这次我们做一个小球的碰撞的游戏，规则是：按下添加按钮，窗口的中心部分会产生一个小球（刚开始默认为黑色），四个方向随机产生，发射小球，再次按下即产生两个小球。当小球碰到窗体边缘的时候会产生反弹，当两个小球接触时会产生碰撞，双方交换速度，向相反方向移动。我们可以选择相应的颜色来改变下一个发射的小球颜色。当按下清除可以清除屏幕上的小球，当按下添加则会继续产生小球。最后我们还添加了自动产生小球的功能，按下开关，在屏幕中间会定时产生小球。接下来，我们来展示代码部分。
public class Jframe {private Ball[] arrayball = new Ball[100];public static void main(String[] args) {Jframe frame = new Jframe();frame.showUI();}public void showUI() {javax.swing.JFrame jf = new javax.swing.JFrame();jf.setSize(1000, 1000);jf.getContentPane().setBackground(Color.WHITE);jf.setTitle("小球");jf.setDefaultCloseOperation(3);// 设置居中显示jf.setLocationRelativeTo(null);JPanel jp1 =new JPanel();JButton jb1 = new JButton("添加");jp1.add(jb1);// jb1.setBounds(100,50, 40, 20);JButton jb2 = new JButton("暂停");jp1.add(jb2);// jb1.setBounds(200,50, 40, 20);JButton jb3 = new JButton("清除");jp1.add(jb3);// jb1.setBounds(300,50, 40, 20);JButton jb4 = new JButton("自动添加");jp1.add(jb4);jf.add(jp1,BorderLayout.NORTH);Mouse mouse = new Mouse(); Color[] color = {Color.RED,Color.BLUE,Color.BLACK,Color.GREEN,Color.YELLOW};for(int i=0;i<color.length;i++){JButton jbu = new JButton();jbu.setBackground(color[i]);jbu.setPreferredSize(new Dimension(30, 30));jp1.add(jbu);jbu.addActionListener(mouse);}jb1.addActionListener(mouse);jb2.addActionListener(mouse);jb3.addActionListener(mouse);jb4.addActionListener(mouse);jf.addMouseListener(mouse);jf.addMouseMotionListener(mouse);BallJpanel cp = new BallJpanel();cp.setBackground(Color.WHITE);jf.add(cp,BorderLayout.CENTER);jf.setVisible(true); Graphics g = cp.getGraphics();mouse.setcp(cp);mouse.setg(g);mouse.setarrayball(arrayball);mouse.setmouse(mouse);cp.setarrayball(arrayball);}}
这是窗体的基本配置，采用边框布局，上方放置按钮，中间是画布。我们为按钮添加了动作监听器，并使用了一系列的方法来把对象传递到其他类中。
public class Ball {public int size = 90; // 小球的直径public int x = 500; // 小球所在的x坐标public int y = 500; // 小球所在的y坐标public int vx = 5;public int vy = 5;public BallJpanel cp;public Color color = Color.BLACK;public int max_x, max_y, Min_x, Min_y;private Ball[] arrayball;public void setcp(BallJpanel cp) {this.cp = cp;}public void setarrayball(Ball[] arrayball) {this.arrayball = arrayball;}public void setX(int x) {this.x = x;}public int getX() {return x;}public void setY(int y) {this.y = y;}public int setY() {return y;}public Ball(int x, int y, int vx, int vy, Color color) {this.x = x;this.y = y;this.vx = vx;this.vy = vy;this.color = color;}public void ballMove(Graphics g) {x += vx;y += vy;max_y = cp.getHeight();max_x = cp.getWidth();if (x <= size / 2) {x = size / 2;vx = -vx;}if (y <= size / 2) {y = size / 2;vy = -vy;}if (x + size / 2 >= max_x) {x = max_x - size / 2;vx = -vx;}if (y + size / 2 >= max_y) {y = max_y - size / 2;vy = -vy;}for (int i = 0; i < arrayball.length; i++) {if (arrayball[i] == null)break;Ball ball = arrayball[i];if (this.equals(ball))continue;if ((ball.x - this.x) * (ball.x - this.x) + (ball.y - this.y) * (ball.y - this.y) <= size * size){int tempvx = this.vx;int tempvy = this.vy;this.vx = ball.vx;this.vy = ball.vy;ball.vx = tempvx;ball.vy = tempvy;while ((ball.x - this.x) * (ball.x - this.x) + (ball.y - this.y) * (ball.y - this.y) <= size * size){this.x += this.vx;this.y += this.vy;System.out.println("等待");}}}}}
考虑到这是一个小球的运动系统，我们为小球写了一个类，添加小球的时候，会创建小球对象，并使其获得位置，颜色，速度等参数，并将其存入数组。小球的方法就是运动，每当执行ballMove方法，便会为小球修改位置坐标（基于其速度），再判断是否撞击边框，以及判断是否和别的小球有坐标重叠，如果有重叠，则跑一个循环，修改位置坐标，使其分离。Ball这部分代码和监听器中的方法有所联系，我们接下来介绍监听器的方法。
