I always thought that the fact that JDialogs accepted a parent frame as a constructor argument was the thing, that would make the dialog’s position relative to that of the frame:
public JDialog(final java.awt.Frame parent, boolean modal)
Unfortunately all of my freshly opened dialogs would appear in the upper left angle of my screen, not centered over the parent JFrame. I looked around and found out that I simply needed one more method call per dialog:
You need to call this method before the JDialog’s show() method. Now everything is perfect and the dialog appears always right above the center of the frame.