APPENDIX

More about JOptionPane Dialog Boxes

In Chapter 2 you learned how to use the JOptionPane class to display message dialog boxes and input dialog boxes. This appendix provides a more detailed discussion of the dialog boxes you can create using JOptionPane. We will discuss the following types of dialog boxes and how you can display them. Message Dialog. This is a dialog box that displays a message. An OK button is also displayed. Input Dialog. This is a dialog box that prompts the user for input. It provides a text field where input is typed. An OK button and a Cancel button are also displayed. Confirm Dialog. This is a dialog box that asks the user a Yes/No question. A Yes button, a No button, and a Cancel button are displayed. Figure J-1 shows an example of each type of dialog box. Figure J-1 Message Box, Input Box, and Confirm Box

J-1

J-2

Appendix J

More about JOptionPane Dialog Boxes

The JOptionPane class, which is in the javax.swing package, provides static methods to display each type of dialog box.

More about Message Dialogs

The showMessageDialog method is used to display a message dialog. There are several overloaded versions of this method. Table J-1 describes two of the versions. Table J-1 The showMessageDialog Method Method
void showMessageDialog (Component parent, Object message)

Descriptions This method displays a message dialog. The argument passed into parent is a reference to the graphical component that the dialog box should be displayed within. If you pass null to this parameter, the dialog box appears in the center of the screen. The object passed to the message parameter contains the message that is to be displayed. This method displays a message dialog. The argument passed into parent is a reference to the graphical component that the dialog box should be displayed within. If you pass null to this parameter, the dialog box appears in the center of the screen. The object passed to the message parameter contains the message that is to be displayed. The string passed to the title parameter is displayed in the dialog boxs title bar. The value passed to messageType indicates the type of icon to display in the message box.

void showMessageDialog (Component parent, Object message, String title, int messageType)

Here is a statement that calls the rst version of the method:

JOptionPane.showMessageDialog(null, "Hello World");

Figure J-2 Message dialog box

The rst argument can be a reference to a graphical component. The dialog box is displayed inside that component. In this statement we pass null as the rst argument. This causes the dialog box to be displayed in the center of the screen. The second argument is the message that we wish to display. This code causes the dialog box in Figure J-2 to appear.

More about Message Dialogs

J-3

Notice that by default the dialog box in Figure J-2 has the string "Message" displayed in its title bar, and an information icon (showing the letter i) is displayed. You can control the text that is displayed in the title bar and the type of icon that is displayed with the second version of the showMessageDialog method. Here is an example:
JOptionPane.showMessageDialog(null, "Invalid Data", "My Message Box", JOptionPane.ERROR_MESSAGE);

In this method call, the third argument is a string that is displayed in the dialog boxs title bar. The fourth argument is a constant that species the type of message that is being displayed, which determines the type of icon that appears in the dialog box. The constant JOptionPane.ERROR_MESSAGE species that an error icon is to be displayed. This statement displays the dialog box shown in Figure J-3. Figure J-3 Message dialog with specified title and icon

The constants that you may use for the message type are JOptionPane.ERROR_MESSAGE, JOptionPane. INFORMATION_MESSAGE, JOptionPane.WARNING_MESSAGE, JOptionPane. QUESTION_MESSAGE, and JOptionPane.PLAIN_MESSAGE. The following statements call the method with each type of message. Figure J-4 shows the dialog boxes displayed by these messages.
// Display an error message. JOptionPane.showMessageDialog(null, "Error Message", "Error", JOptionPane.ERROR_MESSAGE); // Display an information message. JOptionPane.showMessageDialog(null, "Information Message", "Information", JOptionPane.INFORMATION_MESSAGE); // Display a warning message. JOptionPane.showMessageDialog(null, "Warning Message", "Warning", JOptionPane.WARNING_MESSAGE); // Display a question message. JOptionPane.showMessageDialog(null, "Question Message", "Question", JOptionPane.QUESTION_MESSAGE);

J-4

Appendix J

More about JOptionPane Dialog Boxes // Display a plain message. JOptionPane.showMessageDialog(null, "Plain Message", "Message", JOptionPane.PLAIN_MESSAGE);

