Need an hint on disposing a dialog.

Question

Hi there,
Using: Netbeans 4.1 with the Sun JDK 1.5.0_06 on Linux.
I recently started to study Java and so far I love it. Last evening (after my "daily tutorial section") I tried setting up a very basic program only armed with the JDK 1.5 documentation provided by Sun. The task: a dialog with "Hello World".
Now.. I managed to come up with a very basic program using the JFrame class, and it works too:
import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class HelloWorld extends JOptionPane {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        showMessageDialog(frame, "Hello World!");
        frame.dispose();
}And yes, for your fun and amusement I did make the mistake of forgetting all about dispose() when I first started the critter. So far, so good. Now I want more; I read about the method "setDefaultCloseOperation(int)" which allows the JFrame to determine what to do when a user closes the window. So, after some more reading, I came up with:
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.WindowConstants;
public class HelloWorld extends JOptionPane {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        showMessageDialog(frame, "Hello World!");
        // frame.dispose();
}The only problem here is that it doesn't do what I expected and so far I can't put my finger on it. The user clicks 'ok', the dialog disappears and the application continues to run. Just to be sure I tried replacing the "WindowConstants.DISPOSE_ON_CLOSE" with the value 2 (found on the constants list) and even the JFrame field "EXIT_ON_CLOSE" (so instead of the constant or '2' a mere "frame.EXIT_ON_CLOSE") but nothing! I then found on the Sun tutorial "how to make frames" the example "frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);" which I then tried but I got the same results.
So my questions are basicly:
Could anyone hint me where my thinking goes wrong ? I'm not looking for ready made solutions, but trying to understand whats up here.
Second: I read that this is an extension with JFrame and is not found on Frame. Would my conclusion be right (not only based on these experiments) that it would probably be best to keep using dispose() so that I'm not making myself dependend on specific extensions ?
Last, but not least, in my attempts I tried using "frame.EXIT_ON_CLOSE". Isn't that basicly equal to my "JFrame.EXIT_ON_CLOSE" ?
I am probably going to hate myself for that last question since I can't help feel having missed something obvious from the tutorials (which I'm going to re-read later on) but alas..
Thanks for any input.

Answer 1

if you do this
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);and you only have one frame, then when you close that frame it effectively is a System.exit(0); and a call to dispose() is unnecessary.
however, when a frame is created, there are listeners created(among other things) that hold a reference to the frame. if your application has multiple windows and you want to close one of them without exiting the application, then you need to call dispose(). this "disposes" of all the things in the background(trying not to be too technical) and allows the frame and all associated "stuff" to be garbage collected. In this case, you would want to add a window listener and call dispose() in the windowClosing() method.
this is a very simplistic explanation but I think it's appropriate not to get into too much detail at this point. you should be able to find some good reading material about this with a Google search.
kind regards
walken