Simple GUI-Based Input/Output with JOptionPane

Simple GUI Based Input Output with JOptionPane

The applications in Chapters 210 display text at the command window and obtain input from the command window. Most applications you use on a daily basis use windows or dialog boxes (also called dialogs) to interact with the user. For example, e-mail programs allow you to type and read messages in a window provided by the e-mail program. Typically, dialog boxes are windows in which programs display important messages to the user or obtain information from the user. Java's JOptionPane class (package javax.swing) provides prepackaged dialog boxes for both input and output. These dialogs are displayed by invoking static JOptionPane methods. Figure 11.2 presents a simple addition application that uses two input dialogs to obtain integers from the user and a message dialog to display the sum of the integers the user enters.

Figure 11.2. JOptionPane input and message dialogs used to input values from the user and display results.

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Line 3 imports class JOptionPane for use in this application. Lines 1011 declare the local String variable firstNumber and assign it the result of the call JOptionPane static method showInputDialog. This method displays an input dialog (see the first screen capture in Fig. 11.2), using the method's String argument ("Enter first integer") as a prompt to the user.

Look-and-Feel Observation 11.2

The user types characters in the text field, then clicks the OK button or presses the Enter key to submit the String to the program. Clicking OK also dismisses (hides) the dialog. [Note: If you type in the text field and nothing appears, activate the text field by clicking it with the mouse.] Unlike Scanner, which can be used to input values of several types from the user at the keyboard, an input dialog can input only input Strings. This is typical of most GUI components. Technically, the user can type anything in the input dialog's text field. Our program assumes that the user enters a valid integer value. If the user clicks the Cancel button, showInputDialog returns null. If the user either types a noninteger value or clicks the Cancel button in the input dialog, a runtime logic error will occur in this program and it will not operate correctly. Chapter 13, Exception Handling, discusses how to handle such errors. Lines 1213 display another input dialog that prompts the user to enter the second integer.

To perform the calculation in this application, we must convert the Strings that the user entered to int values. Recall from Section 7.12 that the Integer class's static method parseInt converts its String argument to an int value. Lines 1617 assign the converted values to local variables number1 and number2. Then, line 19 sums these values and assigns the result to local variable sum.

Lines 2223 use JOptionPane static method showMessageDialog to display a message dialog (the last screen capture of Fig. 11.2) containing the sum. The first argument helps the Java application determine where to position the dialog box. The value null indicates that the dialog should appear in the center of the computer screen. The first argument can also be used to specify that the dialog should appear centered over a particular window, which we will demonstrate later in Section 11.8. The second argument is the message to displayin this case, the result of concatenating the String "The sum is " and the value of sum. The third argument"Sum of Two Integers"represents the string that should appear in the dialog's title bar at the top of the dialog. The fourth argumentJOptionPane.PLAIN_MESSAGEis the type of message dialog to display. A PLAIN_MESSAGE dialog does not display an icon to the left of the message. Class JOptionPane provides several overloaded versions of methods showInputDialog and showMessageDialog, as well as methods that display other dialog types. For complete information on class JOptionPane, visit java.sun.com/j2se/1.5.0/docs/api/javax/swing/JOptionPane.html.

Look-and-Feel Observation 11.3

JOptionPane Message Dialog Constants

The constants that represent the message dialog types are shown in Fig. 11.3. All message dialog types except PLAIN_MESSAGE display an icon to the left of the message. These icons provide a visual indication of the message's importance to the user. Note that a QUESTION_MESSAGE icon is the default icon for an input dialog box (see Fig. 11.2).