【正文】 ociated with that index, the getComponentAtIndex( ) accessor returns null. Theponent property returns a reference to this (., the menu bar itself)。sJMenuBar class supersedes the AWTMenuBar class. This class creates a horizontal menu bar ponent with zero or more menus attached to it. JMenuBar uses theDefaultSingleSelectionModel as its data model because the user can raise, or activate , only one of its menus at a given time. Once the mouse pointer leaves that menu, the class removes the menu from the screen (or cancels it, in Swing lingo), and all menus again bee eligible to be raised Figure 144 shows the class hierarchy for the JMenuBar ponent. Figure 144. JMenuBar class diagram You can add JMenu objects to the menu bar with theadd( ) method of theJMenuBar class. JMenuBar then assigns an integer index based on the order in which the menus were added. The menu bar displays the menus from left to right on the bar according to their assigned index. In theory, there is one exception: the help menu. You are supposed to be allowed to mark one menu as the help menu。 ( ).add(menuBar, )。 Figure 144 JMenuBar 類圖 程序員可以用 JMenuBar 類的 add()方法添加 JMenu 對(duì)象。尤其要注意的是， Aqua 人機(jī)界面規(guī)范（ Aqua Human Interface Guidelines）要求菜單欄始終可見。 屬性 JMenuBar 類的屬性如表 143 所示。程序員隨后便能用此 Action類創(chuàng)建菜單項(xiàng)，自動(dòng)地設(shè)置菜單項(xiàng)的文字、圖像、快捷方式，等等。本章之前介紹的 和稍后將介紹的 都使用上述方法。當(dāng)然，如果沒有強(qiáng)制性的理由，程序員是不會(huì)照此辦理的。不管怎樣，程序員在使用 Swing 菜單時(shí)千萬不要混淆 setJMenuBar()方法和老版的 AWT setMenuBar()方法，否則編譯器一定吃 不消。t possible to construct a JMenuItem from a n Action directly, adding theAction to a JMenu or JPopupMenu had the same effect: the menu would create and configure an appropriateJMenuItem for you. 6 菜單和工具欄 摘自《 Java Swing》 （ 第二版 ） Marc Loy,Robert Eckstein等著 JMenuBar 類 Swing 的 JMenuBar類是 AWT的 MenuBar類的替代產(chǎn)品 ， 它創(chuàng)建一個(gè)水平菜單欄組件 ， 其中可連接零個(gè)以上菜單。s internal Insets accordingly). Both JApplet and JDialog contain a setJMenuBar( ) method— this means that you can add menu bars to both applets and dialogs. Either way, be sure not to confuse the setJMenuBar( ) method with the older setMenuBar( ) method of AWT when working with Swing menus, or the piler