Trying to write an if statement around the return value of a CANCEL_OPTION

Question

I am writing a program that takes an input String using JOptionPane. The String can be made of only letters of the A-Z alphabet and the space character. I have written an error-checking loop that will pop another input box up along with an error message if any invalid character is detected. The input box has an 'ok' button and a 'cancel' button. As it is, if you hit cancel, the program crashes and you get a NullPointerException. All I want to do is place an if statement inside of the error-checking loop, immediatly after the code to pop up the second input box, that simply does System.exit() and exits the program correctly(rather than crashing) if 'cancel' is clicked.
My attempt at such an if statement is visible in the following code(letter is a boolean variable defined earlier, and message is the input String that has already been read in):
               letter = isValidMessage(message);
          while(!letter)
               message = JOptionPane.showInputDialog(null, "Message must contain only A-Z, a-z, or space character...");
                        if(JOptionPane.CANCEL_OPTION > 0  //or ==0 or ==1 or ==2, nothing works//)
                            JOptionPane.showMessageDialog(null, "No valid message entered - program will terminate...");
                            System.exit(JOptionPane.CANCEL_OPTION);
               letter = isValidMessage(message);
                }The if statement doesn't work correctly. If you click 'cancel' when the input box comes up, the program does terminate as it is supposed to. The problem is that if I type something in the box and click 'ok', it also causes the program to terminate rather than continuing. I've tried changing the > to ==. I've tried changing 0 to 1 to 2 to -1(I really am not sure which int return value of CANCEL_OPTION corresponds to which event). I think once I got to do the exact opposite...that is to say, it would continue on with the program whether I clicked 'cancel' or 'ok'. Which is equally bad. I don't know what I'm doing wrong.
It's probably something simple that I'm just overlooking.

Answer 1

Well, here's what CANCEL_OPTION is, according to the JOptionPane class:
/** Return value from class method if CANCEL is chosen. */
    public static final int         CANCEL_OPTION = 2;However, I believe you're going about this in the wrong way.
With JOptionPane, whenever you show a dialog, it usually returns whatever OPTION the user chose.
For instance,
    int choice = JOptionPane.showConfirmDialog(parentComponent, "hello");That will bring up a dialog centered on parentComponent, with a message of "hello".
It will have the default options (YES_NO_OPTION).
What it returns is the option the user chose.
There are 4 possible things it can return:
YES_OPTION, when user clicks "Yes"
NO_OPTION, when user clicks "No"
ERROR_OPTION, when an unforseen error occurs and the dialog closes
CANCEL_OPTION, when the user clicks "Cancel" (if that OPTION type were being used), or if they close the dialog
So this is how you would check for CANCEL_OPTION:
    int choice = JOptionPane.showConfirmDialog(parentComponent, "hello");
    if (choice == JOptionPane.CANCEL_OPTION) {
        // User cancelled
    else if (choice == JOptionPane.YES_OPTION) {
        // User chose yes
    // EtcSo you see, going about it like this:
if(JOptionPane.CANCEL_OPTION > 0  //or ==0 or ==1 or ==2, nothing works//)Is really the wrong way, because you're not checking what the user DID, but just what the constant variable "CANCEL_OPTION" is.
However, it seems as if you're in a unique situation where you don't want the OPTION chosen by the user, but an input String.
This is all well and good, but I think this limits how you can interact with the user, as the "showInputDialog" methods return the String the user input- not the OPTION they chose, and you need to use both.
What I've done in the past is created a simple JPanel with its own JTextField.
I pass the JPanel in as the "message" parameter of a showConfirmDialog call, and get both the user input and their chosen OPTION based off a getText call to the JTextField and the OPTION constant returned by showConfirmDialog.
Basically,
JPanel promptPanel = new JPanel(new BorderLayout());
JTextField promptField = new JTextField();
promptPanel.add(promptField, BorderLayout.CENTER);
int choice = JOptionPane.showConfirmDialog(parentComponent, promptPanel, "Input", JOptionPane.OK_CANCEL_OPTION, JOptionPane.QUESTION_MESSAGE);
if (choice == JOptionPane.OK_OPTION) {
   // User chose OK, see if invalid characters were entered
   String input = promptField.getText();
else if (choice == JOptionPane.CANCEL_OPTION) {
   // User cancelled
}Hope that helps.
(note that I'm unsure if showConfirmDialog can accept OK_CANCEL_OPTION as its OPTION parameter, as it says only YES_NO_OPTION and YES_NO_CANCEL_OPTION can be used, but I seem to remember using it in the past. if OK_CANCEL_OPTION doesn't want to work, then use one of the other two and adjust your if statements accordingly)
Cheers!
Edited by: LukeFoss on Oct 9, 2007 10:39 PM