ありがちなボールを跳ね返すゲーム風のプログラム

ballgame.groovy

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

/** ありがちなボールを跳ね返すゲーム風のプログラムです．
    全くGroovyっぽくないのは，ご愛嬌ということで． */
class BallGameFrame extends JFrame implements Runnable {
  double ball_x = 10, ball_y = 10;
  double ball_v_x = 100.0, ball_v_y = 0.0;
  double ball_g = 9.8, dt = 0.05;
  int ball_size = 32;
  int racket_x = 320, racket_y = 400;
  int racket_height = 16, racket_width = 80;

  BallGameFrame() {
    setSize(640, 480);
    mouseMoved = { e ->
      racket_x = e.getX();
    };
    Thread th = new Thread(this);
    th.start();
  }

  void run() {
    while (1) {
      if (ball_y <= 0) {
        ball_v_y = -1 * ball_v_y;
      }
      if (ball_x <= 0 || ball_x + ball_size >= 640) {
        ball_v_x = -1 * ball_v_x;
      }
      if (racket_x <= ball_x && 
          ball_x + ball_size <= racket_x + racket_width && 
          ball_y + ball_size >= racket_y) {
        ball_v_y = -1 * ball_v_y;
      }
      ball_x += dt * ball_v_x;
      ball_v_y += dt * ball_g;
      ball_y += ball_v_y;    
      repaint();
      sleep(50);
    }
  }
  
  void paint(Graphics g) {
    super.paint(g);
    g.fillRect(racket_x, racket_y, racket_width, racket_height);
    g.fillOval((int)ball_x, (int)ball_y, ball_size, ball_size);
  }
}

static void main(String[] args) {
  BallGameFrame f = new BallGameFrame();
  f.setVisible(true);
}