Java Reference
In-Depth Information
The dialog pops up when you select the
About
item in the
Help
menu. Until you select the
OK
button in
the
About Sketcher
dialog, you can't interact with the application window at all since we created this as
a modal dialog. By changing the last argument in the call to the superclass constructor in the
AboutDialog
constructor, you can make it non-modal and see how that works. This kind of dialog is
usually modal though.
If you resize the application window before you display the
About
dialog, you'll see that its position of
the dialog relative to the application window is adjusted accordingly.
How It Works
This is stuff that should be very familiar by now. We create a
JMenu
object for the
Help
item on the
menu bar, and add a shortcut for it by calling its
setMnemonic()
member. We create a
JMenuItem
object which is the
About
menu item and call its
addActionListener()
method to make the
SketchFrame
object the listener for the item. After adding the menu item to the
Help
menu, we add
the
helpMenu
object to the
menubar
object.
We create an
AboutDialog
object in the
actionPerformed()
method for the
SketchFrame
object, as this will be called when the
About
menu item is clicked. Before we display the dialog, we
verify that the source of the event is the menu item,
aboutItem
. This is not important now, but we will
add other menu items later, and we will want to handle their events using the same
actionPerformed()
method. The dialog object is self-contained and disposes of itself when the
OK
button is clicked. The dialog that we want to display here will always display the same message, so
there's no real point in creating and destroying it each time we want to display it. You could arrange for
the dialog box object to be created once, and the reference stored as a member of the
SketchFrame
class. Then you make it visible in the
actionPerformed()
method for the menu item and make it
invisible in the
actionPerformed()
method responding to the dialog
OK
button event.
This is all very well, but it was a lot of work just to get a dialog with a message displayed. Deriving a
class from
JDialog
gives you complete flexibility as to how the dialog works, but we didn't really need
it in this case. Didn't we say there was an easier way?
Instant Dialogs
The
JOptionPane
class in the
javax.swing
package defines a number of static methods that will
create and display standard modal dialogs for you. The simplest dialog you can create this way is a
message dialog rather like our
About
message dialog. The following methods produce message dialogs:
