Question

I iterate through a tree structure to collect the paths of the leaf nodes. Which way do you prefer to collect the result of the operation:

a) merge the results of the children and return this

private Collection<String> extractPaths(final Element element, final IPath parentPath) {
    final IPath path = parentPath.append(element.getLabel());
    final Collection<Element> children = getElementChildren(element);
    if (children.isEmpty())
        return Collections.singletonList(path.toString());

    final Set<String> result = new TreeSet<String>();
    for (final Element child : children)
        result.addAll(extractPaths(child, path));
    return result;
}

b) provide the result collection as a parameter and add new elements in each recursion step

private void extractPaths(final Element element, final IPath parentPath, final Set<String> result) {
    final IPath path = parentPath.append(element.getLabel());
    final Collection<Element> children = getElementChildren(element);
    if (children.isEmpty())
        result.add(path.toString());

    for (final Element child : children)
       extractPaths(child, path, result);
}

Answer 1

The final solution I found after some refactoring is to implement variant b) but to pass a Visitor instead of the result collection:

private void traverse(final Element element, final Visitor... visitors) {
   for (final Visitor visitor : visitors)
       // push e.g. the parent path to the stack
       visitor.push(visitor.visit(element)); 

   for (final Element child: getElementChildren(element))
       traverse(child, visitors);

   for (final Visitor visitor : visitors)
       visitor.pop();
}

The Visitor provides also a stack to carry the information about the parent path. This solution allows me to separate the traversal logic from the collection logic, without the need of the more complex TreeIterator implementation.

private class CollectPathsVisitor extends ElementVisitor {
    public final Set<String> paths = new TreeSet<String>();
    public Object visit(Element element) {
        final IPath parentPath = (IPath) peek();
        final IPath path = parentPath.append(element.getLabel());
        if (!hasChildren(element))
            paths.add(path);
        return path;
    }
}