See: http://processing.org/learning/topics/bouncybubbles.html

MenteCode, since you post a lot (which is good ), I suggest you to read the article https://forum.processing.org/topic/where-to-place-threads-and-other-forum-practices
Particularly about thread placement: if you ask a generic question, without specific code to analyze, just post in General Discussion section.
A bit strict and arbitrary, but simple policy, since such question can range from "how do I split a string" to "how to make a complex program described succinctly", with questions like your in the middle!

Beside the good answer given above, to answer your specific question: " How do I have the same class interact with itself?"
You can call a class' method with a parameter of the type of the same class:

1. class C
2. {
3.   void isCollidingWith(C c)
4.   {
5.     return dist(x, c.x, y, c.y) < radius + c.radius;
6.   }
7. }
8. C c1, c2;
   
9. // [...]
10. if (c1.isCollidingWith(c2))
11. {
12.   // do stuff
13. }
    

(Dummy, untested example, to give you the spirit.)