[Chapter 10] 10.7 MenuBar

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| Chapter 10 Would You Like to Choose from the Menu? |

## 10.7 MenuBar

The `MenuBar` is the component you add to the `Frame` that is displayed on the top line of the `Frame`; the `MenuBar` contains menus. A `Frame` can display only one `MenuBar` at a time. However, you can change the `MenuBar` based on the state of the program so that different menus can appear at different points. The `MenuBar` class extends `MenuComponent` and implements the `MenuContainer` interface.

A `MenuBar` can be used only as a child component of a `Frame`. An applet cannot have a `MenuBar` attached to it, unless you implement the whole thing yourself. Normally, you cannot modify the `MenuBar` of the applet holder (the browser), unless it is Java based. In other words, you cannot affect the menus of Netscape Navigator, but you can customize appletviewer and HotJava, as shown in the following code with the result shown in Figure 10.4. The `getTopLevelParent()` method was introduced in Window with `Window`.

    import java.awt.*;
    public class ChangeMenu extends java.applet.Applet {
        public void init ()  {
            Frame f = ComponentUtilities.getTopLevelParent(this);
            if (f != null) {
                MenuBar mb = f.getMenuBar();
                Menu m = new Menu ("Cool");
                mb.add (m);
            }
        }
    }

#### Figure 10.4: Customizing appletviewer's MenuBar

> NOTE:

When you add a `MenuBar` to a `Frame`, it takes up space that is part of the drawing area. You need to get the top insets to find out how much space is occupied by the `MenuBar` and be careful not to draw under it. If you do, the `MenuBar` will cover what you draw.

### MenuBar Methods

Constructors

public MenuBar()

  The `MenuBar` constructor creates an empty `MenuBar`. To add menus to the `MenuBar`, use the `add()`method.

Menus

public int getMenuCount ()
public int countMenus ()

  The `getMenuCount()` method returns the number of top-level menus within the `MenuBar`.

  `countMenus()` is the Java 1.0 name for this method.

public Menu getMenu (int index)

  The `getMenu()` method returns the `Menu` at position `index`. If `index` is invalid, `getMenu()` throws the run-time exception `ArrayIndexOutOfBoundsException`.

public synchronized Menu add (Menu m)

  The `add()` method puts choice `m` on the `MenuBar`. The label used to create `m` is displayed on the `MenuBar`. If `m` is already in another `MenuBar`, it is removed from it. The order of items added determines the order displayed on the `MenuBar`, with one exception: if a menu is designated as a help menu by `setHelpMenu()`, it is placed at the right end of the menu bar. Only a `Menu` can be added to a `MenuBar`; you can't add a `MenuItem`. In other words, a `MenuItem` has to lie under at least one menu.

public synchronized void remove (int index)

  The `remove()` method removes the `Menu` at position `index` from the `MenuBar`. If `index` is invalid, `remove()` throws the `ArrayIndexOutOfBoundsException` run-time exception. `index` is zero based.

public synchronized void remove (MenuComponent component)

  This version of `remove()` removes the menu `component` from the `MenuBar`. If `component` is not in `MenuBar`, nothing happens. The system calls this method when you add a new `Menu` to make sure it does not exist on another `MenuBar`.

Shortcuts

public MenuItem getShortcutMenuItem (MenuShortcut shortcut)

  The `getShortcutMenuItem()` method retrieves the `MenuItem` associated with the `MenuShortcut` `shortcut`. If `MenuShortcut` does not exist for this `Menu`, the method returns `null`. `getShortcutMenuItem()` walks through the all submenus recursively to try to find `shortcut`.

public synchronized Enumeration shortcuts()

  The `shortcuts()` method retrieves an `Enumeration` of all the `MenuShortcut` objects associated with this `MenuBar`.

public void deleteShortcut (MenuShortcut shortcut)

  The `deleteShortcut()` method removes `MenuShortcut` from the associated `MenuItem` in the `MenuBar`. If the shortcut is not associated with any menu item, nothing happens.

Help menus

It is the convention on many platforms to display help menus as the last menu on the `MenuBar`. The `MenuBar` class lets you designate one of the menus as this special menu. The physical position of a help menu depends on the platform, but those giving special treatment to help menus place them on the right. A `Menu` designated as a help menu doesn't have to bear the label "Help"; the label is up to you.

public Menu getHelpMenu ()

  The `getHelpMenu()` method returns the `Menu` that has been designated as the help menu with `setHelpMenu()`. If the menu bar doesn't have a help menu, `getHelpMenu()` returns `null`.

public synchronized void setHelpMenu (Menu m)

  The `setHelpMenu()` method sets the menu bar's help menu to `m`. This makes `m` the rightmost menu on the `MenuBar`, possibly right justified. If `m` is not already on the `MenuBar`, nothing happens.

Peers

public synchronized void addNotify ()

  The `addNotify()` method creates the `MenuBar` peer with all the menus on it, and in turn their menu items.

public synchronized void removeNotify ()

  The `removeNotify()` method destroys the peer of the `MenuBar` and removes it from the screen. The peers of the items on the `MenuBar` are also destroyed.

### MenuBar Events

A `MenuBar` does not generate any events.

CheckboxMenuItem |  | Putting It All Together