This is a bit of a generalized comment, and arguably to some extent personal preference. It would be nice if fields that aren't accessed anywhere outside of the current field (like defaultBallSpeed) used a local var instead of setting a field on the Bounce object.  I think there a bunch of cases in bounce where this could be done.  Doing so can make it more clear what is/isn't used outside of the file, and generally makes the code less brittle in my experience.  There's probably some opportunity for me to improve this in Flappy as well.  This is more of a nice to have than a requirement, so feel free to ignore if you wish.