JListに固定ヘッダを実装する

class StickyLayerUI extends LayerUI<JScrollPane> {
  private final JPanel panel = new JPanel();

  @SuppressWarnings("unchecked")
  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    if (c instanceof JLayer) {
      JScrollPane scroll = (JScrollPane) ((JLayer<?>) c).getView();
      JViewport viewport = scroll.getViewport();
      JList<String> list = (JList<String>) viewport.getView();
      int cellHeight = list.getFixedCellHeight();
      Rectangle viewRect = viewport.getViewRect();
      Point vp = SwingUtilities.convertPoint(viewport, 0, 0, c);
      Point pt1 = SwingUtilities.convertPoint(c, vp, list);
      int idx1 = list.locationToIndex(pt1);
      Rectangle header1 = new Rectangle(
          vp.x, vp.y, viewRect.width, cellHeight);
      if (idx1 >= 0) {
        Graphics2D g2 = (Graphics2D) g.create();
        int headerIndex1 = getHeaderIndex1(list, idx1);
        Component c1 = getComponent(list, headerIndex1);
        int nhi = getNextHeaderIndex1(list, idx1);
        Point nextPt = list.getCellBounds(nhi, nhi).getLocation();
        if (header1.contains(
            SwingUtilities.convertPoint(list, nextPt, c))) {
          Dimension d = header1.getSize();
          SwingUtilities.paintComponent(
              g2, c1, panel, getHeaderRect(list, idx1, c, d));
          Component cn = getComponent(list, nhi);
          SwingUtilities.paintComponent(
              g2, cn, panel, getHeaderRect(list, nhi, c, d));
        } else {
          SwingUtilities.paintComponent(g2, c1, panel, header1);
        }
        g2.dispose();
      }
    }
  }

  private static int getHeaderIndex1(JList<String> list, int start) {
    return list.getNextMatch("0", start, Position.Bias.Backward);
  }

  private static int getNextHeaderIndex1(JList<String> list, int start) {
    return list.getNextMatch("0", start, Position.Bias.Forward);
  }

  private static Rectangle getHeaderRect(
      JList<?> list, int i, Component dst, Dimension d) {
    Rectangle r = SwingUtilities.convertRectangle(
        list, list.getCellBounds(i, i), dst);
    r.setSize(d);
    return r;
  }

  private static <E> Component getComponent(JList<E> list, int idx) {
    E value = list.getModel().getElementAt(idx);
    ListCellRenderer<? super E> renderer = list.getCellRenderer();
    Component c = renderer.getListCellRendererComponent(
        list, value, idx, false, false);
    c.setBackground(Color.GRAY);
    c.setForeground(Color.WHITE);
    return c;
  }

  // ...
}