EVENT-DRIVEN PROGRAMMING - Introduction to Java Programming - page 605

Let's revisit Listing 16.1, HandleEvent.java. Since a JButton object fires ActionEvent ,
a listener object for ActionEvent must be an instance of ActionListener , so the listener
class implements ActionListener in line 34. The source object invokes
addActionListener(listener) to register a listener, as follows:

JButton jbtOK = new JButton( "OK" ); // Line 7 in Listing 16.1

create source object

OKListenerClass listener1
= new OKListenerClass(); // Line 18 in Listing 16.1

create listener object

jbtOK.addActionListener(listener1); // Line 20 in Listing 16.1

register listener

When you click the button, the JButton object fires an ActionEvent and passes it to invoke
the listener's actionPerformed method to handle the event.
The event object contains information pertinent to the event, which can be obtained using
the methods, as shown in Figure 16.7. For example, you can use e.getSource() to obtain
the source object that fired the event. For an action event, you can use e.getWhen() to
obtain the time when the event occurred.

java.util.EventObject

+getSource(): Object

Returns the source object for the event.

java.awt.event.AWTEvent

java.awt.event.ActionEvent

+getActionCommand(): String Returns the command string associated with this action. For a

button, its text is the command string.
Returns the modifier keys held down during this action event.
Returns the timestamp when this event occurred. The time is the

+getModifiers(): int
+getWhen(): long

number of milliseconds since January 1, 1970, 00:00:00 GMT.

F IGURE 16.7

You can obtain useful information from an event object.

We now write a program that uses two buttons to control the size of a circle, as shown in
Figure 16.8.

F IGURE 16.8

The user clicks the Enlarge and Shrink buttons to enlarge and shrink the size

of the circle.

We will develop this program incrementally. First we will write the program in Listing 16.2
that displays the user interface with a circle in the center (line 14) and two buttons on the bot-
tom (line 15).

first version