The Action interface provides a useful extension to the ActionListener interface in cases where the same functionality may be accessed by several controls.

In addition to the actionPerformed method defined by the ActionListener interface, this interface allows the application to define, in a single place:

• One or more text strings that describe the function. These strings can be used, for example, to display the flyover text for a button or to set the text in a menu item.
• One or more icons that depict the function. These icons can be used for the images in a menu control, or for composite entries in a more sophisticated user interface.
• The enabled/disabled state of the functionality. Instead of having to separately disable the menu item and the toolbar button, the application can disable the function that implements this interface. All components which are registered as listeners for the state change then know to disable event generation for that item and to modify the display accordingly.

This interface can be added to an existing class or used to create an adapter (typically, by subclassing AbstractAction). The Action object can then be added to multiple Action-aware containers and connected to Action-capable components. The GUI controls can then be activated or deactivated all at once by invoking the Action object's setEnabled method.

Note that Action implementations tend to be more expensive in terms of storage than a typical ActionListener, which does not offer the benefits of centralized control of functionality and broadcast of property changes. For this reason, you should take care to only use Actions where their benefits are desired, and use simple ActionListeners elsewhere.

Swing Components Supporting Action

Many of Swing's components have an Action property. When an Action is set on a component, the following things happen:

• The Action is added as an ActionListener to the component.
• The component configures some of its properties to match the Action.
• The component installs a PropertyChangeListener on the Action so that the component can change its properties to reflect changes in the Action's properties.

The following table describes the properties used by Swing components that support Actions. In the table, button refers to any AbstractButton subclass, which includes not only JButton but also classes such as JMenuItem. Unless otherwise stated, a null property value in an Action (or a Action that is null) results in the button's corresponding property being set to null.

JPopupMenu, JToolBar and JMenu all provide convenience methods for creating a component and setting the Action on the corresponding component. Refer to each of these classes for more information.

Action uses PropertyChangeListener to inform listeners the Action has changed. The beans specification indicates that a null property name can be used to indicate multiple values have changed. By default Swing components that take an Action do not handle such a change. To indicate that Swing should treat null according to the beans specification set the system property swing.actions.reconfigureOnNull to the String value true.

The key used for storing a small Icon, such as ImageIcon. This is typically used with menus such as JMenuItem.

If the same Action is used with menus and buttons you'll typically specify both a SMALL_ICON and a LARGE_ICON_KEY. The menu will use the SMALL_ICON and the button will use the LARGE_ICON_KEY.

The key used for storing a Boolean that corresponds to the selected state. This is typically used only for components that have a meaningful selection state. For example, JRadioButton and JCheckBox make use of this but instances of JMenu don't.

This property differs from the others in that it is both read by the component and set by the component. For example, if an Action is attached to a JCheckBox the selected state of the JCheckBox will be set from that of the Action. If the user clicks on the JCheckBox the selected state of the JCheckBox and the Action will both be updated.

Note: the value of this field is prefixed with 'Swing' to avoid possible collisions with existing Actions.

The key used for storing an Integer that corresponds to the index in the text (identified by the NAME property) that the decoration for a mnemonic should be rendered at. If the value of this property is greater than or equal to the length of the text, it will treated as -1.

Note: the value of this field is prefixed with 'Swing' to avoid possible collisions with existing Actions.

The key used for storing an Icon. This is typically used by buttons, such as JButton and JToggleButton.

If the same Action is used with menus and buttons you'll typically specify both a SMALL_ICON and a LARGE_ICON_KEY. The menu will use the SMALL_ICON and the button the LARGE_ICON_KEY.

Note: the value of this field is prefixed with 'Swing' to avoid possible collisions with existing Actions.