Building graphical user interfaces - Objects First with Java: A Practical Introduction Using BlueJ - page 395

Dialogs can be implemented in a similar way to our main JFrame . They often use the class
JDialog to display the frame.

For modal dialogs with a standard structure, however, there are some convenience methods in
class JOptionPane that make it very easy to show such dialogs. JOptionPane has, among
other things, static methods to show three types of standard dialog. They are:

■ Message dialog: This is a dialog that displays a message and has an OK button to close the
dialog.
■ Confirm dialog: This dialog usually asks a question and has buttons for the user to make a
selection—for example, Ye s , No , and Cancel .
■ Input dialog: This dialog includes a prompt and a text field for the user to enter some text.

Our “About” box is a simple message dialog. Looking through the JOptionPane documenta-
tion, we find that there are static methods named showMessageDialog to do this.

Exercise 11.32 Find the documentation for showMessageDialog . How many methods
with this name are there? What are the differences between them? Which one should we use
for the “About” box? Why?

Exercise 11.33 Implement the showAbout method in your ImageViewer class, using a
call to a showMessageDialog method.

Exercise 11.34 The showInputDialog methods of JOptionPane allow a user to be
prompted for input via a dialog when required. On the other hand, the JTextField compo-
nent allows a permanent text input area to be displayed within a GUI. Find the documentation
for this class. What input causes an ActionListener associated with a JTextField to be
notified? Can a user be prevented from editing the text in the field? Is it possible for a listener
to be notified of arbitrary changes to the text in the field? ( Hint : What use does a JTextField
make of a Document object?)

You can find an example of a JTextField in the calculator project in Chapter 7.

After studying the documentation, we can now implement our “About” box by making a call to
the showMessageDialog method. The code is shown in Code 11.9. Note that we have intro-
duced a string constant named VERSION to hold the current version number.

Code 11.9
Displaying a modal
dialog

private void showAbout()
{

JOptionPane.showMessageDialog(frame,
"ImageViewer\n" + VERSION,
"About ImageViewer" ,
JOptionPane.INFORMATION_MESSAGE);

}