一、我的思路

- 创建窗体、设定窗体大小

- 在窗体中添加画布，并将画布添加至线程

- 线程run()方法持续执行，坐标不断变化，画布随之重绘，形成小球下落的效果，并随时判断小球X轴、Y轴坐标，触底后，通过flag值调整坐标加减方向

代码一

 1 import java.awt.*;
 2 public class BallHitWall{
 3     public static void main(String args[]){
 4         Frame w = new Frame();
 5         w.setSize(1024,750);
 6 
 7         MyPanelBall mpb = new MyPanelBall();
 8         w.add(mpb);
 9 
10         Thread t = new Thread(mpb);
11         t.start();
12     
13         w.setVisible(true);
14         
15     }
16 }
17 
18 class MyPanelBall extends Panel implements Runnable{
19     int x=10,y=10,flag=0;
20     public void paint(Graphics g){
21         g.setColor(Color.red);
22         g.fillOval(x,y,10,10);
23     }
24     public void run(){
25         while(true){
26             switch(flag){
27                 case 0: x++;y++;
28                 case 1: x++;y--;
29                 case 2: x--;y++;
30             }
31             if(y>750){
32                 flag=1;
33             }
34             if(x>1024){
35                 flag=2;
36             }
37             if(x<2||y<2){
38                 flag=0;
39             }
40             System.out.println(flag);
41             System.out.println("x:"+x);
42             System.out.println("y:"+y);
43             try{
44                 Thread.sleep(10);
45             }catch(Exception e){}
46             repaint();
47         }
48     }
49 }

执行结果

小球触底后没有反弹，X\Y轴坐标一直停留在751没变化

冥思苦想无果，下班路上走了一圈，以为是flag重新赋值的代码块位置不对，调整至switch前面，运行，结果依旧。。。查百度——switch用错了？

https://m.runoob.com/java/java-switch-case.html

Break,break,break……

改代码如下：

 1 import java.awt.*;
 2 public class BallHitWall{
 3     public static void main(String args[]){
 4         Frame w = new Frame();
 5         w.setSize(1024,750);
 6 
 7         MyPanelBall mpb = new MyPanelBall();
 8         w.add(mpb);
 9 
10         Thread t = new Thread(mpb);
11         t.start();
12     
13         w.setVisible(true);
14         
15     }
16 }
17 
18 class MyPanelBall extends Panel implements Runnable{
19     int x=10,y=10,flag=0;
20     public void paint(Graphics g){
21         g.setColor(Color.red);
22         g.fillOval(x,y,10,10);
23     }
24     public void run(){
25         while(true){
26             switch(flag){
27                 case 0: x++;y++;
28                 case 1: x++;y--;
29                 case 2: x--;y++;
30             }
31             if(y>750){
32                 flag=1;
33             }
34             if(x>1024){
35                 flag=2;
36             }
37             if(x<2||y<2){
38                 flag=0;
39             }
40             System.out.println(flag);
41             System.out.println("x:"+x);
42             System.out.println("y:"+y);
43             try{
44                 Thread.sleep(10);
45             }catch(Exception e){}
46             repaint();
47         }
48     }
49 }

到底是什么问题

 1 import java.awt.*;
 2 public class BallHitWall{
 3     public static void main(String args[]){
 4         Frame w = new Frame();
 5         w.setSize(400,300);
 6 
 7         MyPanelBall mpb = new MyPanelBall();
 8         w.add(mpb);
 9 
10         Thread t = new Thread(mpb);
11         t.start();
12 
13         w.setVisible(true);
14 
15     }
16 }
17 
18 class MyPanelBall extends Panel implements Runnable{
19     int x=0,y=0,flag=0;
20     public void paint(Graphics g){
21         g.setColor(Color.red);
22         g.fillOval(x,y,10,10);
23     }
24     public void run(){
25         while(true){
26             switch(flag){
27                 case 0: x++;y++;break;
28                 case 1: x++;y--;break;
29                 case 2: x--;y--;break;
30         case 3: x--;y++;
31             }
32         if(y>=300){
33             if(x>=300){
34                 flag=1;
35             }
36 
37         }
38             if(x>=290&&x<=390&&y>=290){
39                 flag=1;
40             }
41             if(x>=390&&y>=190){
42                 flag=2;
43             }
44             if(y<=1&&x>=200&&x<=390){
45                 flag=3;
46             }
47             if(y<=290&&x<=1){
48                 flag=0;
49             }
50             System.out.println(flag);
51             System.out.println("x:"+x);
52             System.out.println("y:"+y);
53             try{
54                 Thread.sleep(10);
55             }catch(Exception e){}
56             repaint();
57         }
58     }
59 }