JCheckBoxに不定状態のアイコンを追加する

class TriStateCheckBox extends JCheckBox {
  private transient TriStateActionListener listener;
  private transient Icon icon;

  protected TriStateCheckBox(String title) {
    super(title);
  }

  public void updateStatus(Status s) {
    switch (s) {
      case SELECTED:
        setSelected(true);
        setIcon(null);
        break;
      case DESELECTED:
        setSelected(false);
        setIcon(null);
        break;
      case INDETERMINATE:
        setSelected(false);
        setIcon(icon);
        break;
      default:
        throw new AssertionError("Unknown Status");
    }
  }

  @Override public void updateUI() {
    setIcon(null);
    removeActionListener(listener);
    super.updateUI();
    listener = new TriStateActionListener();
    icon = new IndeterminateIcon();
    listener.setIcon(icon);
    addActionListener(listener);
    if (Objects.nonNull(getIcon())) {
      setIcon(icon);
    }
  }
}

class IndeterminateIcon implements Icon {
  // private  FOREGROUND = UIManager.getColor("CheckBox.foreground");
  private static final Color FOREGROUND = Color.BLACK;
  private static final int MARGIN = 4;
  private static final int HEIGHT = 2;
  private final Icon icon = UIManager.getIcon("CheckBox.icon");

  @Override public void paintIcon(Component c, Graphics g, int x, int y) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.translate(x, y);
    icon.paintIcon(c, g2, 0, 0);
    g2.setPaint(FOREGROUND);
    g2.fillRect(
      MARGIN,
      (getIconHeight() - HEIGHT) / 2,
      getIconWidth() - MARGIN - MARGIN,
      HEIGHT);
    g2.dispose();
  }

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

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

enum Status {
  SELECTED, DESELECTED, INDETERMINATE
}