程序清单如下:
package cn.china.gaoji1;

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

/**
* @author a
*/
public class gaoji1 extends MIDlet implements CommandListener
{
private Display display;
private TextBox textbox;
private Command exit;

public void checkcolor()
{
display=Display.getDisplay(this);
exit=new Command("退出",Command.SCREEN,1);
String message=null;

if(display.isColor())
{
message="彩色显示屏";
}
else
{
message="黑白显示屏";
};

textbox=new TextBox("检验屏幕是否是彩色显示屏",message,17,0);
textbox.addCommand(exit);
textbox.setCommandListener(this);
}

public void startApp()
{
display.setCurrent(textbox);
}

public void pauseApp()
{
}

public void destroyApp(boolean unconditional)
{
}

public void commandAction(Command command,Displayable displayable)
{
if(command==exit)
{
destroyApp(true);
notifyDestroyed();
}
}
}

编译通过,但是运行时出现问题

报告如下:
Starting emulator in execution mode
正在通过存储根 C:\Documents and Settings\a\j2mewtk\2.5.2\appdb\DefaultColorPhone 来运行
Running with locale: Chinese_People's Republic of China.936
Running in the identified_third_party security domain
startApp threw an Exception
java.lang.NullPointerException
java.lang.NullPointerException
at cn.chinamobile.gaoji1.gaoji1.startApp(gaoji1.java:43)
at javax.microedition.midlet.MIDletProxy.startApp(MIDletProxy.java:43)
at com.sun.midp.midlet.Scheduler.schedule(Scheduler.java:374)
at com.sun.midp.main.Main.runLocalClass(Main.java:466)
at com.sun.midp.main.Main.main(Main.java:120)

请问各位高手,是什么问题?该如何修改使其在模拟器上运行正常.

开发环境是NETBEANS IDE6.5

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