Re: JDialog Question...

Ramon <>
Tue, 01 Dec 2009 23:20:48 +0100
What a silly mistake :) Thank you Daniel for your useful tips.

Have a good day/night.

Daniel Pitts wrote:

Please don't top-post. Harder to read it makes the post.
Ramon wrote:

Daniel Pitts wrote:

Ramon wrote:


I'm trying to make a class that inherits from JDialog. The dialog
has two buttons: Ok and Cancel. When the OK button is clicked, the
main form is trying to read a result that is stored in the dialog's

Question: Is there a safe way how to read a dialog's instance
variable (using a getter) after the dialog is set to invisible (i.e.
this.setVisible(false)) ?

PS: In general I am able to read the dialog's variables but,
apparently, sometimes the dialog is destroyed by the garbage


If you have a reference to the JDialog, then it will not be garbage
collected. Are having a particular problem? What are the symptoms?

Provide an SSCCE if you want to get the most help from this
newsgroup. See <> if you don't know how or why to
provide an SSCCE.

The following is a short version of my Jdialog...

public class InputDialog extends JDialog
    private Boolean okButtonPressed_; // true = OK button pressed

    /* init here ... */

     * OK button even handler
    private void okButtonActionPerformed(java.awt.event.ActionEvent evt)
       this.okButtonPressed_ = true;

This is a method of the class which is calling my InputDialog:

    private void cmdUserSettings()
        InputDialog inputDialog = new InputDialog(null, true);

        // if ok button was pressed
        if (inputDialog.getOkButtonPressed()) // #####
        { /* ... */ }
        { /* ... */ }

        if (inputDialog != null)

The line marked with ##### is sometimes throwing a
NullPointerException, thus indicating that somehow the garbage is
destroying inputDialog...

While this isn't an SSCCE, I do believe I'm able to tell you the solution.

The garbage collector will not set a value to null, so the NPE is caused
by something else. In this case, your okButtonPressed_ is null.

If null doesn't make sense for your value, you should use the primitive
version, in this case "boolean" with a lower-case 'b'.

A couple of other comments. In Java, the convention is to avoid "_" in
names, except constants, which are ALL_UPPERCASE_LETTERS. Thus, a
better declaration for your case is:

private boolean okPressed;

public boolean isOkPressed() {
   return okPressed;

One other comment, for your use-case, you might be happier using
JOptionPane. some quick googling should give you some hints on how to
use it.

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