class BounceBall extends Ball {
  float FRICTION = 0.99;
  float SPRING = 0.001;
  
  BounceBall (int ix, int iy, float ivx, float ivy, float iradius) {
    // just use Ball's constructor, it's good enough
    super(ix, iy, ivx, ivy, iradius);
  }
  
  // this overwrites Ball's update function
  void update() {
    vx *= FRICTION;
    vy *= FRICTION;
    spring(width/2, height/2);
    // but we call it here anyways
    super.update();
    limit();
  }
  
  void spring(float targX, float targY) {
    vx += (targX-x) * SPRING;
    vy += (targY-y) * SPRING;
  }
  
  
  void limit() {
    if ((y > height - radius) || (y < radius)) {
      vy = -vy;
      y = constrain(y, radius, height-radius);
    }
    if ((x < radius) || (x > width - radius)) {
      vx = -vx;
      x = constrain(x, radius, width - radius);
    }
  }
}