JAVA (III)

1.11. Displaying Text in a Message Dialog Box

The program in Listing 1.1 displays the text on the console, as shown in Figure 1.13. You can rewrite the program to display the text in a message dialog box. To do so, you need to use the showMessageDialog method in the JOptionPane class. JOptionPane is one of the many predefined classes in the Java system that you can reuse rather than "reinventing the wheel." You can use the showMessageDialog method to display any text in a message dialog box, as shown in Figure 1.14. The new program is given in Listing 1.2.

Figure 1.14. "Welcome to Java!" is displayed in a message box.

Listing 1.2. WelcomeInMessageDialogBox.java

1 /** This application program displays Welcome to Java! 2 * in a message dialog box. 3 */ 4 import javax.swing.JOptionPane; 5 6 public class WelcomeInMessageDialogBox { 7 public static void main(String[] args) { 8 // Display Welcome to Java! in a message dialog box 9 JOptionPane.showMessageDialog(null, "Welcome to Java!", 10 "Display Message", JOptionPane.INFORMATION_MESSAGE); 11 } 12 }

The line numbers are not part of the program, but are displayed for reference purposes.

This program uses a Java class JOptionPane (line 9). Java's predefined classes are grouped into packages. JOptionPane is in the javax.swing package. JOptionPane is imported to the program using the import statement in line 4 so that the compiler can locate the class. Recall that you have used the System class in the statement System.out.println("Welcome to Java"); in Listing 1.1. The System class is not imported because it is in the java.lang package. All the classes in the java.lang package are implicitly imported in every Java program.

Note

If you replace JOptionPane on line 9 with javax.swing.JOptionPane, you don't need to import it in line 4. javax.swing.JOptionPane is the full name for the JOptionPane class.

The showMessageDialog method is a static method. Such a method should be invoked by using the class name followed by a dot operator (.) and the method name with arguments. "Methods." The showMessageDialog method can be invoked with four arguments, as in lines 9–10.

The first argument can always be null. null is a Java keyword that will be fully introduced in Part II, "Object-Oriented Programming." The second argument can be a string for text to be displayed. The third argument is the title of the message box. The fourth argument can be JOptionPane.INFORMATION_MESSAGE, which causes the icon  to be displayed in the message box.

Note

There are several ways to use the showMessageDialog method. For the time being, all you need to know are two ways to invoke it. One is to use a statement, as shown in the example:

JOptionPane.showMessageDialog(null, x,
y, JOptionPane.INFORMATION_MESSAGE);

where x is a string for the text to be displayed, and y is a string for the title of the message box. The other is to use a statement like this one:

JOptionPane.showMessageDialog(null, x);

where x is a string for the text to be displayed.