How to make objects of the same class interact with each other?

Question

I have a single class Part and two objects from this class part1 and part2. I would like to be able to have these objects bounce off of each other when they collide using methods. I am able to do this with functions outside the class, but would like to be able to do it with methods purely inside the class, so I can add objects without having to add each object to my watchforpartcollision function.

This is code works but I would like the function to be a method:

void draw() {
  background(255);
  part1.drawpart();
  part2.drawpart();
  part1.watchforcollision();
  part1.applymovement();
  part2.watchforcollision();
  part2.applymovement();
  watchforpartcollision(); //function I would like to make a method
}
...
void watchforpartcollision() {
  if (dist(part1.x + part1.size/2, part1.y + part1.size/2,
  part2.x + part2.size/2, part2.y + part2.size/2) <= (part1.size)) {
      partcolide();
    }
}
void partcolide() {
  part1.xspeed = (part1.xspeed * part2.oldxspeed) + part2.oldxspeed;
  part1.yspeed = (part1.yspeed * part2.oldyspeed) + part2.oldyspeed;
  part2.xspeed = (part2.xspeed * part1.oldxspeed) + part1.oldxspeed;
  part2.yspeed = (part2.yspeed * part1.oldyspeed) + part1.oldyspeed;
}

This is what I would like:

void draw() {
  background(255);
  drawcursor();
  part1.drawpart();
  part2.drawpart();
  part1.watchforcollision(); //method I would like to add function too
  part1.applymovement();
  part2.watchforcollision(); //method I would like to add function too
  part2.applymovement();
}
...
class Part {
 ... 
  void watchforcollision() {
    ...
    if (dist(x + size/2, y + size/2, x + size/2, y + size/2) <= (size)) {
/*This ^ is the line that messes it up because it's checking the distance
  between the same two things*/
      partcolide();
      }
  }
  ...
  void partcolide() {
    xspeed = (xspeed * oldxspeed) + oldxspeed;
    yspeed = (yspeed * oldyspeed) + oldyspeed;
    xspeed = (xspeed * oldxspeed) + oldxspeed;
    yspeed = (yspeed * oldyspeed) + oldyspeed;
  }
}

Any help would be much appreciated!
Thank you!

Answer 1

If you want to move the collision logic into the class, then you'll need to pass in the object you want to collide with when you make the method call:

part1.watchforcollision(part2);

Inside the class:

void watchforpartcollision(Part obj) { // `obj` is the Part object passed in
  if (dist(x + size/2, y + size/2,
  obj.x + obj.size/2, obj.y + obj.size/2) <= (size)) {
      partcolide();
      obj.partcolide(); // apply the collision to the second object too
    }
}