Section 3.9. (Optional) GUI and Graphics Case Study: Using Dialog Boxes

Displaying Text in a Dialog Box

Although the programs presented in this book thus far display output in the command window, many Java applications use windows or dialog boxes (also called dialogs ) to display output. For example, World Wide Web browsers such as Netscape or Microsoft Internet Explorer display Web pages in their own windows. E-mail programs allow you to type and read messages in a window. Typically, dialog boxes are windows in which programs display important messages to the user of the program. Class JOptionPane provides prepackaged dialog boxes that enable programs to display windows containing messages to userssuch windows are called message dialogs . Figure 3.17 displays the string "Welcome\nto\nJava" in a message dialog.

Figure 3.17. Using JOptionPane to display multiple lines in a dialog box.

1

// Fig. 3.17: Dialog1.java

2

// Printing multiple lines in dialog box.

3


import

javax.swing.JOptionPane;

// import class JOptionPane


4
 5

public class

Dialog1
 6  {
 7

public static void

main( String args[] )
 8     {
 9

// display a dialog with the message

10

JOptionPane.showMessageDialog(

null

,

"Welcome\nto\nJava"

);

11     }

// end main

12  }

// end class Dialog1

Line 3 indicates that our program uses class JOptionPane from package javax.swing . This package contains many classes that help Java programmers create graphical user interfaces ( GUIs ) for applications. GUI components facilitate data entry by a program's user, and formatting or presenting data outputs to the user. In method main , line 10 calls method showMessageDialog of class JOptionPane to display a dialog box containing a message. The method requires two arguments. The first argument helps the Java application determine where to position the dialog box. When the first argument is null , the dialog box appears in the center of the computer screen. The second argument is the String to display in the dialog box.

Method showMessageDialog is a special method of class JOptionPane called a static method . Such methods often define frequently used tasks that do not explicitly require creating an object. For example, many programs display messages to users in dialog boxes. Rather than require programmers to create code that performs this task, the designers of Java's JOptionPane class declared a static method for this purpose. Now, with a simple method call, all programmers can make a program display a dialog box containing a message. A static method typically is called by using its class name followed by a dot (.) and the method name , as in

ClassName.methodName( arguments )

Chapter 6, Methods: A Deeper Look will cover calling static methods in greater detail.

Entering Text in a Dialog Box

Our next application (Fig. 3.18) demonstrates input using dialogs. This program uses another predefined dialog box from class JOptionPane called an input dialog that allows the user to enter data for use in the program. The program asks for the user's name and responds with a greeting containing the name entered by the user.

Figure 3.18. Obtaining user input from a dialog.

1

// Fig. 3.18: NameDialog.java

2

// Basic input with a dialog box.

3

import

javax.swing.JOptionPane;
 4
 5

public class

NameDialog
 6  {
 7

public static void

main( String args[] )
 8     {
 9

// prompt user to enter name

10

String name =

11

JOptionPane.showInputDialog(

"What is your name?"

);

12
13

// create the message

14

String message =

15

String.format(

"Welcome, %s, to Java Programming!"

, name );

16
17

// display the message to welcome the user by name

18        JOptionPane.showMessageDialog(

null

, message );
19     }

// end main

20  }

// end class NameDialog

---

[Page 107]

Lines 1011 use method showInputDialog of class JOptionPane to display a simple input dialog containing a prompt and a field for the user to enter text, known as a text field . The argument to showInputDialog is the prompt that indicates what the user should enter. The user types characters in the text field, then clicks the OK button or presses the Enter key to return the String to the program. Method showInputDialog returns a String containing the characters typed by the user, which we store in variable name . [ Note: If you press the Cancel button in the dialog, the method returns null and the program displays the word "null" as the name.]

Lines 1415 use static String method format to return a String containing a greeting with the name entered by the user. Method format is similar to method System.out.printf , except that format returns a formatted String rather than displaying it in a command window. Line 18 displays the greeting in a message dialog.

GUI and Graphics Case Study Exercise

3.1

Modify the addition program in Fig. 2.7 to use dialog-based input with JOptionPane instead of console-based input using Scanner . Since method showInputDialog only returns a String , you must convert the String the user enters to an int for use in calculations. Method Integer.parseInt( String s ) takes a String argument representing an integer (e.g., the result of JOptionPane.showInputDialog ) and returns the value as an int . If the String does not contain a valid integer, then the program will terminate with an error.