You just need a direction field that alternates between +1 and -1 that gives the speed value direction when multiplied by it.. You will need checks in your act method (or a method it calls) for the conditions required to changed direction and when the conditions are met, multiply the direction field by -1 (which changes it to the other number; -1 to 1 and 1 to -1). The conditions for changing directions at the top or the bottom are two parters. For example, if the direction is currently up and the upper limit is reached, change directions; and similar for the lower limit. Actually, you do need the direction field, as you can just multiply the speed itself by -1 when the conditions are met. Still, however, the speed will have to be kept in a field.