JDialog position

Question

Hello,
I wonder how the position of a JDialog is decided.
In the following code it is positioned below the parent frame. But when using the commented line with setLocationRelativeTo(DialogTest.this), the dialog is nicely placed in the parent's center.
Quite often I had also dialogs with the top left corner centered in the parent. Is there any rule as for what happens when?
Greetings
Jörg
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class DialogTest extends JFrame {
  DialogTest dt;
  public DialogTest() {
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    setSize(300,300);
    setLocation(300,100);
    Container cp= getContentPane();
    JButton b= new JButton("Show dialog");
    b.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
     JDialog dlg= new JDialog(DialogTest.this);
//     JDialog dlg= new JDialog(dt);
     dlg.setSize(200,100);
     dlg.setModal(true);
//     dlg.setLocationRelativeTo(DialogTest.this);
     dlg.setLocationRelativeTo(dt);
     dlg.setResizable(false);
     dlg.setTitle("The dialogue");
     dlg.setVisible(true);
    cp.add(b, BorderLayout.SOUTH);
    setVisible(true);
  public static void main(String args[]) {
    java.awt.EventQueue.invokeLater(new Runnable() {
      public void run() {
     DialogTest dt= new DialogTest();
//     new DialogTest();
}

Answer 1

dlg.setLocationRelativeTo(dt);well dt is "null" so you are really saying
dlg.setLocationRelativeTo(null);which means center the dialog in the center of the screen.
Quite often I had also dialogs with the top left corner centered in the parentThe location of the dialog is determined at the time the method is invoked, not at the time the dialog is made visible. So this would happen if the size of the frame is (0, 0) when the method is invoked.
However, this may be in older versions of Java. It does not appear to happen in JDK6 on XP:
import javax.swing.*;
public class DialogTest5
  public static void main(String args[])
       JFrame frame1 = new JFrame();
     frame1.setVisible(true);
     JDialog dialog1 = new JDialog(frame1, false);
     dialog1.setLocationRelativeTo(frame1);
//     frame1.setSize(200, 200); // has no effect on dialog location
     dialog1.setVisible(true);
       JFrame frame2 = new JFrame();
       frame2.setBounds(500, 500, 200, 200);
       frame2.setVisible(true);
     JDialog dialog2 = new JDialog(frame2, false);
//     dialog2.setSize(150, 100); // uncomment to see how this affects placement
     dialog2.setLocationRelativeTo(frame2);
     dialog2.setVisible(true);
}