Swing/CommonParentTreePath のバックアップ(No.1)

  private void showCommonParent() {
private void showCommonParent() {
  JTree tree = (JTree) getInvoker();
  Optional.ofNullable(tree.getSelectionPaths())
      .filter(paths -> paths.length > 1)
      .map(TreeUtils::findCommonParent)
      .ifPresent(p -> {
        Object node = p.getLastPathComponent();
        String title = "common parent";
        JOptionPane.showMessageDialog(
            tree, node, title, JOptionPane.INFORMATION_MESSAGE);
      });
}

// class TreeUtils ...
public static TreePath findCommonParent(TreePath... paths) {
  return Stream.of(paths)
      .map(TreePath::getPath)
      .reduce(TreeUtils::getCommonPath)
      .map(TreePath::new)
      .orElse(null);
}

public static <T> T[] getCommonPath(T[] node1, T[] node2) {
  int min = Math.min(node1.length, node2.length);
  for (int len = min; len > 0; len--) {
    T[] a1 = Arrays.copyOf(node1, len);
    T[] a2 = Arrays.copyOf(node2, len);
    if (Arrays.deepEquals(a1, a2)) {
      return a1;
    }
  }
  return Arrays.copyOf(node1, 1);
}