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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 include combo boxes, lists, radio buttons, 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 the Swing components that implement each part of the menu system.

[PENDING: Labels to add: JMenuBar(in the API reference documentation), JMenu(in the API reference documentation), JMenuItem(in the API reference documentation), JCheckBoxMenuItem(in the API reference documentation), JRadioButtonMenuItem(in the API reference documentation), and JSeparator(in the API reference documentation). Leave out JPopupMenu.]

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

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

[PENDING: Show action code and point to action page.]

Here is the code that created the menus shown near the beginning of this menu section. You can find the entire program in MenuLookDemo.java. To run the program, you need to have the following image file: images/middle.gif image file. Because this code has no event handling, the menus do nothing useful except look like 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. 

//in the constructor for a JFrame subclass:
JMenuBar menuBar;
JMenu menu, submenu;
JMenuItem menuItem;
JCheckBoxMenuItem cbMenuItem;
JRadioButtonMenuItem rbMenuItem;
...
//Create the menu bar.
menuBar = new JMenuBar();
setJMenuBar(menuBar);

//Build the first menu.
menu = new JMenu("A Menu");
menuBar.add(menu);

//a group of JMenuItems
menuItem = new JMenuItem("A text-only menu item");
menu.add(menuItem);
menuItem = new JMenuItem("Both text and icon", 
new ImageIcon("images/middle.gif"));
menu.add(menuItem);
menuItem = new JMenuItem(new ImageIcon("images/middle.gif"));
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);
group.add(rbMenuItem);
menu.add(rbMenuItem);
rbMenuItem = new JRadioButtonMenuItem("Another one");
group.add(rbMenuItem);
menu.add(rbMenuItem);

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

//a submenu
menu.addSeparator();
submenu = new JMenu("A submenu");
menuItem = new JMenuItem("An item in the submenu");
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");
menuBar.add(menu);

As the code shows, to set the menu bar for a JFrame, you use the setJMenuBar method. To add a JMenu to a JMenuBar, you use the add(JMenu) method. To add menu items and submenus to a JMenu, you use the add(JMenuItem) method. These methods and more are listed in The Menu API.

Handling Events from Menu Items

To detect when the user selects a JMenuItem, you can listen for action events (just as you would for a JButton). To detect when the user selects a JRadioButtonMenuItem, you can listen for either action events or item events, as described in How to Use Radio Buttons. For JCheckBoxMenuItems, you generally listen for item events, as described in How to Use Check Boxes.

The following picture shows a program that adds event detection to the preceding example. The program's code is in MenuDemo.java. Like MenuLookDemo, MenuDemo uses the images/middle.gif image file.

Here is the code that implements the event handling: 
public class MenuDemo ... implements ActionListener, ItemListener {
    ...
    public MenuDemo() {
        ...//for each JMenuItem instance:
        menuItem.addActionListener(this);

        ...//for each JRadioButtonMenuItem: 
        rbMenuItem.addActionListener(this);

        ...//for each JCheckBoxMenuItem: 
        cbMenuItem.addItemListener(this);
        ...
    }

    public void actionPerformed(ActionEvent e) {
        ...//Get information from the action event...
        ...//Display it in the text area...
    }

    public void itemStateChanged(ItemEvent e) {
        ...//Get information from the item event...
        ...//Display it in the text area...
    }
For examples of handling action and item events, see the button, radio button, and check box sections, as well as the list of examples at the end of this section.

Bringing Up a Popup Menu

To bring up a popup menu ( JPopupMenu(in the API reference documentation)), you must register a mouse listener on each component that the popup menu should be associated with. The mouse listener must detect user requests that the popup menu be brought up. For Windows and Motif platforms, the user brings up a popup menu by pressing the right mouse button while the cursor is over a component that has a popup menu.

The mouse listener brings up the popup menu by invoking setVisible(true) on the appropriate JPopupMenu instance. The following code, taken from PopupMenuDemo.java, shows how to create and show popup menus: [PENDING: Check. This is the same mouse-press-detecting code as was used for 1.1 AWT popup menus. Is it appropriate to use the same code for all L&Fs?] 

