appel is that true? Wouldn't that mean a very fast moving small object ( a bullet with a tiny hitbox ) might move past the boundary in one instant and never "collide"
dzzd how does that work for a circle within a circle
You could fix (somewhat) with AppGameContainer.setMaximumLogicUpdateInterval(int).
However, if the game bullet of size 10x10 moves super-fast, say 400 pixels per 1 delta, then it's not possible to detect collision this way. You'll need to find another way. One would be to check if the line composing of previous location and new location intersect any of the lines of the target shape.