JComboBoxのドロップダウンリストでアイテム選択状態表示をラウンド矩形に変更する

class RoundedCornerListCellRenderer<E> implements ListCellRenderer<E> {
  private static final Icon GAP = new GapIcon();
  private final DefaultListCellRenderer renderer = new DefaultListCellRenderer() {
    @Override protected void paintComponent(Graphics g) {
      if (getIcon() != null) {
        Graphics2D g2 = (Graphics2D) g.create();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setPaint(getBackground());
        Rectangle r = SwingUtilities.calculateInnerArea(this, null);
        g2.fill(new RoundRectangle2D.Float(r.x, r.y, r.width, r.height, 10f, 10f));
        super.paintComponent(g2);
        g2.dispose();
      } else {
        super.paintComponent(g);
      }
    }
  };

  @Override public Component getListCellRendererComponent(JList<? extends E> list, E value, int index, boolean isSelected, boolean cellHasFocus) {
    Component c = renderer.getListCellRendererComponent(
        list, value, index, isSelected, cellHasFocus);
    if (c instanceof JLabel) {
      JLabel label = (JLabel) c;
      label.setOpaque(false);
      label.setBorder(BorderFactory.createEmptyBorder(2, 2, 2, 2));
      label.setIconTextGap(0);
      label.setIcon(index >= 0 ? GAP : null);
      // Nimbus DerivedColor bug?
      Color fgc = isSelected
          ? new Color(list.getSelectionForeground().getRGB())
          : list.getForeground();
      label.setForeground(fgc);
    }
    return c;
  }
}

class GapIcon implements Icon {
  @Override public void paintIcon(Component c, Graphics g, int x, int y) {
    /* Empty paint */
  }

  @Override public int getIconWidth() {
    return 2;
  }

  @Override public int getIconHeight() {
    return 18;
  }
}