前面已经实现了一个球在一个窗口中随机运动,下面将在前面的基础上实现两个球的随机运动及碰撞,此次的代码中用到的算法参考了Twinsen写的一文中关于两球碰撞时的算法.程序仍旧有两个类,一个Ball类,一个BallCanvas类,Ball类与前面相同,而BallCanvas类作了修改,修改后的代码如下:

import java.awt.Canvas;

import java.awt.Color;

import java.awt.Graphics;

import java.util.Random;

public class BallCanvas extends Canvas implements Runnable {

private int ballRadius = 40;   //球的半径

private int ballAX,ballAY;        //当前A,B球的位置

private int ballBX,ballBY;

private double ballAXMoveLength= 7;     //A,B球当前速度在X,Y轴上的分速度

private double ballAYMoveLength = 9;    //为了减少计算中数据丢失,用double型而不用int型

private double ballBXMoveLength = 6;    //

private double ballBYMoveLength = 5;     //如果用int型两球最终将停止运动

private Random r;

private boolean move = true;       //标志球是否移动

private int screenWidth, screenHeight;        //屏幕宽及高

private int ballARX, ballARY;  //A球圆心位置

private int ballBRX, ballBRY;   //B球圆心位置

public BallCanvas(int screenWidth, int screenHeight){

r = new Random();

ballAX = r.nextInt(screenWidth – 2*ballRadius);     //随机初始化A球的初始位置

ballAY = r.nextInt(screenHeight – 2*ballRadius);

do{        //随机初始化B球的初始位置,且确定A球跟B球不发生碰撞

ballBX = r.nextInt(screenWidth – 2*ballRadius);

ballBY = r.nextInt(screenHeight – 2*ballRadius);

}while((int)Math.sqrt((ballAX-ballBX)*(ballAX-ballBX) +

(ballAY-ballBY)*(ballAY-ballBY)) < 2*ballRadius);

this.screenHeight = screenHeight;

this.screenWidth = screenWidth;

}

public void paint(Graphics g){

//g.setColor(Color.WHITE);            //此处使用背景色而不用白色,如果要自己清空屏幕,则需加上缓存,不然会产生画面抖动

//g.fillRect(0, 0, screenWidth, screenHeight);

g.setColor(Color.RED);

g.fillArc((int)ballAX, (int)ballAY, (int)ballRadius*2, (int)ballRadius*2, 0, 360);

g.fillArc((int)ballBX, (int)ballBY,(int) ballRadius*2, (int)ballRadius*2, 0, 360);

}

/**

* 退出

*/

public void exit(){

move = false;

}

public void run(){

while(move){

//处理A球

if(ballAX + ballAXMoveLength + 2*ballRadius > screenWidth ||

ballAX + ballAXMoveLength < 0){         //当在X轴上碰到墙时,X轴行进方向改变

ballAXMoveLength*=-1;

}else{

ballAX += ballAXMoveLength;                 //没碰壁时继续前进

}

if(ballAY + ballAYMoveLength + 2*ballRadius > screenHeight ||

ballAY + ballAYMoveLength < 0){         //当在Y轴上碰到墙时,Y轴行进方向改变

ballAYMoveLength*=-1;

}else{

ballAY += ballAYMoveLength;

}

//处理B球

if(ballBX + ballBXMoveLength + 2*ballRadius > screenWidth ||

ballBX + ballBXMoveLength < 0){         //当在X轴上碰到墙时,X轴行进方向改变

ballBXMoveLength*=-1;

}else{

ballBX += ballBXMoveLength;                 //没碰壁时继续前进

}

if(ballBY + ballBYMoveLength + 2*ballRadius > screenHeight ||

ballBY + ballBYMoveLength < 0){         //当在Y轴上碰到墙时,Y轴行进方向改变

ballBYMoveLength*=-1;

}else{

ballBY += ballBYMoveLength;

}

ballsCollide();   //检查两球是否碰撞

repaint();    //更新画面

try{

Thread.sleep(10);

}catch(InterruptedException e){

}

}

}

/**

* 检查并处理两球碰撞

* 此处用向量来计算两球碰撞后的速度,具体算法分析请参见”向量几何在游戏编程中的使用”

*/

public void ballsCollide(){

if(Math.sqrt((ballAX-ballBX)*(ballAX-ballBX) +              //并没有产生碰撞

(ballAY-ballBY)*(ballAY-ballBY)) > 2*ballRadius){

return;

