JTreeの各ノードタイトルに章番号を自動追加して表示する

JTree tree = new JTree(makeModel()) {
  @Override public void updateUI() {
    setCellRenderer(null);
    super.updateUI();
    setCellRenderer(new ChapterNumberingTreeCellRenderer());
    setRootVisible(false);
  }
};
// ...
class ChapterNumberingTreeCellRenderer extends DefaultTreeCellRenderer {
  private static final String MARK = "\u00a7"; // "§";

  @Override public Component getTreeCellRendererComponent(
      JTree tree, Object value, boolean selected, boolean expanded,
      boolean leaf, int row, boolean hasFocus) {
    JLabel l = (JLabel) super.getTreeCellRendererComponent(
        tree, value, selected, expanded, leaf, row, hasFocus);
    if (value instanceof DefaultMutableTreeNode) {
      TreeNode[] tn = ((DefaultMutableTreeNode) value).getPath();
      String s = IntStream.range(1, tn.length).mapToObj(i -> {
        TreeNode n = tn[i];
        TreeNode p = n.getParent();
        return String.valueOf(1 + p.getIndex(n));
      }).collect(Collectors.joining("."));
      l.setText(String.format("%s%s %s", MARK, s, value));
    }
    return l;
  }
}