Java Reference
In-Depth Information
19 add(panel);
20
21
// Create a listener
22 ButtonListener listener =
new
ButtonListener();
23
24
// Register listener with buttons
25 jbtNew.addActionListener(listener);
26 jbtOpen.addActionListener(listener);
27 jbtSave.addActionListener(listener);
28 jbtPrint.addActionListener(listener);
29 }
30
31
class
ButtonListener
implements
ActionListener {
32 @Override
33
create listener
register listener
listener class
public void
actionPerformed(ActionEvent e) {
handle event
if
(e.getSource() == jbtNew)
34
35 System.out.println(
"Process New"
);
36
37 System.out.println(
"Process Open"
);
38
39 System.out.println(
"Process Save"
);
40
41 System.out.println(
"Process Print"
);
42 }
43 }
44
45
/** Main method */
46
public static void
main(String[] args) {
47 JFrame frame =
new
DetectSourceDemo();
48 frame.setTitle(
"DetectSourceDemo"
);
49 frame.setLocationRelativeTo(
null
);
// Center the frame
50 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
51
52 frame.setVisible(
true
);
53 }
54 }
else if
(e.getSource() == jbtOpen)
else if
(e.getSource() == jbtSave)
else if
(e.getSource() == jbtPrint)
frame.pack();
This program defines just one inner listener class (lines 31-43), creates a listener from the
class (line 22), and registers it to four buttons (lines 25-28). When a button is clicked, the but-
ton fires an
ActionEvent
and invokes the listener's
actionPerformed
method. The
actionPerformed
method checks the source of the event using the
getSource()
method
for the event (lines 34, 36, 38, 40) and determines which button fired the event.
Defining one listener class for handling a large number of events is efficient. In this case,
you create just one listener object. Using anonymous inner classes, you would create four lis-
tener objects.
You could also rewrite Listing 16.4 by defining the custom frame class that implements
ActionListener
, as shown in Listing 16.6.
L
ISTING
16.6
FrameAsListenerDemo.java
1
import
javax.swing.*;
2
import
java.awt.event.*;
3
4
public class
FrameAsListenerDemo
extends
JFrame
5
implements
ActionListener
{
implement
ActionListener
6
// Create four buttons
7
private
JButton jbtNew =
new
JButton(
"New"
);
8
private
JButton jbtOpen =
new
JButton(
"Open"
);
9
private
JButton jbtSave =
new
JButton(
"Save"
);
