The zombies are being moved by the player's knockback, as though it's their own knockback. Ideally the zombies would only be moved by their own knockback and not the player's.

Zombies won't be directly affected by the variables and logic in another class. What you're saying isn't really possible. Is it perhaps the case that both the player and the zombie are getting knocked back at the same time? Is this the scenario - http://www.greenfoot.org/users/9686 - ? since I don't see the problem happening.

Firstly, yes I realize that this is not possible. There must be some other way that the zombies are being affected. The player and any intersecting zombies will knock each other back, yes, but every single zombie on the map is being affected, not just the intersecting ones. What's more, other actions like the player shooting a shotgun, which also affect the player's knockback, will knock all existing zombies as well. It is indeed the scenario. The reason that you are not seeing the problem is because it only happens in multiplayer, and even then it is an uncommon glitch. I have now altered the code to make the knockback variables from both the player and the zombie to make them private. Hopefully this will fix the problem, but I haven't tested it enough yet.

I would comment out the code for the player (or just change the name of the act method) and then check the behavior of the zombies to see if it is really the knockback of the player that is causing the zombies to be moved.

I commented out the lines that set the players knockback and the glitch no longer happened. I added the lines back in and it straight away happened again. I have also noticed that once the red player dies the glitch no longer happens. The zombies are only affected by the blue player's knockback.

Ah, I finally fixed it. It turns out that the player base class was always setting the playerBlueX variable of the world to it's X coordinate, and doing the same with the Y coordinate. In short, both the blue player and the red player would have been setting the world's blue player coordinates to their own in their move code, which they don't execute when they are being knocked back. Thanks for the help anyway! :)

Your blue player and red player are actors that have a 'getX' and a 'getY' method for getting the current coordinates. Having fields for the blue player and the red player would be much more useful in your world class than having multiple fields for the individual states of each actor. Also, this will avoid having to update the value of the fields each act cycle.

Are you saying that the player would update the world's player location instead of it's own? How would that help?

No. I am saying that with player references, you do not need location fields at all. Instead of: you would have: and then locally you can use: And, if needed, you can add 'get' methods to return each player.