Figure J-4 Different types of messages

If the previous code were written into a program just as it appears and then executed, the ve dialog boxes shown in Figure J-4 would be displayed one at a time. The user would have to click the OK button on the rst dialog box to close it before the second dialog box would appear. The same would be true for all of the dialog boxes that follow. The dialog boxes displayed by the JOptionPane class are modal dialog boxes. A modal dialog box suspends execution of any other statements until the dialog box is closed. For example, when the JOptionPane.showMessageDialog method is called, the statements that appear after the method call do not execute until the user closes the message box. This is illustrated in Figure J-5. Figure J-5 Execution of statement after displaying a modal dialog box

More about Input Dialogs

J-5

More about Input Dialogs

An input dialog is a quick and simple way to ask the user to enter data. Table J-2 describes two overloaded versions of the static showInputDialog method, which displays an input dialog. The following code calls the rst version of the showInputDialog method:
String name; name = JOptionPane.showInputDialog("Enter your name.");

Table J-2 The showInputDialog Method Method

String showInputDialog (Object message)

Description This method displays an input dialog that provides a text field for the user to type input. The object passed to the message parameter contains the message that is to be displayed. If the user clicks on the OK button, this method returns the string that was entered by the user. If the user clicks on the Cancel button, this method returns null. This method displays an input dialog that provides a text input field for the user to type input. The argument passed into parent is a reference to the graphical component that the dialog box should be displayed within. If you pass null to this parameter, the dialog box appears in the center of the screen. The object passed to the message parameter contains the message that is to be displayed. The string passed to the title parameter is displayed in the dialog boxs title bar. The value passed to messageType indicates the type of icon to display in the message box. If the user clicks on the OK button, this method returns the string that was entered by the user. If the user clicks on the Cancel button, this method returns null.

String showInputDialog (Component parent, Object message, String title, int messageType)

The argument passed to the method is the message to display. This statement causes the dialog box shown in Figure J-6 to be displayed in the center of the screen. If the user clicks on the OK button, name references the string value entered by the user into the text eld. If the user clicks the Cancel button, name references null. Figure J-6 Input dialog box

J-6

Appendix J

More about JOptionPane Dialog Boxes

By default the input dialog box has the string Input in its title bar and displays a question icon. The second version of the method shown in Table J-2 allows you to control the text displayed in the input dialogs title bar and the type of icon displayed. It takes the same arguments as the second version of the showMessageDialog method in Table J-1. Here is an example:
String value; value = JOptionPane.showInputDialog(null, "Enter the value again.", "Enter Carefully!", JOptionPane.WARNING_MESSAGE);

This statement displays the input dialog shown in Figure J-7. If the user clicks on the OK button, value references the string value entered by the user into the text eld. If the user clicks on the Cancel button, value references null. Figure J-7 Input dialog box

Displaying Confirm Dialogs

A conrm dialog box typically asks the user a Yes or No question. By default a Yes button, a No button, and a Cancel button are displayed. The showConfirmDialog method is used to display a conrm dialog box. There are several overloaded versions of this method. Table J-3 describes two of them. The following code calls the rst version of the method:
int value; value = JOptionPane.showConfirmDialog(null, "Are you sure?");

The rst argument can be a reference to a graphical component, and the dialog box is displayed inside that component. In this statement we pass null, which causes the dialog box to be displayed in the center of the screen. The second argument is the message that we wish to display. This code causes the dialog box in Figure J-8 to appear. By default the conrm dialog box displays Select an Option in its title bar, a Yes button, a No button, and a Cancel button. The showConfirmDialog method returns an integer that represents the button clicked by the user. You can determine which button the user clicked by comparing the methods return value to one of the following constants: JOptionPane.YES_OPTION, JOptionPane.NO_OPTION, or JOptionPane.CANCEL_OPTION.

