J2ME中CustomItem的traverse方法使用有关问题

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

public class MainMidlet extends MIDlet {

    private static MainMidlet instance;
    private static Display display;

    public MainMidlet() {
        instance = this;
        display = Display.getDisplay(this);
        CalendarForm calendarForm = new CalendarForm("日期Form");
        display.setCurrent(calendarForm);
    }

    public void startApp() {
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }

    public static void quitApp() {
        instance.destroyApp(true);
        instance.notifyDestroyed();
        instance = null;
    }

    public static Display getDisplay() {
        return display;
    }
}