Interface Action

public interface Action
extends ActionListener

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.

Component Property	Components	Action Key	Notes
enabled	All	The isEnabled method
toolTipText	All	SHORT_DESCRIPTION
actionCommand	All	ACTION_COMMAND_KEY
mnemonic	All buttons	MNEMONIC_KEY	A null value or Action results in the button's mnemonic property being set to '\0'.
text	All buttons	NAME	If you do not want the text of the button to mirror that of the Action, set the property hideActionText to true. If hideActionText is true, setting the Action changes the text of the button to null and any changes to NAME are ignored. hideActionText is useful for tool bar buttons that typically only show an Icon. JToolBar.add(Action) sets the property to true if the Action has a non-null value for LARGE_ICON_KEY or SMALL_ICON.
displayedMnemonicIndex	All buttons	DISPLAYED_MNEMONIC_INDEX_KEY	If the value of DISPLAYED_MNEMONIC_INDEX_KEY is beyond the bounds of the text, it is ignored. When setAction is called, if the value from the Action is null, the displayed mnemonic index is not updated. In any subsequent changes to DISPLAYED_MNEMONIC_INDEX_KEY, null is treated as -1.
icon	All buttons except of JCheckBox, JToggleButton and JRadioButton.	either LARGE_ICON_KEY or SMALL_ICON	The JMenuItem subclasses only use SMALL_ICON. All other buttons will use LARGE_ICON_KEY; if the value is null they use SMALL_ICON.
accelerator	All JMenuItem subclasses, with the exception of JMenu.	ACCELERATOR_KEY
selected	JToggleButton, JCheckBox, JRadioButton, JCheckBoxMenuItem and JRadioButtonMenuItem	SELECTED_KEY	Components that honor this property only use the value if it is non-null. For example, if you set an Action that has a null value for SELECTED_KEY on a JToggleButton, the JToggleButton will not update it's selected state in any way. Similarly, any time the JToggleButton's selected state changes it will only set the value back on the Action if the Action has a non-null value for SELECTED_KEY. Components that honor this property keep their selected state in sync with this property. When the same Action is used with multiple components, all the components keep their selected state in sync with this property. Mutually exclusive buttons, such as JToggleButtons in a ButtonGroup, force only one of the buttons to be selected. As such, do not use the same Action that defines a value for the SELECTED_KEY property with multiple mutually exclusive buttons.

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.

Fields

DEFAULT

static final String DEFAULT

Not currently used.

See Also:

Constant Field Values

NAME

static final String NAME

The key used for storing the String name for the action, used for a menu or button.

See Also:

Constant Field Values

SHORT_DESCRIPTION

static final String SHORT_DESCRIPTION

The key used for storing a short String description for the action, used for tooltip text.

See Also:

Constant Field Values

LONG_DESCRIPTION

static final String LONG_DESCRIPTION

The key used for storing a longer String description for the action, could be used for context-sensitive help.

See Also:

Constant Field Values

SMALL_ICON

static final String SMALL_ICON

See Also:

Constant Field Values

ACTION_COMMAND_KEY

static final String ACTION_COMMAND_KEY

The key used to determine the command String for the ActionEvent that will be created when an Action is going to be notified as the result of residing in a Keymap associated with a JComponent.

See Also:

Constant Field Values

ACCELERATOR_KEY

static final String ACCELERATOR_KEY

The key used for storing a KeyStroke to be used as the accelerator for the action.

Since:

1.3

See Also:

Constant Field Values

MNEMONIC_KEY

static final String MNEMONIC_KEY

The key used for storing an Integer that corresponds to one of the KeyEvent key codes. The value is commonly used to specify a mnemonic. For example: myAction.putValue(Action.MNEMONIC_KEY, KeyEvent.VK_A) sets the mnemonic of myAction to 'a', while myAction.putValue(Action.MNEMONIC_KEY, KeyEvent.getExtendedKeyCodeForChar('?')) sets the mnemonic of myAction to Cyrillic letter "Ef".

Since:

1.3

See Also:

Constant Field Values

SELECTED_KEY

static final String SELECTED_KEY

Since:

1.6

See Also:

Constant Field Values

DISPLAYED_MNEMONIC_INDEX_KEY

static final String DISPLAYED_MNEMONIC_INDEX_KEY

Since:

1.6

See Also:

AbstractButton.setDisplayedMnemonicIndex(int), Constant Field Values

LARGE_ICON_KEY

static final String LARGE_ICON_KEY

Since:

1.6

See Also:

Constant Field Values

Methods

getValue

Object getValue(String key)

Gets one of this object's properties using the associated key.

See Also:

putValue(java.lang.String, java.lang.Object)

putValue

void putValue(String key,
              Object value)

Sets one of this object's properties using the associated key. If the value has changed, a PropertyChangeEvent is sent to listeners.

Parameters:

key - a String containing the key

value - an Object value

setEnabled

void setEnabled(boolean b)

Sets the enabled state of the Action. When enabled, any component associated with this object is active and able to fire this object's actionPerformed method. If the value has changed, a PropertyChangeEvent is sent to listeners.

Parameters:

b - true to enable this Action, false to disable it

isEnabled

boolean isEnabled()

Returns the enabled state of the Action. When enabled, any component associated with this object is active and able to fire this object's actionPerformed method.

Returns:

true if this Action is enabled

addPropertyChangeListener

void addPropertyChangeListener(PropertyChangeListener listener)

Adds a PropertyChange listener. Containers and attached components use these methods to register interest in this Action object. When its enabled state or other property changes, the registered listeners are informed of the change.

Parameters:

listener - a PropertyChangeListener object

removePropertyChangeListener

void removePropertyChangeListener(PropertyChangeListener listener)

Removes a PropertyChange listener.

Parameters:

listener - a PropertyChangeListener object

See Also:

addPropertyChangeListener(java.beans.PropertyChangeListener)