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Action listeners are probably the easiest and most common event handlers to implement. You implement an action listener to respond to the user's indication that some implementation-dependent action should occur.When the user clicks a button, chooses a menu item or presses Enter in a text field, an action event occurs. The result is that an
actionPerformed
message is sent to all action listeners that are registered on the relevant component.Here is the action event handling code from an applet named
Beeper
:Thepublic class Beeper ... implements ActionListener { ... //where initialization occurs: button.addActionListener(this); ... public void actionPerformed(ActionEvent e) { Toolkit.getDefaultToolkit().beep(); } }Beeper
applet is described in this trail's introduction to events, Introduction to Event Listeners. You can find the entire program inBeeper.java
. The other example described in that section,MultiListener.java
, has two action sources and two action listeners, with one listener listening to both sources and the other listening to just one.
Because
ActionListener
has only one method, it has no corresponding adapter class.
Method Purpose actionPerformed(actionEvent)
Called just after the user informs the listened-to component that an action should occur.
Method Purpose String getActionCommand()
Returns the string associated with this action. Most objects that can fire action events support a method called setActionCommand
that lets you set this string.int getModifiers()
Returns an integer representing the modifier keys the user was pressing when the action event occurred. You can use the ActionEvent
-defined constantsSHIFT_MASK
,CTRL_MASK
,META_MASK
, andALT_MASK
to determine which keys were pressed. For example, if the user Shift-selects a menu item, then the following expression is nonzero:actionEvent.getModifiers() & ActionEvent.SHIFT_MASKObject getSource()
(injava.util.EventObject
)Returns the object that fired the event.
The following table lists some of the many examples that use action listeners.
Example Where Described Notes Beeper
This section and Introduction to Event Listeners Contains one button with one action listener that beeps when you click the button. MultiListener
Introduction to Event Listeners Registers two different action listeners on one button. Also registers the same action listener on two different buttons. RadioButtonDemo
How to Use Radio Buttons Registers the same action listener on five radio buttons. The listener uses the getActionCommand
method to determine which radio button fired the event.MenuDemo
How to Use Menus Shows how to listen for action events on menu items. DragPictureDemo2
Adding Cut/Copy/Paste Support to Data Transfer Uses setActionCommand
to attach the cut, copy, and paste actions to the menu. Then uses as action listener to forward the cut/copy/paste actions to the currently focused component.TextDemo
How to Use Text Fields An applet that registers an action listener on a text field. IconDemoApplet
How to Use Icons Loads an image in an action listener. Because loading an image can take a while, this program uses a SwingWorker
to load the image in a background thread.TableDialogEditDemo
How to Use Tables Registers an action listener through a factory method on the OK button of a color chooser dialog. SliderDemo
How to Use Sliders Registers an action listener on a timer that controls an animation loop.
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