[Chapter 10] 10.5 Menu

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

## 10.5 Menu

`Menu`s are the pull-down objects that appear on the `MenuBar` of a `Frame` or within other menus. They contain `MenuItem`s or `CheckboxMenuItem`s for the user to select. The `Menu` class subclasses `MenuItem` (so it can appear on a `Menu`, too) and implements `MenuContainer`. Tear-off menus are menus that can be dragged, placed elsewhere on the screen, and remain on the screen when the input focus moves to something else. Java supports tear-off menus if the underlying platform does. Motif (UNIX) supports tear-off menus; Microsoft Windows platforms do not.

### Menu Methods

Constructors

public Menu ()

  The first constructor for `Menu` creates a menu that has no label and cannot be torn off. To set the label at a later time, use `setLabel()`.

public Menu (String label)

  This constructor for `Menu` creates a `Menu` with `label` displayed on it. The `Menu` cannot be torn off.

public Menu (String label, boolean tearOff)

  This constructor for `Menu` creates a `Menu` with `label` displayed on it. The handling of `tearOff` is platform dependent.

  Figure 10.3 shows a tear-off menu for Windows NT/95 and Motif. Since Windows does not support tear-off menus, the Windows menu looks and acts like a regular menu.

#### Figure 10.3: Tear-off menu

Items

public int getItemCount()
public int countItems ()

  The `getItemCount()` method returns the number of items within the `Menu`. Only top-level items are counted: if an item is a submenu, this method doesn't include the items on it.

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

public MenuItem getItem (int index)

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

public synchronized MenuItem add (MenuItem item)

  The `add()` method puts `item` on the menu. The label assigned to `item` when it was created is displayed on the menu. If `item` is already in another menu, it is removed from that menu. If `item` is a `Menu`, it creates a submenu. (Remember that `Menu` subclasses `MenuItem`.)

public void add (String label)

  This version of `add()` creates a `MenuItem` with `label` as the text and adds that to the menu. If `label` is the `String` "-", a separator bar is added to the `Menu`.

public synchronized void insert(MenuItem item, int index)

  The `insert()` method puts `item` on the menu at position `index`. The label assigned to `item` when it was created is displayed on the menu. Positions are zero based, and if `index` < 0, `insert()` throws the `IllegalArgumentException` run-time exception.

public synchronized void insert(String label, int index)

  This version of `insert()` method creates a `MenuItem` with `label` as the text and adds that to the menu at position `index`. If `label` is the String "--", a separator bar is added to the `Menu`. Positions are zero based, and if `index` < 0, this method throws the `IllegalArgumentException` run-time exception.

public void addSeparator ()

  The `addSeparator()` method creates a separator `MenuItem` and adds that to the menu. Separator menu items are strictly cosmetic and do not generate events when selected.

public void insertSeparator(int index)

  The `insertSeparator()` method creates a separator `MenuItem` and adds that to the menu at position `index`. Separator menu items are strictly cosmetic and do not generate events when selected. Positions are zero based. If `index` < 0, `insertSeparator()` throws the `IllegalArgumentException` run-time exception.

public synchronized void remove (int index)

  The `remove()` method removes the `MenuItem` at position `index` from the `Menu`. If `index` is invalid, `remove()` throws the `ArrayIndexOutOfBoundsException` run-time exception. `index` is zero based, so it can range from 0 to `getItemCount()-1`.

public synchronized void remove (MenuComponent component)

  This version of `remove()` removes the menu item `component` from the `Menu`. If component is not in the `Menu`, nothing happens.

public synchronized void removeAll()

  The `removeAll()` removes all `MenuItems` from the `Menu`.

Peers

public synchronized void addNotify ()

  The `addNotify()` method creates the `Menu` peer with all the `MenuItems` on it.

public synchronized void removeNotify ()

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

Miscellaneous methods

public boolean isTearOff ()

  The `isTearOff()` method returns `true` if this `Menu` is a tear-off menu, and `false` otherwise. Once a menu is created, there is no way to change the tear-off setting. This method can return `true` even on platforms that do not support tear-off menus.

public String paramString ()

  The `paramString()` method of `Menu` should be protected like other `paramString()` methods. However, it is public so you have access to it. When you call the `toString()` method of a `Menu`, the default `toString()` method of `MenuComponent` is called. This in turn calls `paramString()`, which builds up the string to display. At the `Menu` level, the setting for `TearOff` (from constructor) and whether or not it is the help menu (from `MenuBar.setHelpMenu()`) for the menu bar are added. If the constructor for the `Menu` was `new Menu (`File`)`, the results of `toString()` would be:

    java.awt.Menu [menu0,label=File,tearOff=false,isHelpMenu=false]

### Menu Events

A `Menu` does not generate any event when it is selected. An event is generated when a `MenuItem` on the menu is selected, as long as it is not another `Menu`. You can capture all the events that happen on a `Menu` by overriding `postEvent()`.

MenuItem |  | CheckboxMenuItem