小球碰撞的关键是 判断小球是否碰撞过：
1.当两小球间距小于小球直径时发生碰撞。
2.当小球碰到边缘时发生碰撞。

import static org.junit.Assert.*;
import javax.swing.*
import org.junit.Test;
import java.awt.*;
import java.util.ArrayList;
import java.util.Scanner;
public class Draw {
    static int length;   //小球数量
    static int[][] BallList;   //小球属性数组 0,1为x，y     3，4为X,Y的标识符
    public static void main(String[] args){
        Draw draw=new Draw();
        System.out.println("输入小球个数");
        Scanner sc=new Scanner(System.in);
        length=sc.nextInt();
        BallList=new int[length][5];
        draw.addBall(length);
        JFrame frame=new JFrame();
        frame.setSize(1024, 768);
        frame.getContentPane().setBackground(Color.white);
        Draw.Ball w=new Draw.Ball();
        frame.add(w);
        Thread t=new Thread(w);
        t.start();
                frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    
    }
    public void addBall(int length1){   //随机位置生成小球
        for(int i=0;i<length1;i++){
            BallList[i][0]=(int)(Math.random()*800+100);
            BallList[i][1]=(int)(Math.random()*500+200);
            BallList[i][3]=1;
            BallList[i][4]=1;
        }
    }
    public static class Ball extends Panel implements Runnable{ 
        int a,b,c;   //管理颜色的变量
        ArrayList<Color> colo=new ArrayList<Color>(length);  //管理颜色的数组
        int[][] juge=new int[length][2];   //判断每个小球是否碰撞
        public Ball(){                //给判断变量和颜色变量赋初值
            for(int i=0;i<length;i++){
                juge[i][0]=1;
                juge[i][1]=1;
                a=(int)(Math.random()*100);
                b=(int)(Math.random()*100);
                c=(int)(Math.random()*100);
                colo.add(i, new Color(a,b,c));     //给每个小球上初色
            }
        }
        public void paint(Graphics g){
            for(int i=0;i<length;i++){
            if(BallList[i][3]!=juge[i][0]||BallList[i][4]!=juge[i][1])    //当小球碰撞后改变颜色
            {
                a=(int)(Math.random()*100);
                b=(int)(Math.random()*100);
                c=(int)(Math.random()*100);
                colo.set(i, new Color(a,b,c));      
            }
            juge[i][0]=BallList[i][3];     //改变颜色后将判断变量的值设为小球的标识符
            juge[i][1]=BallList[i][4];
            
            g.setColor(colo.get(i));
            //g.setFont(new Font("",0,30));
            g.fillOval(BallList[i][0], BallList[i][1], 20, 20);
            }
        }
        public void run()
        {
            while(true)
            {
            
                for(int i=0;i<length;i++){    //判断小球行驶轨迹 标识符为1时向右（向上）行驶
                if(BallList[i][3]==1&&BallList[i][4]==1){
                    BallList[i][0]++;BallList[i][1]++;
                    if(BallList[i][0]>1000) BallList[i][3]=0;  //当小球撞墙时标识符变化
                    if(BallList[i][1]>720) BallList[i][4]=0;
                    try{
                        Thread.sleep(1);
                    }catch(Exception ex){}
                    repaint();  
                }
                else if(BallList[i][3]!=1&&BallList[i][4]==1)
                {
                    BallList[i][0]--;BallList[i][1]++;
                    if(BallList[i][0]<10) BallList[i][3]=1;
                    if(BallList[i][1]>720) BallList[i][4]=0;
                    try{
                        Thread.sleep(1);
                    }catch(Exception ex){}
                    repaint();
                }
                else if(BallList[i][3]==1&&BallList[i][4]!=1){
                    BallList[i][0]++;
                    BallList[i][1]--;
                    if(BallList[i][0]>1000) BallList[i][3]=0;
                    if(BallList[i][1]<10) BallList[i][4]=1;
                    try{
                        Thread.sleep(1);
                    }catch(Exception ex){}
                    repaint();
                }
                else{
                    BallList[i][0]--;
                    BallList[i][1]--;
                    if(BallList[i][0]<10) BallList[i][3]=1;
                    if(BallList[i][1]<10) BallList[i][4]=1;
                    try{
                        Thread.sleep(1);
                    }catch(Exception ex){}
                    repaint();
                }
                for(int p=0;p<length;p++){     //小球两两判断是否相撞
                    for(int q=p+1;q<length;q++){
                if(Math.abs(BallList[p][0]-BallList[q][0])<=20&&Math.abs(BallList[p][1]-BallList[q][1])<=20){  //当两小球球心间距小于等于20时判定相撞
                  //相撞后改变标识符
                    if(BallList[p][3]==1) BallList[p][3]=0;
                    else BallList[p][3]=1;
                    if(BallList[p][4]==1) BallList[p][4]=0;
                    else BallList[p][4]=1;
                    if(BallList[q][3]==1) BallList[q][3]=0;
                    else BallList[q][3]=1;
                    if(BallList[q][4]==1) BallList[q][4]=0;
                    else BallList[q][4]=1;
                    try{
                        Thread.sleep(1);
                    }catch(Exception ex){}
                    repaint();
                }
                }
                }
        
                }
                
                }
        }
    }
    
}