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How to Write an Action Listener

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:

public class Beeper ...  implements ActionListener {
    ...
    //where initialization occurs:
        button.addActionListener(this);
    ...
    public void actionPerformed(ActionEvent e) {
	Toolkit.getDefaultToolkit().beep();
    }
}

The Beeper applet is described in this trail's introduction to events, Some Simple Event-Handling Examples. You can find the entire program in Beeper.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.

The Action Event API

The ActionListener interface contains a single method, and thus has no corresponding adapter class. Here is the lone ActionListener method:

void actionPerformed(ActionEvent)

Called just after the user informs the listened-to component that an action should occur.

The actionPerformed method has a single parameter: an ActionEvent object. The ActionEvent class defines two useful methods:

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 constants SHIFT_MASK, CTRL_MASK, META_MASK, and ALT_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_MASK
    

Also useful is the getSource method, which ActionEvent inherits from EventObject.

Examples that Use Action Listeners

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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