飛び回る球 - Code Samples @ ウィキ

package test;
 
import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
 
public class BasicApplet extends Applet implements ActionListener {
 
	Ball ball;
	Rectangle rect;
 
	//コンストラクタはやっつけ
	public BasicApplet(){
		this.setSize(120,120);
		this.setBackground(Color.black);
		ball = new Ball(10);
		rect = new Rectangle(10,10,100, 100);
		new javax.swing.Timer(50, this).start();
	}
 
	public void paint(Graphics g){
		ball.draw(g);
	}
 
	//タイマーイベントで、ballの座標を更新する
	public void actionPerformed(ActionEvent arg0) {
		ball.move(rect);
		repaint();
	}
 
	//飛び回るボールのクラス
	public class Ball {
		private double velocity, arg;	//速度,x軸となす角
		private final double RAD = Math.PI / 180;
		private float hue;//ボールの色を変えるため
		private int x, y, size, sizeOffset;//ボールの位置とサイズ
		private Color color;//色変え用
 
		public Ball(int size) {
			this.size = size;
			sizeOffset = size / 2;
			x = 10;
			y = 10;
			velocity = 10.0;
			arg = 33.0;
			hue = 0.f;
			color = new Color(0xffffff);
		}
 
		/**
		 * @param rect ボールの移動範囲となる四角いエリア
		 * */
		public void move(Rectangle rect) {
			int xdist, ydist;
			Point next;
			xdist = (int) (velocity * Math.cos(arg * RAD));
			ydist = (int) (velocity * Math.sin(arg * RAD));
 
			next = new Point(x + xdist, y + ydist);
 
			// x range check
			if (next.x < rect.x) {
				next.x = -next.x;
				arg = 180 - arg;
			} else if (rect.width < next.x) {
				next.x = rect.width - (next.x - rect.width);
				arg = 180 - arg;
			}
 
			// y range check
			if (next.y < rect.y) {
				next.y = -next.y;
				arg = 360 - arg;
			} else if (rect.height < next.y) {
				next.y = rect.height - (next.y - rect.height);
				arg = 360 - arg;
			}
			x = next.x;
			y = next.y;
		}
 
		//ボールを描画する
		public void draw(Graphics g) {
			if (1.0 <= (hue += 0.05f)) {
				hue = 0;
			}
			g.setColor(new Color(color.HSBtoRGB(hue, 1.f, 1.f)));//色を付ける
			g.fillOval(x - sizeOffset, y - sizeOffset, size, size);
		}
	}
}