JInternalFrame em um JDesktopPane

 private void centralizaForm(JInternalFrame frame) {
        Dimension desktopSize = frmMdi.getSize();
        Dimension jInternalFrameSize = frame.getSize();
        frame.setLocation((desktopSize.width - jInternalFrameSize.width) / 2,
                (desktopSize.height - jInternalFrameSize.height) / 2);
    }