Java Reference
In-Depth Information
Directory "Sketcher 2 displaying an About dialog"
The constructor first calls the base
JDialog
class constructor to create a modal dialog with the title bar
given by the
title
argument. It then defines the position of the dialog relative to the position of the frame.
NOTE
Whenyoucreateaninstanceofthe
AboutDialog
classintheSketcher programalittle
later in this chapter, you specify the
SketcherFrame
object as the parent for the dialog. The
parent relationship between the application window and the dialog implies a lifetime depend-
ency. When the
SketcherFrame
object is destroyed, the
AboutDialog
object is, too, because
it is a child of the
SketcherFrame
object. This doesn't just apply to
JDialog
objects — any
Window
object can have another
Window
object as a parent.
By default the
AboutDialog
window is positioned relative to the top-left corner of the screen. To position
the dialog in a more sensible position relative to the application window, you set the coordinates of the top-
left corner of the dialog as one quarter of the distance across the width of the application window, and one
quarter of the distance down from the top-left corner of the application window.
You add the components you want to display in a dialog to the content pane for the
JDialog
object. The
content pane has a
BorderLayout
manager by default, just like the content pane for the application window,
and this is quite convenient for the
AboutDialog
layout. The dialog contains two
JPanel
objects that are
created in the constructor, one to hold a
JLabel
object for the message that is passed as an argument to the
constructor and the other to hold the OK button that closes the dialog. The
messagePane
object is added so
that it fills the center of the dialog window. The
buttonPane
position is specified as
BorderLayout.SOUTH
,
so it is at the bottom of the dialog window. Both
JPanel
objects have a
FlowLayout
manager by default.
You want the
AboutDialog
object to be the listener for the OK button so you pass the
this
variable as
the argument to the
addActionListener()
method call for the button.
The
pack()
method is inherited from the
Window
class. This method packs the components in the win-
dow, setting the window to an optimal size for the components it contains before laying out the components.
Note that if you don't call
pack()
here, the size for your dialog is not set and you aren't able to see it.
The
actionPerformed()
method is called when the OK button is selected. This just disposes of the dia-
log by calling the
dispose()
method for the
AboutDialog
object so the dialog window disappears from the
screen and the resources it was using are released.
Creating an About Menu Item
To add a Help menu with an About item to the Sketcher application, you need to insert some code into the
SketcherFrame
class constructor. You can make the
SketcherFrame
object the listener for the About menu
item by making it implement the
ActionListener
interface and adding the
actionPerformed()
method
implementation. Let's see it working.
TRY IT OUT: Displaying an About Dialog
