1. Event Handling CS 21a: Introduction to Computing I
Department of Information Systems and Computer Science
Ateneo de Manila University

2. Event handling in Java
User interaction with applets and frames involves handling events
Event examples:
Clicking on a button
Typing text on a text field
Dragging a mouse along an area in the applet/frame
Events are handled through listeners
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

3. Buttons and listeners
To program functionality for a button, you need to associate the button object to a listener object
private JButton click; … click = new Button( “Click me” ); … click.addActionListener( new SomeListener() );
A listener class needs to be defined
The class will contain a method which will be executed when the button is clicked
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

4. ActionListener
In the java library (java.awt.event.*), there is an interface called ActionListener
public interface ActionListener { public void actionPerformed( ActionEvent ae ); }
The listener object to be associated to a button must be an instance of a class that implements this interface
public class SomeListener implements ActionListener { public void actionPerformed( ActionEvent ae ) { // contains statements to be executed when the button is clicked } }
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

5. Define listener classes as inner classes
Inner class: class defined within a class
Useful when you need to have access to data within objects of the containing class
The inner class is not a public class, and is used only within the containing class/method, often to create just one instance
Listeners need access to the visual components within the frame or applet
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

6. Button listener Instance fields of the frame/applet
private JButton button;
private JTextField textField;
private JLabel label;
...
button = new JButton( "Click Me" );
c.add( button );
class ButtonListener implements ActionListener {
public void actionPerformed( ActionEvent ae )
String text = textField.getText();
label.setText( “Hello,”+text ); } }
button.addActionListener( new ButtonListener() );
Instance fields of the frame/applet
Need access to these fields within the listener class
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

7. Handling multiple buttons
When there are multiple buttons to listen to, create a different listener class for each button
A different actionPerformed() method is executed for each button
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

8. Organizing applet or frame code
For a complex applet or frame, the init() method or the constructor of the frame could be unmanageably long if all component instantiation, calls to add, and listener classes are defined in that method
Better to have separate methods for different visual objects or groups of visual objects
The init() method or the constructor then contains calls to these methods
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

9. Organizing code private Container c; ... public HelloWorldFrame()
{ c = getContentPane();
createLabel();
createTextField();
createButton(); }
...
public void createButton() {
button = new JButton( "Click Me" );
c.add( button );
class ButtonListener implements ButtonListener
{ public void actionPerformed( ActionEvent ae )
{ String text = textField.getText();
label.setText( “Hello,”+text ); }
c.addActionListener( new ButtonListener() );
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

10. Organizing code
private Container c; ...
public HelloWorldFrame()
{ c = getContentPane();
createLabel();
createTextField();
createButton(); }
...
public void createButton() {
button = new JButton( "Click Me" );
c.add( button );
class ButtonListener implements ButtonListener
{ public void actionPerformed( ActionEvent ae )
{ String text = textField.getText();
label.setText( “Hello,”+text ); }
c.addActionListener( new ButtonListener() );
define Container c; as an instance field so all methods could refer to it
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved

11. Listeners in other contexts
Can use ActionListener and actionPerformed() for JTextFields
For mouse operations/movements, use MouseListener or MouseMotionListener
For keyboard actions, use KeyListener
ActionListeners are sufficient for CS 21a
Copyright 2008, by the authors of these slides, and Ateneo de Manila University. All rights reserved