How to use Menus

How to Use Menus

A menu provides a space-saving way to let the user choose one of several options. Other components with which the user can make a one-of-many choice includecombo boxes, lists, radio buttons, spinners, and tool bars. If any of your menu items performs an action that is duplicated by another menu item or by a tool-bar button, then in addition to this section you should read How to Use Actions.

Menus are unique in that, by convention, they aren't placed with the other components in the UI. Instead, a menu usually appears either in a menu bar or as a popup menu. A menu bar contains one or more menus and has a customary, platform-dependent location — usually along the top of a window. A popup menu is a menu that is invisible until the user makes a platform-specific mouse action, such as pressing the right mouse button, over a popup-enabled component. The popup menu then appears under the cursor.

The following figure shows many menu-related components: a menu bar, menus, menu items, radio button menu items, check box menu items, and separators. As you can see, a menu item can have either an image or text, or both. You can also specify other properties, such as font and color.

 The rest of this section teaches you about the menu components and tells you how to use various menu features:

• The menu component hierarchy
• Creating menus
• Handling events from menu items
• Enabling keyboard operation
• Bringing up a popup menu
• Customizing menu layout
• The Menu API
• Examples that use menus

The Menu Component Hierarchy

Here is a picture of the inheritance hierarchy for the menu-related classes:

As the figure shows, menu items (including menus) are simply buttons. You might be wondering how a menu, if it's only a button, shows its menu items. The answer is that when a menu is activated, it automatically brings up a popup menu that displays the menu items.

Creating Menus

The following code creates the menus shown near the beginning of this menu section. The bold lines of code create and connect the menu objects; the other code sets up or customizes the menu objects. You can find the entire program in MenuLookDemo.java. Other required files are listed in the example index.

Because this code has no event handling, the menus do nothing useful except to look as they should. If you run the example, you'll notice that despite the lack of custom event handling, menus and submenus appear when they should, and the check boxes and radio buttons respond appropriately when the user chooses them.

//Where the GUI is created:
JMenuBar menuBar;
JMenu menu, submenu;
JMenuItem menuItem;
JRadioButtonMenuItem rbMenuItem;
JCheckBoxMenuItem cbMenuItem;

//Create the menu bar.
menuBar = new JMenuBar();

//Build the first menu.
menu = new JMenu("A Menu");
menu.setMnemonic(KeyEvent.VK_A);
menu.getAccessibleContext().setAccessibleDescription(
        "The only menu in this program that has menu items");
menuBar.add(menu);

//a group of JMenuItems
menuItem = new JMenuItem("A text-only menu item",
                         KeyEvent.VK_T);
menuItem.setAccelerator(KeyStroke.getKeyStroke(
        KeyEvent.VK_1, ActionEvent.ALT_MASK));
menuItem.getAccessibleContext().setAccessibleDescription(
        "This doesn't really do anything");
menu.add(menuItem);

menuItem = new JMenuItem("Both text and icon",
                         new ImageIcon("images/middle.gif"));
menuItem.setMnemonic(KeyEvent.VK_B);
menu.add(menuItem);

menuItem = new JMenuItem(new ImageIcon("images/middle.gif"));
menuItem.setMnemonic(KeyEvent.VK_D);
menu.add(menuItem);

//a group of radio button menu items
menu.addSeparator();
ButtonGroup group = new ButtonGroup();
rbMenuItem = new JRadioButtonMenuItem("A radio button menu item");
rbMenuItem.setSelected(true);
rbMenuItem.setMnemonic(KeyEvent.VK_R);
group.add(rbMenuItem);
menu.add(rbMenuItem);

rbMenuItem = new JRadioButtonMenuItem("Another one");
rbMenuItem.setMnemonic(KeyEvent.VK_O);
group.add(rbMenuItem);
menu.add(rbMenuItem);

//a group of check box menu items
menu.addSeparator();
cbMenuItem = new JCheckBoxMenuItem("A check box menu item");
cbMenuItem.setMnemonic(KeyEvent.VK_C);
menu.add(cbMenuItem);

cbMenuItem = new JCheckBoxMenuItem("Another one");
cbMenuItem.setMnemonic(KeyEvent.VK_H);
menu.add(cbMenuItem);

//a submenu
menu.addSeparator();
submenu = new JMenu("A submenu");
submenu.setMnemonic(KeyEvent.VK_S);

menuItem = new JMenuItem("An item in the submenu");
menuItem.setAccelerator(KeyStroke.getKeyStroke(
        KeyEvent.VK_2, ActionEvent.ALT_MASK));
submenu.add(menuItem);

menuItem = new JMenuItem("Another item");
submenu.add(menuItem);
menu.add(submenu);

//Build second menu in the menu bar.
menu = new JMenu("Another Menu");
menu.setMnemonic(KeyEvent.VK_N);
menu.getAccessibleContext().setAccessibleDescription(
        "This menu does nothing"); 

menuBar.add(menu);