List界面无法显示啊~~新手求教

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*/

package com.mygame.GoBang;
import javax.microedition.lcdui.*;
import javax.microedition.midlet.MIDlet;
/**
*
* @author Administrator
*/
public class GoBang extends MIDlet implements CommandListener{

private static GoBang instance=null;
private Display display=null;
public static final String[] titles={"Connect to net work","Play with Computer","settings"};
private List menu=null;
/*public static final String titles1="网络对战";
public static final String titles2="和电脑对战";
public static final String titles3="关于";*/
public void startApp(){
if(display==null){
display=Display.getDisplay(this);
instance=this;
menu=new List("GoBang",List.IMPLICIT,titles,null);
/*menu.append(titles1, null);
menu.append(titles2, null);
menu.append(titles3, null);*/
menu.setCommandListener(this);
menu.addCommand(new Command("EXIT",Command.EXIT,1));
}
display.setCurrent(menu);
}

public Displayable getMenu(){
return menu;
}

public static void SetCurrent(Alert alert,Displayable displayable){
instance.display.setCurrent(alert,displayable);
}

public static void SetCurrent(Displayable displayable){
instance.display.setCurrent(displayable);
}

public void pauseApp(){
}

public void destroyApp(boolean unconditional){
}

public void commandAction(Command command,Displayable displayable){
if(command.getCommandType()==Command.EXIT){
destroyApp(false);
notifyDestroyed();
}else if(command==List.SELECT_COMMAND){
int index=menu.getSelectedIndex();
if(index==0){
Form form=new Form("连接服务器");
form.append("Pleasewait....application is connecting to another game server");
display.setCurrent(form);
new Thread(new GameEngine(this)).start();
}else if(index==1){ //与电脑对战
new Thread(new GameEngine(this)).start();
}else{
Form form=new Form("关于作者");
form.append("此软件供HNU08级计算机系内部使用"+"\n");
form.append("作者：");
}
}
}
}