8. วัตถุสิ่งของ

import jlab.graphics.DWindow;
public class TestBall {
  public static void main(String[] args) {
    DWindow w = new DWindow(200, 200);
    Ball b1 = createRandomBall(200,200);
    Ball b2 = createRandomBall(200,200);
    while (true) {
      w.clearBackground();
      move(w, b1); move(w, b2);
      draw(w, b1); draw(w, b2);
      w.sleep(50);
    }
  }
  private static void draw(DWindow w, Ball b) {
    w.fillEllipse(DWindow.BLACK, b.x, b.y, 2*b.r, 2*b.r);
  }
  private static double random(double a, double b) {  
    return a + (b - a) * Math.random();
  }
  private static void move(DWindow w, Ball b) {
    b.x += b.dx; b.y += b.dy;
    if (b.x <= b.r || b.x >= w.getWidth() - b.r) b.dx = -b.dx;  
    if (b.y <= b.r || b.y >= w.getHeight()- b.r) b.dy = -b.dy;  
    b.x = Math.min(Math.max(b.x, b.r), w.getWidth()  - b.r);
    b.y = Math.min(Math.max(b.y, b.r), w.getHeight() - b.r);        
  }
  private static Ball createRandomBall(double w, double h) {
    Ball b = new Ball();
    b.x = w/2; b.y = h/2;
    b.dx = random(-5, 5);  b.dy = random(-5, 5);
    b.r =  random(3, 8);
    return b;
  }
}