You should check to see if the lobsters are too close to the crab. I believe this is possible through the constructor of your world though I have not been studying java through Greenfoot so no guarantees.

Math maniac you know it's really hard to make a virus in java and the viruses can really only harm other java files so what are you worried about. Your only downloading the source not and executable and finally if the program was a virus your computer would have already crashed because you already ran the file in you browser.

The main problem with this scenario is that when you initiated the game you made the lobster right next to the crab. So they can't actually play instead you could make every position random except for the crab. If they die right at the start you can just re-initialize everything