Ball ball1;
Ball ball2;
void setup(){
  size(400,400);
  smooth();
  ball1=new Ball(64);
  ball2=new Ball(32);
}
void draw(){
  background(255);
  ball1.move();
  ball2.move();
  if (ball1.intersect (ball2)){
    ball1.highlight();
    ball2.highlight();
  }
  
  ball1.display();
  ball2.display();
}
class Ball{
  float r;
  float x,y;
  float xspeed, yspeed;
  color c = color(100,50);
  
  Ball(float tempR){
    r=tempR;
    x=random(width);
    y=(height);
    xspeed=random(-5,5);
    yspeed=random(-5,5);
  }
  
  void move(){
    x += xspeed;
    y += yspeed;
    
    if (x>width || x<0){
      xspeed *= -1;
    }
    
    if (y>height || y<0){
      yspeed *= -1;
    }
  }
  
  void highlight(){
    c=color(0,150);
  }
  
  void display(){
    stroke(0);
    fill(c);
    ellipse(x,y,r*2,r*2);
    c=color(100,50);
  }
  
  boolean intersect(Ball b){
    float distance = dist(x,y,b.x,b.y);
    if (distance < r + b.r){
      
      return true;
    }else{
      return false;
    }
  }
}