Graphical User Interfaces - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 204

7.2.3 Button events
Buttons provide the most common event generating components in user inter-
faces. (See the JButton and other button classes in the Swing hierarchy in
Figure 6.2.) Clicking on a button should initiate some action and in fact the term
action is used in the names for the button event handling machinery. A button
maintains a list of objects that implement the ActionListener interface. When
the button is pressed, it invokes the actionPerformed() method for all of the
ActionListener instances in its list. To include an ActionListener in its
list, you invoke the button's addActionListener(ActionListener al)
method.
ActionListener classes must provide an implementation of the method
void actionPerformed (ActionEvent ae)
Yo u use this method to code for the particular operation required when the
user clicks the button. An ActionEvent object is passed as the parameter of
actionPerformed() and from it you can extract information about the event
such as which button initiated the event.
We mentioned above the getSource() method inherited from EventOb-
ject that can identify what object sent the event. With buttons you can also use
the method getActionCommand() in ActionEvent to identify the button
that sent the event. This method returns either the text string displayed on the but-
ton or a string set directly with the setActionCommand() method (inherited
from AbstractButton class).
The PlainButtonApplet program below implements the ActionLis-
tener interface and so provides an actionPerformed() method. We create
two instances of JButton and then add the applet to the buttons' ActionLis-
tener lists with the statements:
buttonA.addActionListener (this);
buttonB.addActionListener (this);
Here the “ this „ reference refers to the applet instance.
Each time the GUI button is clicked with the mouse button, the GUI button's
actionPerformed() method is invoked. From the ActionEvent we obtain
the identity of the button via the getActionCommand() method and decide
Figure 7.1 The PlainButton applet running illustrates event handling for buttons.
Each click on a button sends a message to the status bar.