JCheckBoxに不定状態のアイコンを追加する

JCheckBox checkBox = new JCheckBox("TriState JCheckBox") {
  protected TriStateActionListener listener = null;
  class TriStateActionListener implements ActionListener {
    protected Icon icon;
    public void setIcon(Icon icon) {
      this.icon = icon;
    }

    @Override public void actionPerformed(ActionEvent e) {
      JCheckBox cb = (JCheckBox) e.getSource();
      if (!cb.isSelected()) {
        cb.setIcon(icon);
      } else if (cb.getIcon() != null) {
        cb.setIcon(null);
        cb.setSelected(false);
      }
    }
  }

  @Override public void updateUI() {
    final Icon oi = getIcon();
    removeActionListener(listener);
    setIcon(null);
    super.updateUI();
    EventQueue.invokeLater(new Runnable() {
      @Override public void run() {
        if (listener == null) {
          listener = new TriStateActionListener();
        }
        Icon icon = new IndeterminateIcon();
        listener.setIcon(icon);
        addActionListener(listener);
        if (oi != null) {
          setIcon(icon);
        }
      }
    });
  }
};

class IndeterminateIcon implements Icon {
  private final Color color = UIManager.getColor("CheckBox.foreground");
  private final Icon icon = UIManager.getIcon("CheckBox.icon");
  @Override public void paintIcon(Component c, Graphics g, int x, int y) {
    icon.paintIcon(c, g, x, y);
    int w = getIconWidth(), h = getIconHeight();
    int a = 4, b = 2;
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setPaint(Color.BLACK);
    g2.translate(x, y);
    g2.fillRect(a, (h - b) / 2, w - a - a, b);
    g2.dispose();
  }

  @Override public int getIconWidth() {
    return icon.getIconWidth();
  }

  @Override public int getIconHeight() {
    return icon.getIconHeight();
  }
}