class Circle {
  int x;    // x coordinates
  int y;    // y coordinates
  float sx; // speed in x direction
  float sy; // speed in y direction
  int d;    // diameters
  color c;  // colors

  Circle(int scale, color c) {
    x = width/2;
    y = height/2;
    d = scale;
    while (sx==0)
      sx = random(-10, 10);
    while (sy==0)
      sy = random(-10, 10);
    this.c = c;
  }

  void move() {
    // Update circle's position
    x += sx; 
    y += sy;
    // Bounce back when touch the wall
    if ((sx<0 && x<d/2) || (sx>0 && x>width-d/2))
      sx *= -1;
    if ((sy<0 && y<d/2) || (sy>0 && y>height-d/2))
      sy *= -1;
  }

  void display() {
    fill(c);
    ellipse(x, y, d, d);
  }
}