MultiListener

The Java 1.1 event model, which you saw at its simplest in the Beeper 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.

You can download the source in MultiListener.java.

Java Source

// Multilistener.java
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.event.ActionEvent;

public class MultiListener extends JApplet 
                           implements ActionListener {

    JTextArea topTextArea;
    JTextArea bottomTextArea;
    JButton button1, button2;

    public void init() {
        JLabel l = null;
		Container container = getContentPane();
        GridBagLayout gridbag = new GridBagLayout();
        GridBagConstraints c = new GridBagConstraints();
        container.setLayout(gridbag);

        c.fill = GridBagConstraints.BOTH;
        c.gridwidth = GridBagConstraints.REMAINDER;
        l = new JLabel("What MultiListener hears:");
        gridbag.setConstraints(l, c);
        container.add(l);

        c.weighty = 1.0;
        topTextArea = new JTextArea(5, 20);
        topTextArea.setEditable(false);
		JScrollPane topScroller = new JScrollPane(topTextArea);
        gridbag.setConstraints(topScroller, c);
        container.add(topScroller);

        c.weightx = 0.0;
        c.weighty = 0.0;
        l = new JLabel("What Eavesdropper hears:");
        gridbag.setConstraints(l, c);
        container.add(l);

        c.weighty = 1.0;
        bottomTextArea = new JTextArea(5, 20);
        bottomTextArea.setEditable(false);
		JScrollPane bottomScroller = new JScrollPane(bottomTextArea);
        gridbag.setConstraints(bottomScroller, c);
        container.add(bottomScroller);

        c.weightx = 1.0;
        c.weighty = 0.0;
        c.gridwidth = 1;
        c.insets = new Insets(10, 10, 0, 10);
        button1 = new JButton("Blah blah blah");
        gridbag.setConstraints(button1, c);
        container.add(button1);

        c.gridwidth = GridBagConstraints.REMAINDER;
        button2 = new JButton("You don't say!");
        gridbag.setConstraints(button2, c);
        container.add(button2);

        button1.addActionListener(this);
        button2.addActionListener(this);

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

    public void actionPerformed(ActionEvent e) {
        topTextArea.append(e.getActionCommand() + "\n");
    }
}

class Eavesdropper implements ActionListener {
    JTextArea myTextArea;
    public Eavesdropper(JTextArea ta) {
        myTextArea = ta;
    }

    public void actionPerformed(ActionEvent e) {
        myTextArea.append(e.getActionCommand() + "\n");
    }
}

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