swing 飞机大战 二 实现地图的滚动

我的想法是先创建一个，保存图片和y坐标，主要就是靠画图片的y坐标来制造地图向下移动的感觉

创建地图类，4张背景图片可供选择

public class Map{
	public int y;
	public ImageIcon img1;
	public ImageIcon img2;
	public ImageIcon img3;
	public ImageIcon img4;
	public Map(int y){
		this.y = y;
		img1 = new ImageIcon("img/bj-1.jpg");
		img2 = new ImageIcon("img/bj-2.jpg");
		img3 = new ImageIcon("img/bj-3.jpg");
		img4 = new ImageIcon("img/bj-4.jpg");
	}
}

在创建地图对象，需要两个对象，轮换刷图，当一个向下滚动看不见后，就将它的y坐标改成负的，通过线程，增加y值

package pass;

import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.util.Vector;

import javax.swing.JFrame;
import javax.swing.JPanel;

import Game.FoeFly;
import Game.Map;
import Game.MyFly;

/**
 * 
 * @author Administrator
 *关卡类，
 */
public class No1 { //初始化面板
	PanelNo1 mp;
	public No1(JFrame jf){
		mp = new PanelNo1();
		jf.add(mp);
		jf.setSize(400, 600);
		mp.setBounds(0, 0, 500, 600);
	}
}
class PanelNo1 extends JPanel implements Runnable{
	MyFly myfly;
	Vector foefly;
	Vector