More on listeners - An Introduction to Graphical User Interfaces with Java Swing - page 83

ActionEvent this does not make sense. It is not of importance where a button
was pressed. An ActionEvent on the other hand contains information on which
button had been pressed. The different event types supply methods to access the
information contained in the event, e.g. getX() and getY() allow one to get the co-
ordinates of a MouseEvent , and getActionCommand() allows one to find the source
of an action event.

9.2

■

Implementing listeners

We have so far implemented listeners in classes of their own. We now see
three more ways of implementing them. We return to our first GUI, the counter
from Chapter 3. There, a class CounterListener is defined that implements an
ActionListener .Weillustrate the other methods for implementing listeners on
this example.

9.2.1 Listeners as internal classes

In the counter example we defined the listener in a separate class CounterLis-
tener . The listener is assigned to two buttons and has to take actions when one
of them is pressed. The reaction was to update the CounterPanel . The counter
listener has to know which panel to update. Therefore we pass a reference to a
CounterPanel in the constructor of the listener. Alternatively we can define the
listener as an internal class inside CounterPanel . Then the methods of Counter-
Panel are known to the listener and can be called directly, i.e. the listener can use
decrement() instead of countPane.decrement() . Therefore, no reference to a
panel is passed to the listener. The constructor of the listener has an empty body. In
fact, it can be omitted altogether; then the default constructor of ActionListener
takes over. This is implemented in the package its.CounterInternalGUI , where
the class CounterInternalPanel with an internally defined listener replaces both
CounterPanel and CounterListener .Weonly print the listing of this class.

File: its/CounterInternalGUI/CounterInternalPanel.java

1.

package its.CounterInternalGUI;

2.
3.

import javax.swing.JPanel;

4.

import javax.swing.JButton;

5.

import javax.swing.JLabel;

6.

import java.awt.BorderLayout;

import javax.swing.SwingConstants; 7.

import java.awt.event.ActionListener; 8.

import java.awt.event.ActionEvent; 9.

10.
11.

public class CounterInternalPanel extends JPanel {

12.
13. private CounterModel counter;