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If you have two or more components that perform the same function, consider using an
Action
object to implement the function. AnAction
object is an action listener that provides not only action-event handling, but also centralized handling of the state of action-event-firing components such as tool bar buttons, menu items, common buttons, and text fields. The state that an action can handle includes text, icon, mnemonic, and enabled status.You typically attach an action to a component using the
setAction
method. Here's what happens whensetAction
is invoked on a component:Here's an example of creating a tool-bar button and menu item that perform the same function:
- The component's state is updated to match the state of the
Action
. For example, if theAction
's text and icon values were set, the component's text and icon are set to those values.- The
Action
object is registered as an action listener on the component.- If the state of the
Action
changes, the component's state is updated to match theAction
. For example, if you change the enabled status of the action, all components it's attached to change their enabled states to match the action.Action leftAction = new LeftAction(); //LeftAction code is shown later ... button = new JButton(leftAction) ... menuItem = new JMenuItem(leftAction);
Version Note: Prior to 1.3, the only way for a button or menu item to get the full benefit of using anAction
was to create the component using theadd(Action)
method ofJToolBar
,JMenu
, orJPopupMenu
. This was because the pre-1.3 releases have no API exceptaddActionListener(ActionListener)
to connect anAction
to an already existing component. Although you could useaddActionListener
to add anAction
object as an action listener to any button, for example, the button wouldn't be notified when the action was disabled.To create an
Action
object, you generally create a subclass ofAbstractAction
and then instantiate it. In your subclass, you must implement theactionPerformed
method to react appropriately when the action event occurs. Here's an example of creating and instantiating anAbstractAction
subclass:leftAction = new LeftAction("Go left", anIcon, "This is the left button.", new Integer(KeyEvent.VK_L)); ... class LeftAction extends AbstractAction { public LeftAction(String text, ImageIcon icon, String desc, Integer mnemonic) { super(text, icon); putValue(SHORT_DESCRIPTION, desc); putValue(MNEMONIC_KEY, mnemonic); } public void actionPerformed(ActionEvent e) { displayResult("Action for first button/menu item", e); } }When the action created by the preceding code is attached to a button and a menu item, the button and menu item display the text and icon associated with the action. The
L
character is used for mnemonics on the button and menu item, and their tool-tip text is set to theSHORT_DESCRIPTION
string followed by a representation of the mnemonic key.For example, we have provided a simple example,
ActionDemo.java
, which defines three actions. Each action is attached to a button and a menu item. Thanks to the mnemonic values set for each button's action, the key sequenceAlt-L
activates the left button,Alt-M
the middle button, andAlt-R
the right button. The tool tip for the left button displays This is the left button. Alt-L. All of this configuration occurs automatically, without the program making explicit calls to set the mnemonic or tool-tip text. As we'll show later, the program does make calls to set the button text, but only to avoid using the values already set by the actions.
Here is what the user sees when the "Go left" action is disabled:
Try this:
- Run ActionDemo using JavaTM Web Start. Or, to compile and run the example yourself, consult the example index.
- Choose the top item from the left menu (Menu > Go left).
The text area displays some text identifying both the event source and the action listener that received the event.- Click the leftmost button in the tool bar.
The text area again displays information about the event. Note that although the source of the events is different, both events were detected by the same action listener: theAction
object attached to the components.- Choose the top item from the Action State menu.
This disables the "Go left"Action
object, which in turn disables its associated menu item and button.
Here's the code that disables the "Go left" action:
After you create components using anboolean selected = ...//true if the action should be enabled; //false, otherwise leftAction.setEnabled(selected);Action
, you might well need to customize them. For example, you might want to customize the appearance of one of the components by adding or deleting the icon or text. For example,ActionDemo.java
has no icons in its menus, and no text in its buttons. Here's the code that accomplishes this:menuItem = new JMenuItem(); menuItem.setAction(leftAction); menuItem.setIcon(null); //arbitrarily chose not to use icon in menu ... button = new JButton(); button.setAction(leftAction); button.setText(""); //an icon-only buttonWe chose to create an icon-only button and a text-only menu item from the same action by setting the icon property to
null
and the text to an empty string. However, if a property of theAction
changes, the widget may try to reset the icon and text from theAction
again.
The following tables list the commonly usedAction
constructors and methods. The API for usingAction
objects falls into three categories:
Components that Support set/getAction Class Purpose AbstractButton
JComboBox
JTextField
As of release 1.3, these components and their subclasses may have an action directly assigned to them via setAction
. For further information about components that are often associated with actions, see the sections on tool bar buttons, menu items, common buttons, and text fields. For details on which properties each component takes from theAction
, see the API documentation for the relevant class'sconfigurePropertiesFromAction
method.
Creating and Using an AbstractAction Constructor or Method Purpose AbstractAction()
AbstractAction(String)
AbstractAction(String, Icon)
Create an Action
object. Through arguments, you can specify the text and icon to be used in the components that the action controls.void setEnabled(boolean)
boolean isEnabled()
Set or get whether the components the action controls are enabled. Invoking setEnabled(false)
disables all the components that the action controls. Similarly, invokingsetEnabled(true)
enables the action's components.void putValue(String, Object)
Object getValue(String)
Set or get an object associated with a specified key. Used for setting and getting properties associated with an action.
This table defines the properties that can be set on an action. The second column lists which components automatically use the properties (and what method is specifically called). For example, setting the
ACCELERATOR_KEY
on an action that is then attached to a menu item, means thatJMenuItem.setAccelerator(KeyStroke)
is called automatically.
Property Auto-Applied to:
Class
(Method Called)Purpose ACCELERATOR_KEY
JMenuItem
(setAccelerator)The KeyStroke
to be used as the accelerator for the action. For a discussion of accelerators versus mnemonics, see Enabling Keyboard Operation. Introduced in 1.3.ACTION_COMMAND_KEY
AbstractButton
,JCheckBox
,JRadioButton
(setActionCommand)The command string associated with the ActionEvent
.LONG_DESCRIPTION
None The longer description for the action. Can be used for context-sensitive help. MNEMONIC_KEY
AbstractButton
,JMenuItem
,JCheckBox
,JRadioButton
(setMnemonic)The mnemonic for the action. For a discussion of accelerators versus mnemonics, see Enabling Keyboard Operation. Introduced in 1.3. NAME
AbstractButton
,JMenuItem
,JCheckBox
,JRadioButton
(setText)The name of the action. You can set this property when creating the action using the AbstractAction(String)
orAbstractAction(String, Icon)
constructors.SHORT_DESCRIPTION
AbstractButton
,JCheckBox
,JRadioButton
(setToolTipText)The short description of the action. SMALL_ICON
AbstractButton
,JMenuItem
(setIcon)The icon for the action used in the tool bar or on a button. You can set this property when creating the action using the AbstractAction(name, icon)
constructor.
The following examples useAction
objects.
Example Where Described Notes ActionDemo
This section Uses actions to bind buttons and menu items to the same function. TextComponentDemo
Text Component Features Uses text actions to create menu items for text editing commands, such as cut, copy, and paste, and to bind key strokes to caret movement. Also implements custom AbstractAction
subclasses to implement undo and redo. The text action discussion begins in Concepts: About Editor Kits.
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