Swing Menu System

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The Swing menu system is designed as a superset of the AWT menu system. The changes that have been made are designed to take advantage of the capabilities of lightweight components while simplifying the menu class hierarchy and increasing maintainability. This document describes the design of the Swing menu system as a whole.

Comprehensive documentation for the Swing menu system is currently under development. In this document, the individual components of the Swing menu system are briefly described in detail in their own specifications. Consult the menu-related JavaDoc APIs for current details.

The Problem

The 1.1 AWT provides a simple but fairly restrictive set of capabilities for menus. In most cases, the restrictions were necessary because of restrictions in the native windowing systems' menu controls. The basis of the 1.1 AWT menu class hierarchy is the abstract superclass MenuComponent. Various problem arise as a result of this design:

MenuComponent duplicates much of the functionality found in the Component class.
Where it neglects to duplicate, Component methods are simply not supported.
Maintenance is a nightmare because methods must be added or fixed in multiple hierarchies

Goals

The goals of the Swing menu design were:

To define a superset API for menu bars, menus, menu items, and popup menus.
To provide code examples of how these classes are used.
To provide external events for state changes within the menu system (something that developers have asked for).
To include an easy, automated method for end-user customization of menus.
To provide a design for a menu-merging API

An Overview of the Menu System

The section describes the menu system at a high level. For specific information on individual component classes, see their respective documents.

A set of menus is usually used in an application to give the end user a kind of "table of contents" for actions that are available to him or her. A menu bar is most often displayed at the top of the application's main window (JFrame). It presents a set of menus that represent the main categories or groupings of actions. The actions themselves are usually presented as menu items on the menus, but in some cases they are grouped together on submenus that are subordinate to the top-level menus. The Swing menu system is made up of several component classes:

JMenuBar: A container for menus. This class is used as a sort of table of contents for actions available in an application.
JMenuItem: A button-like component that fires an action event when selected by the application user.
JCheckboxMenuItem: A checkbox- like menu item.
JRadioButtonMenuItem: A radio button-like menu item.
JPopupMenu: A free-floating window that may contain menu items and/or other components.
JMenu: A menu item that automatically pops up a JPopupMenu.

These classes are used to create a menu hierarchy. This can be thought of in terms of a containment hierarchy, but has one important difference: Menus contain their children in a separate physical window. This arrangement tends to wreak havoc on our traditional ideas of what happens in the containment relation. At the same time, it is extremely useful to deal with the menu hierarchy as if it were a containment hierarchy.

Sample Code

Creating menus in Swing is very similar to creating menus using AWT. This example shows a simple case in which AWT code has been converted to Swing by adding a "J" to the menu class names:

   JMenuBar menuBar = new JMenuBar();
   JMenu menu = new JMenu("Roman");
   menuBar.add(menu);
	menu.add(new JMenuItem("Caesar"));
	menu.add(new JMenuItem("Catullus"));
	JMenu test2 = (JMenu) menu.add(new JMenu("Horace"));
	test2.add(new JCheckboxMenuItem("Odes"));
	test2.add(new JCheckboxMenuItem("Epodes"));
	
   menu = new JMenu("Swedish");
   menuBar.add(menu);
   menu.add(new JMenuItem("Per Lagerkvist"));
   menu.add(new JMenuItem("Selma Lagerlöf"));
   menu.add(new JMenuItem("Åsa Moberg"));
   menu.addSeparator();
   JMenuItem skeelos = menu.add(new JMenuItem("Jojje"));
   menuBar.add(menu);

With the additional capabilities of Swing components, it is possible to add images to menu items:

	Icon georges  = new ImageIcon("images/georges.gif");
	JMenuItem skeelos = menu.add(new JMenuItem("Jojje", georges));
	skeelos.setHorizontalTextPosition(JButton.RIGHT);
	skeelos.setHorizontalAlignment(JButton.LEFT);
	skeelos.setVerticalTextPosition(JButton.CENTER);