Box2D-for-Processing | Processing JBox2D helper library and examples | Learning library

First of all, check this out (link): it's an example program taken from the Processing Box2D library on github, which displays a box at the center of the screen (it can be moved around with the mouse) while a cascade of little balls fall on it; whenever a ball hits the box, that ball turns red. I copypasted the four .pde files into a single .pde sketch, run it, and it works perfectly.

Now, onto my problem. I'm currently making a game in Processing: you have a ball, a player 1 and a player 2 (both of which can be moved around using the keyboard). Box2D is in charge of the physical interactions between each player and the ball, and I must say it handles them pretty well. Each of the three main objects has its own class. Now, I want stuff to happen as soon as player 1 makes contact with the ball. And that example code seems perfect for this scope, right? It works on my computer, after all.

So, I started copying the relevant parts:

• I copypasted all the 'import's at the beginning of my code;
• I added box2d.listenForCollisions(); inside setup();

I also added these two functions at the very bottom of my sketch: