Expand your world to include brain-eating zombies. Zombies will have a different set of rules than humans.
Redefine how Humans act so they will respond to zombies by attempting to run away. If a human is in an adjacent square to a zombie, the human's brains are eaten and it becomes a new zombie.
Improve the UI of the simulation to support keyboard short cuts to reset the simulation and mouse clicking to "drop" a new zombie at a particular location.
When the simulation starts, a single zombie should be created in a random location.
Zombies are slower thinkers than humans, so zombies should only make a move 1/5th as often as Humans.
If a zombie sees another being (i.e., a human or a zombie) within 10 squares of the direction it is facing (i.e., up, down, left, or right), then it moves towards that being
Otherwise, a Zombie has a 20% chance of turning in a random direction. The zombie moves forward whether it turns or not.
If a Zombie is adjacent to a human (not including diagonals), then it infects the human. The human then becomes a new zombie. (Note: While zombies only move once every five simulation cycles, you should still check to see if they can infect any humans at each tick…. or maybe you could have humans check their surroundings to see if they are adjacent to a zombie.)
Updated Human Rules:
If a Human sees a Zombie within 10 squares of the direction it is facing, then the Human faces the opposite direction and attempts to run two squares away (subsequent moves will only be one square, unless the Human sees another zombie).
Otherwise the Human acts as before (10% random chance of turning, then move forward).
Humans that are adjacent to zombies get turned into new zombies as described above.
Pressing the space bar should cause the zombie simulator to reset (regenerate buildings and humans, and add one zombie).
Clicking the mouse on the map should add a zombie at that location (the [url removed, login to view] class has all the methods you will need to find out if a mouse has been clicked and the current pointer location).
In particular, look at isKeyPressed(), mousePressed(), mouseX(), and mouseY().
One more thing…
Finally, you must add one more feature of your choice to your simulator. Be creative: add a new rule for the humans or zombies, add extra functionality to the UI… anything you like as long as it is interesting.
Sorry for the short notice, but this must be done by 3:00 PM EST
I have updated the code to where the humans spawn and go around the walls.