Java Reference
In-Depth Information
to be displayed on the screen. Thus, a window is a valuable resource that should be given
back to the system when it's no longer needed. Class
Window
(an indirect superclass of
JFrame
) declares method
dispose
for this purpose. When a
Window
is no longer needed in
an application, you should explicitly dispose of it. This can be done by calling the
Window
's
dispose
method or by calling method
setDefaultCloseOperation
with the argument
WindowConstants.DISPOSE_ON_CLOSE
. Terminating an application also returns window
resources to the system. Using
DO_NOTHING_ON_CLOSE
indicates that the program will de-
termine what to do when the user attempts to close the window. For example, the program
might want to ask whether to save a file's changes before closing a window.
Displaying and Positioning Windows
By default, a window is not displayed on the screen until the program invokes the win-
dow's
setVisible
method (inherited from class
java.awt.Component
) with a
true
argu-
ment. A window's size should be set with a call to method
setSize
(inherited from class
java.awt.Component
). The position of a window when it appears on the screen is speci-
fied with method
setLocation
(inherited from class
java.awt.Component
).
Window Events
When the user manipulates the window, this action generates
window events
. Event lis-
teners are registered for window events with
Window
method
addWindowListener
. The
WindowListener
interface provides seven window-event-handling methods—
window-
Activated
(called when the user makes a window the active window),
windowClosed
(called after the window is closed),
windowClosing
(called when the user initiates closing
of the window),
windowDeactivated
(called when the user makes another window the ac-
tive window),
windowDeiconified
(called when the user restores a minimized window),
windowIconified
(called when the user minimizes a window) and
windowOpened
(called
when a program first displays a window on the screen).
Menus
are an integral part of GUIs. They allow the user to perform actions without un-
necessarily cluttering a GUI with extra components. In Swing GUIs, menus can be at-
tached only to objects of the classes that provide method
setJMenuBar
. Two such classes
are
JFrame
and
JApplet
. The classes used to declare menus are
JMenuBar
,
JMenu
,
JMenu-
Item
,
JCheckBoxMenuItem
and class
JRadioButtonMenuItem
.
Look-and-Feel Observation 22.1
Menus simplify GUIs because components can be hidden within them. These components
will be visible only when the user looks for them by selecting the menu.
Overview of Several Menu-Related Components
Class
JMenuBar
(a subclass of
JComponent
) contains the methods necessary to manage a
menu bar
, which is a container for menus. Class
JMenu
(a subclass of
javax.swing.JMenu-
Item
) contains the methods necessary for managing menus. Menus contain menu items
and are added to menu bars or to other menus as submenus. When a menu is clicked, it
expands to show its list of menu items.
