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What is Zone? and How is it Used?
Zone = playing field.
Java has a datastructure called a Map, and since this program is written in Java, it would very quickly get confusing what we were talking about if the playing area was called a Map.
So instead we call playing areas Zones. We have a MapZone which uses a prerendered picture for the background, and we have an UnboundedZone which tiles a background image in a (reasonably) unbounded space.