...//where instance variables are declared:
JPopupMenu popup; 

    ...//where the GUI is constructed:
    //Create the popup menu.
    popup = new JPopupMenu();
    menuItem = new JMenuItem("A popup menu item");
    menuItem.addActionListener(this);
    popup.add(menuItem);
    menuItem = new JMenuItem("Another popup menu item");
    menuItem.addActionListener(this);
    popup.add(menuItem);

    //Add listener to components that can bring up popup menus.
    MouseListener popupListener = new PopupListener();
    output.addMouseListener(popupListener);
    menuBar.addMouseListener(popupListener);
...
class PopupListener extends MouseAdapter {
    public void mousePressed(MouseEvent e) {
        maybeShowPopup(e);
    }

    public void mouseReleased(MouseEvent e) {
        maybeShowPopup(e);
    }

    private void maybeShowPopup(MouseEvent e) {
        if (e.isPopupTrigger()) {
            popup.show(e.getComponent(),
                       e.getX(), e.getY());
        }
    }
}

Popup menus have a few interesting implementation details. One is that every menu has an associated popup menu. When the menu is activated, it uses its associated popup menu to show its menu items.

Another detail is that a popup menu itself uses another component to implement the window containing the menu items. Depending on the circumstances under which the popup menu is displayed, the popup menu might implement its "window" using a lightweight component (such as a JPanel), a "mediumweight" component (such as a Panel(in the Creating a User Interface trail)), or a heavyweight window ( Window(in the API reference documentation)).

Lightweight popup windows are more efficient than heavyweight windows, but they don't work well if you have any heavyweight components inside your GUI. Specifically, when the lightweight popup's display area intersects the heavyweight component's display area, then the heavyweight component is drawn on top. This is one of the reasons we recommend against mixing heavyweight and lightweight components. If you absolutely need to use a heavyweight component in your GUI, then you can use the JPopupMenu setLightWeightPopupEnabled method to disable lightweight popup windows. For details, see Mixing Heavy and Light Components, an article in The Swing Connection.

Customizing Menu Layout

Because menus are made up of ordinary Swing components, you can easily customize them. For example, you can add any lightweight component to a JMenu or JMenuBar. And because JMenuBar uses BoxLayout, you can customize a menu bar's layout just by adding invisible components to it. Here is an example of adding a glue component to a menu bar, so that the last menu is at the right edge of the menu bar: 

...//create and add some menus...
menuBar.add(Box.createHorizontalGlue());
...//create the rightmost menu...
menuBar.add(rightMenu);
Here is a picture of the result, which you can duplicate by compiling and running MenuGlueDemo.java:

Another way of changing the look of menus is to change the layout managers used to control them. For example, you can change a menu bar's layout manager from the default left-to-right BoxLayout to something such as GridLayout. You can also change how an activated menu or other popup menu lays out its items, as MenuLayoutDemo.java demonstrates. Here's a picture of the menu layout that MenuLayoutDemo creates:

The Menu API

The following tables list the commonly used menu constructors and methods. The API for using menus falls into these categories:

Creating and Setting Up Menu Bars
Constructor or Method Purpose
JMenuBar() Creates a menu bar.
void setJMenuBar(JMenuBar)
JMenuBar getJMenuBar()
(in JApplet, JDialog, JFrame, JRootPane) 		Sets or gets the menu bar of an applet, dialog, frame, or root pane. In the next Swing and JDK 1.2 releases, internal frames will support these methods.
void setMenuBar(JMenuBar)
JMenuBar getMenuBar()
(in JInternalFrame) 		Sets or gets the menu bar of an internal frame. In the next Swing and JDK 1.2 releases, these methods will be deprecated, and you should use setJMenuBar/getJMenuBar instead.

Creating and Populating Menus
Constructor or Method Purpose
JMenu() Creates a menu.
JMenuItem add(JMenuItem)
JMenuItem add(Action)
void add(String) 		Adds a menu item to the end of the menu. If the argument is an Action object, then the menu creates a menu item as described in How to Use Actions. If the argument is a string, then the menu automatically creates a JMenuItem object that displays the specified text.
void addSeparator() Adds a separator to the end of the menu.
JMenuItem insert(JMenuItem, int)
JMenuItem insert(Action, int)
void insert(String, int)
void insertSeparator(int) 		Inserts a menu item or separator into the menu at the specified position. The first menu item is at position 0, the second at position 1, and so on. The JMenuItem, Action, and String arguments are treated the same as in the corresponding add methods.
void remove(JMenuItem)
void remove(int)
void removeAll() 		Removes the specified item(s) from the menu. If the argument is an integer, then it specifies the position of the menu item to be removed.

