Swing I - Absolute Java - page 952

The JButton Class

An object of the class JButton is displayed in a GUI as a component that looks like a
button. Click the button with your mouse to simulate pushing it. When creating an object
of the class JButton using new , you can give a string argument to the constructor and the
string will be displayed on the button.
You can add a JButton object to a JFrame by using the method add with the JFrame as
the calling object and the JButton object as the argument. You will later see that you can
also add buttons to other GUI objects (known as “containers”) in a similar way.
A button's action is programmed by registering a listener with the button using the method
addActionListener .
EXAMPLE
JButton niceButton = new JButton("Click here");
niceButton.addActionListener( new SomeActionListenerClass());
someJFrame.add(niceButton);

The Close-Window Button Is Not in the Class

JButton

The buttons that you add to a GUI are all objects of the class JButton . The close-window
button and the other two accompanying buttons on a JFrame are not objects of the class
JButton . They are part of the JFrame object.

Action Listeners and Action Events
Clicking a button with your mouse (or activating certain other items in a GUI) creates
an object known as an event and sends the event object to another object (or objects)
known as the listener(s). This is called firing the event . The listener then performs
some action. When we say that the event is “sent” to the listener object, what we really
mean is that some method in the listener object is invoked with the event object as the
argument. This invocation happens automatically. Your Swing GUI class definition
will not normally contain an invocation of this method. However, your Swing GUI
class definition does need to do two things:
• First, for each button, it needs to specify what objects are listeners that will respond
to events fired by that button; this is called registering the listener.
• Second, it must define the methods that will be invoked when the event is sent to
the listener. Note that these methods will be defined by you, but in normal circum-
stances, you will never write an invocation of these methods. The invocations will
take place automatically.
The following lines from Display 17.2 create an EndingListener object named
buttonEar and register buttonEar as a listener to receive events from the button
named endButton :
EndingListener buttonEar = new EndingListener( );
endButton.addActionListener(buttonEar);

registering a
listener

addAction
Listener