//Полупрозрачная панель
    JPanel otherPanel = new JPanel( new BorderLayout() ) {
        @Override
        public void paintComponent( Graphics g ) {
            super.paintComponent( g );
            Graphics2D g2d = (Graphics2D) g.create();
                                                               //0.5f = 50% прозрачность
            g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f));

            g2d.setColor(getBackground());
            g2d.fillRect( 0, 0, getWidth(), getHeight() );

            g2d.dispose();
        }
    };