Swing II - Java - page 964

EXAMPLE: A Window Listener Inner Class

Display 18.2 gives an example of a class with a window listener class that is an inner

class. The window listener inner class is named

JFrame

. A window listener class need not
be an inner class, but it is frequently convenient to make a window listener class (or other
kind of listener class) an inner class.
The main

CheckOnExit

in Display 18.2 simply displays a message. What is interesting is how the
window listener programs the close-window button. You can apply the window listener used
in this

JFrame

to any . When the close-window button is clicked, a second, smaller

JFrame JFrame

window appears and asks: If the user clicks the

"Are you sure you want to exit?" "Yes"

button, the entire program ends and so both windows go away. If the user clicks the but-

ton, only the smaller window disappears; the program and the main window continue. Let's
look at the programming details.
When the close-window button in the main window is clicked, that fires a window event.
The only registered window listener is the anonymous object that is the argument to

"No"

. Below we repeat the relevant line of code, which is in the constructor for

addWindowListener

:

WindowListenerDemo

addWindowListener( new CheckOnExit());

This anonymous window listener object receives the window event fired when the close-window
button is clicked and then invokes the method

. The method cre- windowClosing windowClosing

ates and displays a window object of the class , which contains the message

ConfirmWindow "Are

as well as the two buttons labeled and . you sure you want to exit?" "Yes" "No"

"Yes" button, the action event fired by that button goes to the
actionPerformed method, which ends the program with a call to System.exit . If the user
clicks the "No" button, then the actionPerformed method invokes the method dispose . The
method dispose , discussed in the next subsection, makes its calling object go away but does
not end the program. The calling object for dispose is the smaller window (which is an object
of the class ConfirmWindow ), so this smaller window goes away but the main window remains.
Notice that even though we have registered a window listener, which says what should hap-
pen when the close-window button is clicked, we still need to invoke the method
setDefaultCloseOperation . When the close-window button is clicked, the policy set by
setDefaultCloseOperation is always carried out in addition to any action by window listeners. If we
do not include any invocation of setDefaultCloseOperation , then the default action is to make the
window go away (but not to end the program). We do not want our main window to go away, so we
set the policy as follows:

If the user clicks the

setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);

This means that clicking the close-window button causes no action other than the actions of
any window listeners. If you are using a window listener to set the action of the close-window
button, you invariably want an invocation of setDefaultCloseOperation with the argument
JFrame.DO_NOTHING_ON_CLOSE .