坦克大战为什么药两个repaint
小弟我最近学到韩顺平视频线程这，有一个疑问请各位豪侠：MyPanel的事件处理KeyListener中已有repaint函数，为什么还要把MyPanel做成线程，在run中再写一个repaint函数，才能把子弹重绘？
package game.tank;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Vector;

public class DrawTank extends JFrame
{
  Mypanel mypanel ;

  public DrawTank()
  {
  mypanel = new Mypanel();
  mypanel.setBackground(Color.black);
   
  Thread t = new Thread(mypanel);
  t.start();
   
  this.add(mypanel);
  this.addKeyListener(mypanel);
  this.setSize(500, 400);
  this.setLocation(500, 350);
  this.setVisible(true);
  this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }

  public static void main(String[] args)
  {
  new DrawTank();
  }
}

// 定义我的面板
class Mypanel extends JPanel implements KeyListener, Runnable
{
  Mytank mytank;
  // 定义敌人的tank数组
  Vector<Enemytank> enemytank = new Vector<Enemytank>();
  // 定义敌人tank出现的数量
  int etSize = 3;

  public Mypanel()
  {
  mytank = new Mytank(200, 340);
  mytank.setColor(0);
  for (int i = 0; i < etSize; i++)
  {
  Enemytank et = new Enemytank((i + 1) * 110, 0);
  et.setColor(1);
  et.setDirect(2);
  enemytank.add(et);
  }
  }

  public void paint(Graphics g)
  {
  super.paint(g);
  g.setColor(Color.cyan);
  this.draw(mytank.getX(), mytank.getY(), g, this.mytank.direct, 0);
  // 画出子弹
  if (mytank.b != null)
  {
  g.draw3DRect(mytank.b.x, mytank.b.y, 1, 1, false);
  }

  // 画出敌人的坦克
  for (int i = 0; i < enemytank.size(); i++)
  {

  this.draw(enemytank.get(i).getX(), enemytank.get(i).getY(), g,
  enemytank.get(i).getDirect(), 1);
  }

  }

  public void draw(int x, int y, Graphics g, int direct, int type)
  {
  switch (type)
  {
  case 0:
  g.setColor(Color.cyan);
  break;
  case 1:
  g.setColor(Color.yellow);
  break;
  }
  switch (direct)
  {
  // 向上
  case 0:
  g.fill3DRect(x, y, 5, 30, false);
  g.fill3DRect(x + 15, y, 5, 30, false);
  g.fill3DRect(x + 5, y + 5, 10, 20, false);
  g.fillOval(x + 5, y + 10, 10, 10);
  g.drawLine(x + 10, y + 15, x + 10, y);
  break;
  // 向右
  case 1:
  g.fill3DRect(x, y, 30, 5, false);
  g.fill3DRect(x, y + 15, 30, 5, false);
  g.fill3DRect(x + 5, y + 5, 20, 10, false);
  g.fillOval(x + 10, y + 4, 10, 10);
  g.drawLine(x + 15, y + 10, x + 30, y + 10);
  break;
  // 向下
  case 2:
  g.fill3DRect(x, y, 5, 30, false);
  g.fill3DRect(x + 15, y, 5, 30, false);
  g.fill3DRect(x + 5, y + 5, 10, 20, false);
  g.fillOval(x + 4, y + 10, 10, 10);