Swing ChangeEvent occur twice

[albert kao]

The existing source codes, which I try to debug, create the MyTabbedPane class from JTabbedPane.
Clicking the tab will generate the ChangeEvent twice inside the addChangeListener.
Why? Is that to make the ChangeEvent occur once only?

public class LangTabbedPane extends JTabbedPane {
   //...
}
   
public class MyTabbedPane extends LangTabbedPane
    implements PropertyChangeListener {
    //...
}


MyTabbedPane tabs;

tabs.addChangeListener(new ChangeListener() {
    public void stateChanged(ChangeEvent e) {
            
    // why ChangeEvent e occur twice ?
    int index = tabs.getSelectedIndex();
    log.debug("addChangeListener index " + index);
    log.debug("tabsCurrentIndex " + tabsCurrentIndex);
    if (tabsCurrentIndex != index) {
        saveState();
        recreateTable();
    } else {
        //....
         }
    }
});

2011-10-11 09:02:00,211 DEBUG Langclient.modules.findPanel - addChangeListener index 1
2011-10-11 09:02:00,211 DEBUG Langclient.modules.findPanel - tabsCurrentIndex 1
2011-10-11 09:02:00,211 DEBUG Langclient.modules.findPanel - addChangeListener index 1
2011-10-11 09:02:00,227 DEBUG Langclient.modules.findPanel - tabsCurrentIndex 1

[Rob Spoor]

Are you sure that you haven't added your change listeners twice? I created a little SSCCE and I only get one event.

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;

public class Test implements ChangeListener
{
    public static void main(String[] args) throws Exception
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                JTabbedPane tabbedPane = new JTabbedPane();
                tabbedPane.addTab("1", new JLabel("1"));
                tabbedPane.addTab("2", new JLabel("2"));
                tabbedPane.addChangeListener(new Test());
                frame.add(tabbedPane);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public void stateChanged(ChangeEvent e)
    {
        JTabbedPane tabbedPane = (JTabbedPane)e.getSource();
        int index = tabbedPane.getSelectedIndex();
        System.out.println(new Date() + ": addChangeListener index " + index);
    }
}