ArrayList balls;
int nrBalls=4;
ball B;

void setup()
{
  frameRate(60);
  size(800,600); 
  smooth();
  balls = new ArrayList();
  for (int i=0;i<nrBalls;i++) 
      {
        B= new ball(random(0,width),random(0,height));
        B.speed = new PVector(4,3);
        balls.add(B);
      }
  stroke(255,255,255,150);
}
void draw()
{
  background(0);
  for (int i=0;i<balls.size();i++)
    {
      B = (ball) balls.get(i);
      strokeWeight(B.mass*2);
      point(B.pos.x,B.pos.y);
      
     // move ball
       B.pos.add(B.speed);
       constrain(B.pos.x,B.mass,width-B.mass);
       constrain(B.pos.y,B.mass,height-B.mass);
     // check boarder
       if (B.pos.x<=B.mass)        B.speed.x=abs(B.speed.x);
       if (B.pos.x>=width-B.mass)  B.speed.x=abs(B.speed.x)*(-1);
       if (B.pos.y<=B.mass)        B.speed.y=abs(B.speed.y);
       if (B.pos.y>=height-B.mass) B.speed.y=abs(B.speed.y)*(-1);
    }
}

class ball
{
  PVector speed;
  PVector pos;
  float mass;
  ball(float x, float y)
     {mass=int (random(10,50));speed=new PVector(0,0);pos=new PVector(x,y);}
}