A flies life

The terminal window is there to assist you. You were getting a message that started with: Line 1: java.lang.NullPointerException Line 2: at Fly.die(Fly.java:135) Line 3: at Fly.act(Fly.java:32) Line 1 tells you why the scenario stopped (the error type) Line 2: tells you where in your code the error occurred (line 135 of the Fly class in the die() method) Line 3: may show where the method was called from (line 32 of the Fly class in the act() method) Lines 2 will not always be immediately after the error type, but will be somewhere in the message (it will be the first one that is recognizably in your scenario)

The 'NullPointerException' is one of the most common run-time error messages. It is telling you that you are trying to do something to nothing as if it were something. That kind of sounds strange, I guess, but in your case, after the fly is removed, it no longer has a world; therefore 'getWorld()' in the next line becomes 'null'; hence, the 'NullPointerException'.