Creating and Populating Popup Menus
Constructor or Method Purpose
JPopupMenu()
JPopupMenu(String) 		Creates a popup menu. The optional string argument specifies the title that a look and feel might display as part of the popup window. [PENDING: check]
JMenuItem add(JMenuItem)
JMenuItem add(Action) 		Adds a menu item to the end of the popup menu. If the argument is an Action object, then the popup menu creates a menu item as described in How to Use Actions.
void addSeparator() Adds a separator to the end of the popup menu.
void insert(Component, int)
JMenuItem insert(Action, int) 		Inserts a menu item into the menu at the specified position. The first menu item is at position 0, the second at position 1, and so on. The Component argument specifies the menu item to add. The Action argument is treated the same as in the corresponding add method.
void remove(JMenuItem)
void remove(int)
void removeAll() 		Removes the specified item(s) from the menu. If the argument is an integer, then it specifies the position of the menu item to be removed.
static void setDefaultLightWeightPopupEnabled(boolean) By default, Swing implements a menu's window using a lightweight component. This can cause problems if you use any heavyweight components in your Swing program, as described in Bringing Up a Popup Menu. (This is one of several reasons to avoid using heavyweight components.) As a workaround, invoke JPopupMenu.setDefaultLightWeightPopupEnabled(false).
void show(Component, int, int) Display the popup menu at the specified X,Y position (specified in that order by the integer arguments) in the coordinate system of the specified component.

Implementing Menu Items
Constructor or Method Purpose
JMenuItem()
JMenuItem(Icon)
JMenuItem(String)
JMenuItem(String, Icon)
JMenuItem(String, int) 		Creates an ordinary menu item. The icon argument, if present, specifies the icon that the menu item should display. Similarly, the string argument specifies the text that the menu item should display. The integer argument specifies the keyboard mnemonic to use. You can specify any of the relevant VK constants defined in the KeyEvent(in the API reference documentation) class. For example, to specify 'a' as the constant, you can use KeyEvent.VK_A.
JCheckBoxMenuItem()
JCheckBoxMenuItem(Icon)
JCheckBoxMenuItem(String)
JCheckBoxMenuItem(String, Icon)
JCheckBoxMenuItem(String, boolean)
JCheckBoxMenuItem(String, Icon, boolean) 		Creates a menu item that looks and acts like a check box. If you specify an icon, then the menu item uses that icon instead of the default check box icons. [PENDING: CHECK] The string argument, if any, specifies the text that the menu item should display. If you specify true for the boolean argument, then the menu item is initially selected (checked). Otherwise, the menu item is initially unselected.
JRadioButtonMenuItem()
JRadioButtonMenuItem(Icon)
JRadioButtonMenuItem(String)
JRadioButtonMenuItem(String, Icon) 		Creates a menu item that looks and acts like a radio button. If you specify an icon, then the menu item uses that icon instead of the default radio button icons. [PENDING: CHECK] The string argument, if any, specifies the text that the menu item should display. The menu item is initially unselected.
void setState(boolean)
boolean getState() 		Set or get the selection state of a menu item.
void setEnabled(boolean) If the argument is true, enable the menu item. Otherwise, disable the menu item.
void setMnemonic(char) Set the keyboard alternative to choosing the menu item with the mouse.
void setActionCommand(String) Set the name of the action performed by the menu item.
void addActionListener(ActionListener)
void addItemListener(ItemListener) 		Add an event listener to the menu item. See Handling Events from Menu Items for details.
Many of the preceding methods are inherited from AbstractButton. See The Button API for information about other useful methods that AbstractButton provides.

Examples that Use Menus

Menus are used in a few Swing examples.

Example Where Described Notes
MenuLookDemo.java This section (Creating Menus) A simple example that creates all kinds of menus except popup menus, but doesn't handle events from the menu items.
MenuDemo.java This section (Handling Events from Menu Items) Adds event handling to MenuLookDemo.
MenuGlueDemo.java This section (Customizing Menu Layout) Demonstrates affecting menu layout by adding an invisible components to the menu bar.
MenuLayoutDemo.java This section (Customizing Menu Layout) Implements sideways-opening menus arranged in a vertical menu bar.
ActionDemo.java How to Use Actions Uses Action objects to implement menu items that duplicate functionality provided by tool bar buttons.
Framework.java [PENDING: nowhere yet; may go away]
InternalFrameDemo.java How to Use Internal Frames Uses a menu item to create windows.

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