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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 Return 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, Some Simple Event-Handling Examples. 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.
TheActionListener
interface contains a single method, and thus has no corresponding adapter class. Here is the lone
ActionListener
method:The
void actionPerformed(ActionEvent)
- Called just after the user informs the listened-to component that an action should occur.
actionPerformed
method has a single parameter: anActionEvent
object. The
ActionEvent
class defines two useful methods:Also useful is the
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. If you don't set the action command explicitly, then it's generally the text displayed in the component. For objects with multiple items, and thus multiple possible actions, the action command is generally the name of the selected item.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_MASKgetSource
method, whichActionEvent
inherits fromEventObject
.
The following table lists some of the many examples that use action listeners.
Example Where Described Notes Beeper
This section and Some Simple Event-Handling Examples Contains one button with one action listener that beeps when you click the button. MultiListener
Some Simple Event-Handling Examples Registers two different action listeners on one button. Also registers the same action listener on two different buttons. RadioButtonDemo.java
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. TextDemo
How to Use Text Fields An applet that registers an action listener on a text field. ActionDemo
How to Use Actions Uses actions to bind buttons and menu items to the same function. 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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