}else{           //碰撞了

ballARX = ballAX + ballRadius;  //A球圆心位置

ballARY = ballAY + ballRadius;

ballBRX = ballBX + ballRadius;   //B球圆心位置

ballBRY = ballBY + ballRadius;

// 求出s'(球心连线上的向量)

double sx = ballARX – ballBRX ;

double sy = ballARY – ballBRY ;

// 求出s1(球心连线上的单位向量)

double s1x = sx / Math.sqrt(sx*sx + sy*sy) ;

double s1y = sy / Math.sqrt(sx*sx + sy*sy) ;

// 求出t'(与球心连线垂直的向量)

double tx = -sy ;

double ty = sx ;

// 求出t1(与球心连线垂直的单位向量)

double t1x = tx / Math.sqrt(tx*tx + ty*ty) ;

double t1y = ty / Math.sqrt(tx*tx + ty*ty) ;

// 求v1a在s1上的投影v1s

double v1s = ballAXMoveLength * s1x + ballAYMoveLength * s1y ;

// 求v1a在t1上的投影v1t

double v1t = ballAXMoveLength * t1x + ballAYMoveLength * t1y ;

// 求v2a在s1上的投影v2s

double v2s = ballBXMoveLength * s1x + ballBYMoveLength * s1y ;

// 求v2a在t1上的投影v2t

double v2t = ballBXMoveLength * t1x + ballBYMoveLength * t1y ;

// 用公式求出v1sf和v2sf

double v1sf = v2s ;

double v2sf = v1s ;

// 最后一步，注意这里我们简化一下，直接将v1sf,v1t和v2sf,v2t投影到x，y轴上，也就是v1’和v2’在x,y轴上的分量

// 先将v1sf和v1t转化为向量

double nsx = v1sf * s1x ;

double nsy = v1sf * s1y ;

double ntx = v1t * t1x ;

double nty = v1t * t1y ;

ballAXMoveLength =(nsx + ntx) ;

ballAYMoveLength =(nsy + nty) ;

// 然后将v2sf和v2t转化为向量

nsx = v2sf * s1x ;

nsy = v2sf * s1y ;

ntx = v2t * t1x ;

nty = v2t * t1y ;

ballBXMoveLength = (nsx + ntx );

ballBYMoveLength =(nsy + nty );

//碰撞之后两球速度变化了,可是两球的位置仍处于碰撞时的位置,此时可能导致程序误以后两球再次发生碰撞

//于是再次处理碰撞,造成程序的死循环,故碰撞后,两球应该以碰撞后的速度迅速产生一段距离避免碰撞的状态

while(Math.sqrt((ballAX-ballBX)*(ballAX-ballBX) +              //仍旧处于碰撞时的位置

(ballAY-ballBY)*(ballAY-ballBY)) < 2*ballRadius){

if(ballAX + ballAXMoveLength + 2*ballRadius > screenWidth ||

ballAX + ballAXMoveLength < 0){         //当在X轴上碰到墙时,X轴行进方向改变

ballAXMoveLength*=-1;

}else{

ballAX += ballAXMoveLength;                 //没碰壁时继续前进

}

if(ballAY + ballAYMoveLength + 2*ballRadius > screenHeight ||

ballAY + ballAYMoveLength < 0){         //当在Y轴上碰到墙时,Y轴行进方向改变

ballAYMoveLength*=-1;

}else{

ballAY += ballAYMoveLength;

}

if(ballBX + ballBXMoveLength + 2*ballRadius > screenWidth ||

ballBX + ballBXMoveLength < 0){         //当在X轴上碰到墙时,X轴行进方向改变

ballBXMoveLength*=-1;

}else{

ballBX += ballBXMoveLength;                 //没碰壁时继续前进

}

if(ballBY + ballBYMoveLength + 2*ballRadius > screenHeight ||

ballBY + ballBYMoveLength < 0){         //当在Y轴上碰到墙时,Y轴行进方向改变

ballBYMoveLength*=-1;

}else{

ballBY += ballBYMoveLength;

}

}

}

}

/**

* 重新调整屏幕大小

*/

public void canvasResize(){

screenWidth = this.getWidth();

screenHeight = this.getHeight();

}

}