want to go from now: Arrays. Or even better: The big brother of the array, the "ArrayList".