Need help creating a JComboBox with rounded corners

public class RoundComboBoxUI extends BasicComboBoxUI 
{
    Color bgColor = new Color( 0,0,0,220 );
 
    protected void installDefaults()
    {
        super.installDefaults();
        comboBox.setOpaque(true );
        comboBox.setBackground( bgColor );
        comboBox.setForeground( Color.white );
        comboBox.setBorder(new SimpleRoundBorder());
    }
 
    protected JButton createArrowButton() 
    {
        JButton button = new BasicArrowButton(BasicArrowButton.SOUTH, bgColor, bgColor, Color.white, bgColor );
        button.setUI( new BorderlessButtonUI() );
        return button;
    }    
 
    class SimpleRoundBorder extends AbstractBorder 
    {
        public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) 
        {
            ((Graphics2D)g).setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            ((Graphics2D)g).setColor( bgColor );
            ((Graphics2D)g).drawRoundRect(x, y, width-2, height-2, 12, 12); 
        }
 
        public Insets getBorderInsets(Component c) 
        {
            return new Insets(3, 6, 3, 3);
        }
 
        public Insets getBorderInsets(Component c, Insets insets) 
        {
            insets.top = insets.left = insets.bottom = insets.right = 3;
            return insets;
        }
    }
}
 
 
I basically do this in the calling function:
JComboBox paletteCombo = new JComboBox();
paletteCombo.setUI( new RoundComboBoxUI() );
paletteCombo.setForeground( Color.white );