how to close app with list choice?

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JavaSE

J2ME

hi i wrote this code and created a list and i want to close app when
user choose Exit Game but it doesn't work!
what is wrong with this code?

package hello;

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

public class Midlet extends MIDlet implements CommandListener {

    private Display display;
    
    List Menu = new List("Choose One:", List.IMPLICIT);
    List Level_choose = new List("Choose One:", List.IMPLICIT);
    
    
    
  

    public void startApp() {
        display = Display.getDisplay(this);
        try {
            
            this.Menu.append("Start Game",null);
            
            this.Menu.append("Top Scores",null);
           
            this.Menu.append("About us",null);
            
            this.Menu.append("Exit Game",null);
            Menu.setCommandListener(this);
            display.setCurrent(Menu);
            Level_choose.setCommandListener(this);

        } catch (Exception e) {
            e.printStackTrace();
        }





    }

    public Display getDisplay() {
        return display;
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
        notifyDestroyed();
    }

    public void exit() {
        System.gc();
        destroyApp(false);
        notifyDestroyed();
    }

    public void commandAction(Command c, Displayable d) {
        List li = (List) display.getCurrent();

        switch (li.getSelectedIndex()) {
            case 0:
                display.setCurrent(this.Level_choose);
                this.choosed = true;
                break;
            case 1:

            case 2:
                break;
            case 3:
                exit();
                

        }
       

        
    }
}