http://www.processing.org/learning/objects/

When a primitive value (integer, float, etc.) is passed in a function, a copy is made. With objects, this is not the case, and the result is a bit more intuitive. If changes are made to an object after it is passed into a function, those changes will affect that object used anywhere else throughout the sketch. This is known as pass by reference since instead of a copy, a reference to the actual object itself is passed into the function.