public class Mouse implements MouseMotionListener, MouseListener, ActionListener {private Graphics g;private BallJpanel cp;private Ball[] arrayball;private int index = 0;private int x;private int y;private int vx;private int vy;private int random=1;private Color color=Color.black;private ThreadBall tb;private Mouse mouse;public int selfFlag=0;public void setmouse(Mouse mouse){this.mouse= mouse;} public void setarrayball(Ball[] arrayball) {this.arrayball = arrayball;}public void setg(Graphics g) {this.g = g;}public void setcp(BallJpanel cp) {this.cp = cp;}public void actionPerformed(ActionEvent e) {if ("添加".equals(e.getActionCommand())) {System.out.println("添加");if (tb == null) {// 创建线程对象tb = new ThreadBall();tb.setcp(cp);tb.setarrayball(arrayball);tb.setg(g);tb.start();tb.setmouse(mouse);}tb.stopFlag=0;addBall();}if ("暂停".equals(e.getActionCommand())) {if(tb!=null){if(tb.stopFlag==0){tb.stopFlag=1;System.out.println("暂停");}else {tb.stopFlag=0;System.out.println("开始");}}}if ("清除".equals(e.getActionCommand())) {tb.stopFlag=1;cp.paint1(g);index=0;System.out.println("清除");}if ("自动添加".equals(e.getActionCommand())){if(selfFlag==0){selfFlag=1;System.out.println("自动添加打开");}else{selfFlag=0;System.out.println("自动添加关闭");}}if("".equals(e.getActionCommand())){JButton jbu=(JButton)e.getSource();color=jbu.getBackground();g.setColor(color);}}public void mouseDragged(MouseEvent e) {}public void mouseMoved(MouseEvent e) {}public void mouseClicked(MouseEvent e) {}public void mousePressed(MouseEvent e) {}public void mouseReleased(MouseEvent e) {}public void mouseEntered(MouseEvent e) {}public void mouseExited(MouseEvent e) {}public void addBall() {x = 500;y = 500;random=1+(int)(Math.random()*4);switch(random){case 1:vx=5;vy=5;break;case 2:vx=-5;vy=-5;break;case 3:vx=5;vy=-5;break;case 4:vx=-5;vy=5;break;}Ball ball = new Ball(x, y,vx , vy, color);arrayball[index++] = ball;}}
监听器中，我们设置了一系列参数来控制一些方法的开启和关闭，以及写了添加小球的方法，为其赋初值，随机一个初始发射方向。这段代码我们用到了线程。线程的使用分为两步，创建线程对象并start线程。
public class ThreadBall extends Thread {private Graphics g;private BallJpanel cp;private Ball[] arrayball;public int stopFlag=0;private int add=0;private Mouse mouse;public void setmouse(Mouse mouse){this.mouse=mouse;}public void setcp(BallJpanel cp) {this.cp = cp; }public void setg(Graphics g){this.g=g;}public void setarrayball(Ball[] arrayball) {this.arrayball = arrayball;}/*** 启动线程执行的方法*/public void run() {while (true) {if(stopFlag==0){for (int i = 0; i < arrayball.length; i++) {if(arrayball[i]==null)break;Ball ball = arrayball[i]; ball.setarrayball(arrayball); ball.setcp(cp);ball.ballMove(g);}cp.paint(g);add++;if(add==5000)add=0;if(add%50==0&&mouse.selfFlag==1)mouse.addBall();}try {Thread.sleep(50);} catch (InterruptedException e) {e.printStackTrace();}}}}
以上是线程的属性和方法，此类继承Thread并重写了run方法。run方法的思路是循环调用ballMove方法修改小球坐标，并调用paint方法更新显示，我们加入了一个延时函数，来控制调用的频率。
public class BallJpanel extends JPanel {private Ball[] arrayball;public void setarrayball(Ball[] arrayball){this.arrayball=arrayball;}public void paint(Graphics g){super.paint(g);for(int i=0;i<arrayball.length;i++){if(arrayball[i]==null){break;}Ball ball=arrayball[i];g.setColor(ball.color);g.fillOval(ball.x-ball.size/2, ball.y-ball.size/2, ball.size, ball.size);}}public void paint1(Graphics g){super.paint(g);for(int i=0;i<arrayball.length;i++){if(arrayball[i]==null){break;}arrayball[i]=null;}}}
BallJpanel类写的是画布，及小球的运动区域，画笔也是从其对象cp上获得。类里用paint写画面的重绘方法（包括画板小球的重绘），paint1用来清空画布及数组。
以上便是java小球运动的全部代码，我们来看一下效果。
