It's just one error. At line 16 of your PlayerPink class, you are dereferencing a null reference. That is, you have obtained a null value and are trying to call a method on it (or access a variable with it). java.lang.NullPointerException - is the type of the error at PlayerPink.act(PlayerPink.java:16) - tells you where it happened The rest: at greenfoot.core.Simulation.actActor(Simulation.java:568) at greenfoot.core.Simulation.runOneLoop(Simulation.java:526) at greenfoot.core.Simulation.runContent(Simulation.java:215) at greenfoot.core.Simulation.run(Simulation.java:205) ... is a "stack trace", showing you where your method was called from. In this case, your method was called from inside Greenfoot ("greenfoot.core.<xxx>") so the information is not useful.