Writing Event Listeners

Some Simple Event-Handling Examples

Here is a bare-bones applet that illustrates event handling. It contains a single button that beeps when you click it.

This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

You can find the entire program in Beeper.java. Here's just the code that implements the event handling for the button:

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

Isn't that simple? The Beeper class implements the ActionListener interface, which contains one method: actionPerformed. Since Beeper implements ActionListener, a Beeper object can register as a listener for the action events that buttons generate. Once the Beeper has been registered using the Button addActionListener method, the Beeper's actionPerformed method is called every time the button is clicked.

A More Complex Example

The event model, which you saw at its simplest in the above example, is quite powerful and flexible. Any number of event listener objects can listen for all kinds of events from any number of event source objects. For example, a program might create one listener per event source. Or a program might have a single listener for all events from all sources. A program can even have more than one listener for a single kind of event from a single event source.

The following applet gives an example of using multiple listeners per object. The applet contains two event sources (JButton instances) and two event listeners. One of the event listeners (an instance of a class called MultiListener) listens for events from both buttons. When it receives an event, it adds the event's "action command" (the text on the button's label) to the top text area. The second event listener (an instance of a class called Eavesdropper) listens for events on only one of the buttons. When it receives an event, it adds the action command to the bottom text area.

This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

You can find the entire program in MultiListener.java. Here's just the code that implements the event handling for the button:

public class MultiListener ... implements ActionListener {
    ...
    //where initialization occurs:
        button1.addActionListener(this);
        button2.addActionListener(this);

        button2.addActionListener(new Eavesdropper(bottomTextArea));
    }

    public void actionPerformed(ActionEvent e) {
        topTextArea.append(e.getActionCommand() + newline);
    }
}

class Eavesdropper implements ActionListener {
    ...
    public void actionPerformed(ActionEvent e) {
        myTextArea.append(e.getActionCommand() + newline);
    }
}

In the above code, both MultiListener and Eavesdropper implement the ActionListener interface and register as action listeners using the JButton addActionListener method. Both classes' implementations of the actionPerformed method are similar: they simply add the event's action command to a text area.

An Example of Handling Another Event Type

So far, the only kind of event you've seen has been action events. Let's take a look at a program that handles another kind of event: mouse events.

The following applet displays a rectangle-edged area and a text area. Whenever a mouse event -- a click, press, release, enter, or exit -- occurs on either the rectangle-edged area (BlankAreaMouseEventDemo), the text area displays a string describing the event.

This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

You can find the entire program in MouseEventDemo.java and BlankArea.java. Here's just the code that implements the event handling:

public class MouseEventDemo ...  implements MouseListener {
    ...
    //where initialization occurs:
        //Register for mouse events on blankArea and applet
        blankArea.addMouseListener(this);
        addMouseListener(this);
    }

    public void mousePressed(MouseEvent e) {
       saySomething("Mouse pressed; # of clicks: "
                    + e.getClickCount(), e);
    }

    public void mouseReleased(MouseEvent e) {
       saySomething("Mouse released; # of clicks: "
                    + e.getClickCount(), e);
    }

    public void mouseEntered(MouseEvent e) {
       saySomething("Mouse entered", e);
    }

    public void mouseExited(MouseEvent e) {
       saySomething("Mouse exited", e);
    }

    public void mouseClicked(MouseEvent e) {
       saySomething("Mouse clicked (# of clicks: "
                    + e.getClickCount() + ")", e);
    }

    void saySomething(String eventDescription, MouseEvent e) {
        textArea.append(eventDescription + " detected on "
                        + e.getComponent().getClass().getName()
                        + "." + newline);
    }
}

You'll see the code explained in How to Write a Mouse Listener, later in this section.

Writing Event Listeners