Displaying Confirm Dialogs

J-7

Table J-3 The showConfirmDialog Method Method

int showConfirmDialog (Component parent, Objectmessage)

Description The argument passed into parent is a reference to the graphical component that the dialog box should be displayed within. If you pass null to this parameter, the dialog box appears in the center of the screen. The object passed to the message parameter contains the message that is to be displayed. The method returns an integer that represents the button clicked by the user. The argument passed into parent is a reference to the graphical component that the dialog box should be displayed within. If you pass null to this parameter, the dialog box appears in the center of the screen. The object passed to the message parameter contains the message that is to be displayed. The string passed to the title parameter is displayed in the dialog boxs title bar. The value passed to optionType indicates the types of buttons to display in the dialog box. The method returns an integer that represents the button clicked by the user.

int showConfirmDialog (Component parent, Object message, String title, int optionType)

Figure J-8 Confirm dialog box

Here is an example:
int value; value = JOptionPane.showConfirmDialog(null, "Are you sure?"); if (value == JOptionPane.YES_OPTION) {

If the user clicked Yes, the code here is executed.

} else if (value == JOptionPane.NO_OPTION) {

If the user clicked No, the code here is executed.

} else if (value == JOptionPane.CANCEL_OPTION) {

If the user clicked Cancel, the code here is executed.

}

J-8

Appendix J

More about JOptionPane Dialog Boxes

The second version of the method shown in Table J-3 allows you to control the text displayed in the conrm dialogs title bar and the type of buttons that are displayed. The rst three arguments are the-same as those used for the second version of the showMessageDialog method in Table J-1. The fourth argument species the types of buttons that are to appear in the dialog box. You may use one of the following constants: JOptionPane.YES_ NO_OPTION or JOptionPane.YES_NO_CANCEL_OPTION. For example, the following code displays a conrm dialog box with only a Yes button and a No button, as shown in Figure J-9.
int value; value = JOptionPane.showConfirmDialog(null, "Are you sure?", "Please Confirm", JOptionPane.YES_NO_OPTION);

Figure J-9 Confirm dialog box with a Yes button and a No button

An Example Program
The program in Code Listing J-1 displays each of the types of dialog boxes we have discussed. Code Listing J-1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 /** This program demonstrates different types of dialog boxes. */ import javax.swing.JOptionPane; public class TestAverageDialog { public static void main(String [] args) { String input; // User input int score1, score2, score3; // test scores double average; // Average test score int repeat; // Confirm dialog button clicked do {

(TestAverageDialog.java)

An Example Program 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 } // Get the three test scores. input = JOptionPane.showInputDialog(null, "Enter score #1."); score1 = Integer.parseInt(input); input = JOptionPane.showInputDialog(null, "Enter score #2."); score2 = Integer.parseInt(input); input = JOptionPane.showInputDialog(null, "Enter score #3."); score3 = Integer.parseInt(input); // Calculate and display the average test score. average = (score1 + score2 + score3) / 3.0; JOptionPane.showMessageDialog(null, "The average is " + average); // Does the user want to average another set? repeat = JOptionPane.showConfirmDialog(null, "Would you like to average another " + "set of test scores?", "Please Confirm", JOptionPane.YES_NO_OPTION); } while (repeat == JOptionPane.YES_OPTION); System.exit(0); }

J-9

When this program executes, the dialog boxes shown in Figure J-10 are displayed, one at a time. Notice the last statement in this program, in line 45:
System.exit(0);

This statement causes the program to end and is required in any GUI program. Unlike a console program, a GUI program does not automatically stop executing when the end of the main method is reached. This is because Swing generates a thread, which is a process running in the computer. If the System.exit method is not called, this thread continues to execute, even after the end of the main method has been reached. The System.exit method requires an integer argument. This argument is an exit code that is passed back to the operating system. Although this code is usually ignored, it can be used outside the program to indicate whether the program ended successfully or as the result of a failure. The value 0 traditionally indicates that the program ended successfully.

J-10

Appendix J

More about JOptionPane Dialog Boxes

Figure J-10 Dialog boxes displayed by the TestAverageDialog program

The first dialog box appears as shown here with example input.

When the user clicks OK, the second dialog box appears, as shown here with example input.

When the user clicks OK, the third dialog box appears, as shown here with example input.

When the user clicks OK, the fourth dialog box appears, as shown here.

When the user clicks OK, the fifth dialog box appears, as shown here. If the user clicks Yes, the loop will iterate again.