It's defined in the look and feel. If I set Windows L&F and show a JDialog I get a dialog with only a close button. If I set Metal L&F I get a dialog with no buttons at all. If I set Motif L&F... I get a Windows dialog with motif widgets inside of it. (Even if I call JDialog.setDefaultLookAndFeelDecorated( true )) Unfortunately my Linix box is at home, so I can't test it out for sure, but I assume I would get the same results as you.
Write once, run anywhere, because there's nowhere to hide! - /. A.C.