// Располагаем экран в центре
            Toolkit kit = Toolkit.getDefaultToolkit();
            Dimension screenSize = kit.getScreenSize();
            setLocation (screenSize.width/ 2 - 450 , screenSize.height/ 2 - 350 );

frame.setSize( 900 , 700 );
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
    Dimension frameSize = frame.getSize();
     if  (frameSize.height > screenSize.height) {
      frameSize.height = screenSize.height;
    }
     if  (frameSize.width > screenSize.width) {
      frameSize.width = screenSize.width;
    }
    frame.setLocation((screenSize.width - frameSize.width) /  2 , (screenSize.height - frameSize.height) /  2 );
    frame.setVisible(true);