8. วัตถุสิ่งของ

import jlab.graphics.DWindow;
public class Ball {
  //------- ตัวแปรประจำอ็อบเจกต์ (object variable) --------
  public double x, y;
  public double dx, dy;
  public double r;
  //------- ตัวสร้าง (constructor) ----------------------
  public Ball(double r0, double x0, double y0) {
    r = r0; x = x0; y = y0;
    dx = random(-3, 3); dy = random(-3, 3);
  }
  public Ball(double r0,double x0,double y0,double dx0,double dy0){
    r = r0; x = x0; y = y0;
    dx = dx0; dy = dy0;
  }
  //------- เมท็อดประจำอ็อบเจกต์ (object method) ----------
  public void move(DWindow w) {
    x += dx; y += dy;
    if (hitWall(x, w.getWidth()))  dx = -dx;  
    if (hitWall(y, w.getHeight())) dy = -dy;  
    x = Math.min(Math.max(x, r), w.getWidth()  - r);
    y = Math.min(Math.max(y, r), w.getHeight() - r);
  }
  public void draw(DWindow w) {
    w.fillEllipse(DWindow.BLACK, x, y, 2*r, 2*r);
  }
  private boolean hitWall(double c, double length) {
    return (c <= r || c >= length-r);
  }
  //------- เมท็อดประจำคลาส (class method) -------------
  private static double random(double a, double b) {  
    return a + (b - a) * Math.random();
  }
  //------- เมท็อด main เพื่อทดสอบการทำงานของ Ball --------
  public static void main(String[] args) {
    DWindow w = new DWindow(200, 200);
    Ball[] balls = new Ball[300];
    for (int i=0; i<balls.length; i++)  
      balls[i] = new Ball(2, 100, 100);
    w.setRepaintDuringSleep(true);
    while (true) {
      w.fade(0.3);
      for (int i=0; i<balls.length; i++) {
        balls[i].move(w);  balls[i].draw(w);
      }
      w.sleep(50);
    }
  }
}