How to write a Change Listener in Java?

[Samarth]

Hi friends,
I am new to this forum, so please ignore my mistakes. I have started doing Java recently. I want to use the change listener, so that I can know when the source object is changed.!! But I don't know about the Change Listener which is used for doing that purpose.?! So please tell me how to write a Change Listener in Java.?? Hoping that someone would help me..!!

[kyosang]

A change listener is registered on an object and the listener is notified when the object has changed. A change listener must have a stateChanged (...) method, which has a ChangeEvent parameter. Focus events are fired whenever a component gains or loses the keyboard focus. You can call the ChangeEvent getSource() method to get the slider which caused this event. You should be knowing the basic thing that component generally gains the focus when the user clicks it.

[Solitario]

The following is a method of the ChangeListener Interface, which is given below :

• stateChanged(ChangeEvent) - This method is called when the listened-to component changes state.

A ChangeListener has only one method. Since it has only one method, there is no corresponding adapter class.
The following is the method of the ChangeEvent Class :

• Object getSource() - This method returns the object that fired the event.

[absolute55]

There are many swing components which relies on change events for basic functionality. Components like sliders, color choosers and spinners depends on change events. If you want to know when the value in a slider changes, you need to register a change listener. When you register a change listener on a spinner, a component introduced will be notified when the spinner's value changes. Hope that you can understand properly when to use the change listener.!!

[Zecho]

I am providing an example of coding which is useful to change event handling for a slider. Have a look on the following example :

   framesPerSecond.addChangeListener(new SliderListener());
    ...
    class SliderListener implements ChangeListener {
        public void stateChanged(ChangeEvent e) {
            JSlider source = (JSlider)e.getSource();
            if (!source.getValueIsAdjusting()) {
                int fps = (int)source.getValue();
    	    ...
            }    
        }
    }

[Allan.d]

I have provided you with an example of coding having the Change Listener. You will find the coding very lengthy but it can be useful for you to understand things in full coding :

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

public class ColorChooserDemo extends JPanel
                              implements ChangeListener {

    protected JColorChooser tcc;
    protected JLabel banner;

    public ColorChooserDemo() {
        super(new BorderLayout());

        banner = new JLabel("This is Study Place!",
                            JLabel.CENTER);
        banner.setForeground(Color.red);
        banner.setBackground(Color.yellow);
        banner.setOpaque(true);
        banner.setFont(new Font("Times", Font.BOLD, 22));
        banner.setPreferredSize(new Dimension(120, 77));

        JPanel bannerPanel = new JPanel(new BorderLayout());
        bannerPanel.add(banner, BorderLayout.CENTER);
        bannerPanel.setBorder(BorderFactory.createTitledBorder("Banner"));

        tcc = new JColorChooser(banner.getForeground());
        tcc.getSelectionModel().addChangeListener(this);
        tcc.setBorder(BorderFactory.createTitledBorder(
                                             "Choose Text Color"));

        add(bannerPanel, BorderLayout.CENTER);
        add(tcc, BorderLayout.PAGE_END);
    }

    public void stateChanged(ChangeEvent e) {
        Color newColor = tcc.getColor();
        banner.setForeground(newColor);
    }

    private static void createAndShowGUI() {
        JFrame frame = new JFrame("ColorChooserDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JComponent newContentPane = new ColorChooserDemo();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {

        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}