This section provides a tutorial example on how to use ActionListener, ChangeListener and ItemListener interfaces to handle different types of events generated on radio buttons.
As you can see from the previous section, a radio button can have 3 types of event listensers:
ActionListener, ChangeListener, and ItemListener. The following sample program shows you
when those listeners are called, and how many times:
/**
* JRadioButtonTest.java
* Copyright (c) 2002 by Dr. Herong Yang, http://www.herongyang.com/
*/
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
public class JRadioButtonTest {
public static void main(String[] a) {
JFrame f = new JFrame("My Radio Buttons");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
ButtonGroup g = new ButtonGroup();
MyRadioButton b1 = new MyRadioButton("On");
g.add(b1);
f.getContentPane().add(b1,BorderLayout.NORTH);
MyRadioButton b2 = new MyRadioButton("Off");
g.add(b2);
f.getContentPane().add(b2,BorderLayout.SOUTH);
f.pack();
f.setVisible(true);
}
private static class MyRadioButton extends JRadioButton
implements ActionListener, ChangeListener, ItemListener {
static int count = 0;
String text = null;
public MyRadioButton(String t) {
super(t);
text = t;
addActionListener(this);
addChangeListener(this);
addItemListener(this);
}
public void actionPerformed(ActionEvent e) {
count++;
System.out.println(count+": Action performed - "+text);
}
public void stateChanged(ChangeEvent e) {
count++;
System.out.println(count+": State changed on - "+text);
}
public void itemStateChanged(ItemEvent e) {
count++;
System.out.println(count+": Item state changed - "+text);
}
}
}
This example program creates two radio buttons and puts them in a single button group.
Each button has 3 listeners to handle 3 different types of events. A counter is
used in the listener class to help to identify the order of events.
If you run this program, you will see two radio buttons: one labeled as "On" and
the other labeled as "Off":
If you press the "On" button and hold it, you will
see 2 messages showing in command window. If you release the "On" button, you
will see 5 more messages. If you continue to press the "Off" button and hold it,
you will see 2 more messages. If you release the "Off" button, you will see 7 more
messages. Here is the list of all the messages:
1: State changed on - On
2: State changed on - On - "On" pressed
3: State changed on - On
4: Item state changed - On
5: State changed on - On
6: Action performed - On
7: State changed on - On - "On" released
8: State changed on - Off
9: State changed on - Off - "Off" pressed
10: State changed on - On
11: Item state changed - On
12: State changed on - Off
13: Item state changed - Off
14: State changed on - Off
15: Action performed - Off
16: State changed on - Off - "Off" released
Note that:
Action event raised only once when you release a button.
Change event (stateChanged method call) is raised 2 times when you press a button;
and raised 2 time again when you release a button. This tells us that a button has more than
2 states: selected and deselected.
Item event (itemStateChanged method call) is raised only once when you release a button.
In a button group, if one button is selected, other selected buttons will be deselected.
Events #10 and #11 show that when "Off" is selected, "On" is deselected.
Sample programs listed in this section have been tested with JDK 1.3.1, 1.4.1, 1.5.0, and 1.6.0.
