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March 22, 2002
n "It Might Be Efficient, But It Ain't OO," I presented an alternative way to implement the java.awt.ActionListener interface. In my opinion, creating one large actionPerformed() method and simply switching between the various action commands does not represent the best approach. I said such an approach
doesn't follow good object-oriented (OO) design.
Such statements always cause controversy, and, needless to say, my comments did! In fact, my answer generated quite a bit of reader feedback. In this Java Q&A, I present concrete examples I hope will justify my position and help you implement this practice in your programs. As an added bonus, my approach applies to any interface implementation you might write.
Let's start with a simple GUI (graphical user interface).
Figure 1. A simple GUI
Figure 1's GUI comprises a panel containing three buttons and a label. When you press a button, the GUI writes a message to the label indicating which button you pressed.
To display the panel, I embedded it within a frame. However, for the purposes of today's Java Q&A, I'll simply focus on the panel code. (Please see Resources for the full source code.)
Implementation
Following the discussion in the original Java Q&A, you can implement the buttons' listeners in two ways. First, the panel can implement the ActionListener interface directly, or, second, you can create three listener classes whose instances listen to the buttons directly. Let's
look at the first choice in which the panel implements one large listener.
Wire the buttons directly to the panel
The following class shows how to wire the buttons directly to the panel:
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;
/**
* The ActionListenerGUI is an example of a simple GUI that directly implements
* the ActionListener Interface.
* @author Tony Sintes
*/
public class ActionListenerPanel extends JPanel implements ActionListener {
public ActionListenerPanel() {
setup();
}
private void setup() {
setLayout( new BorderLayout() );
JButton button1 = new JButton( BUTTON_1 );
button1.setActionCommand( BUTTON_1_ACTION );
button1.addActionListener( this );
JButton button2 = new JButton( BUTTON_2 );
button2.setActionCommand( BUTTON_2_ACTION );
button2.addActionListener( this );
JButton button3 = new JButton( BUTTON_3 );
button3.setActionCommand( BUTTON_3_ACTION );
button3.addActionListener( this );
JPanel buttons = new JPanel();
buttons.add( button1 );
buttons.add( button2 );
buttons.add( button3 );
add( display, BorderLayout.NORTH );
add( buttons, BorderLayout.SOUTH );
}
public void actionPerformed( ActionEvent actionEvent ) {
String actionName = actionEvent.getActionCommand().trim();
if( actionName.equals( BUTTON_1_ACTION ) ) {
display.setText( BUTTON_1 );
}
else if( actionName.equals( BUTTON_2_ACTION ) ) {
display.setText( BUTTON_2 );
}
else if( actionName.equals( BUTTON_3_ACTION ) ) {
display.setText( BUTTON_3 );
}
}
private JLabel display = new JLabel("Press a button");
private static String BUTTON_1 = "Button 1";
private static String BUTTON_2 = "Button 2";
private static String BUTTON_3 = "Button 3";
private static String BUTTON_1_ACTION = "B1";
private static String BUTTON_2_ACTION = "B2";
private static String BUTTON_3_ACTION = "B3";
}
In the code above, notice the setup() and actionPerformed() methods. In setup(), I create three buttons. Each button creation takes the following form:
- Feedback
Resources
- To download this article's example source code, go to
http://www.javaworld.com/javaworld/javaqa/2002-03/morelisteners/02-qa-0322-morelisteners.zip - For Tony's first look at object-oriented listener design, see "It Might Be Efficient, But It Ain't OO" (JavaWorld, February 2002)
http://www.javaworld.com/javaworld/javaqa/2002-02/03-qa-0222-optimization.html - For more on event listeners, read Robert Hasting's "Speed Up Listener Notification" (JavaWorld, February 2000)
http://www.javaworld.com/javaworld/jw-02-2000/jw-02-fast.html - The Object-Oriented Design and Programming section of JavaWorld's Topical Index features numerous articles addressing design
http://www.javaworld.com/channel_content/jw-oop-index.shtml - Want more? See the Java Q&A index page for the full Q&A catalog
http://www.javaworld.com/columns/jw-qna-index.shtml - For more than 100 insightful Java tips from some of the best minds in the business, visit JavaWorld's Java Tips index page
http://www.javaworld.com/columns/jw-tips-index.shtml - Learn the basics of object-oriented programming in our Programming Theory & Practice discussion
http://forums.idg.net/webx?50@@.ee6b806 - Sign up for JavaWorld's free weekly Applied Java email newsletter
http://www.javaworld.com/subscribe - You'll find a wealth of IT-related articles from our sister publications at IDG.net
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