JavaFX - 如何创建菜单栏和子菜单项

import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.control.CheckMenuItem;
import javafx.scene.control.ContextMenu;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.control.RadioMenuItem;
import javafx.scene.control.SeparatorMenuItem;
import javafx.scene.control.ToggleGroup;
import javafx.scene.input.MouseButton;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.BorderPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class Main extends Application {
  private MenuItem exitMenuItem() {
    MenuItem exitMenuItem = new MenuItem("Exit");
    exitMenuItem.setOnAction(actionEvent -> Platform.exit());

    return exitMenuItem;
  }
  private Menu fileMenu() {
    Menu fileMenu = new Menu("File");
    MenuItem newMenuItem = new MenuItem("New");
    MenuItem saveMenuItem = new MenuItem("Save");
    fileMenu.getItems().addAll(newMenuItem, saveMenuItem,
        new SeparatorMenuItem(), exitMenuItem());
    return fileMenu;
  }
  private Menu cameraMenu() {
    Menu cameraMenu = new Menu("Cameras");

    CheckMenuItem cam1MenuItem = new CheckMenuItem("Show Camera 1");
    cam1MenuItem.setSelected(true);

    CheckMenuItem cam2MenuItem = new CheckMenuItem("Show Camera 2");
    cam2MenuItem.setSelected(true);

    cameraMenu.getItems().addAll(cam1MenuItem, cam2MenuItem);

    return cameraMenu;
  }
  private Menu alarmMenu() {
    Menu alarmMenu = new Menu("Alarm");
    RadioMenuItem soundAlarmItem = new RadioMenuItem("Sound Alarm");
    RadioMenuItem stopAlarmItem = new RadioMenuItem("Alarm Off");

    Menu contingencyPlans = new Menu("Plans");
    contingencyPlans.getItems().addAll(new CheckMenuItem("A"),
        new CheckMenuItem("B"), new CheckMenuItem("C"));
    alarmMenu.getItems().add(contingencyPlans);
    ToggleGroup tGroup = new ToggleGroup();
    soundAlarmItem.setToggleGroup(tGroup);
    stopAlarmItem.setToggleGroup(tGroup);
    stopAlarmItem.setSelected(true);
    alarmMenu.getItems().addAll(soundAlarmItem, stopAlarmItem,
        new SeparatorMenuItem());
    return alarmMenu;
  }
  @Override
  public void start(Stage primaryStage) {
    BorderPane root = new BorderPane();
    Scene scene = new Scene(root, 300, 250, Color.WHITE);
    MenuBar menuBar = new MenuBar();
    menuBar.getMenus().addAll(fileMenu(), cameraMenu(), alarmMenu());
    root.setTop(menuBar);
    ContextMenu contextFileMenu = new ContextMenu(exitMenuItem());

    primaryStage.addEventHandler(MouseEvent.MOUSE_CLICKED, (MouseEvent me) -> {
      if (me.getButton() == MouseButton.SECONDARY || me.isControlDown()) {
        contextFileMenu.show(root, me.getScreenX(), me.getScreenY());
      } else {
        contextFileMenu.hide();
      }
    });
    primaryStage.setScene(scene);
    primaryStage.show();
  }
  public static void main(String[] args) {
    launch(args);
  }
}