code: public CollectionTree getTree(String collectionId, String userId) throws JinxException {
      Map<String, String> params = new TreeMap<>();
      params.put("method", "flickr.collections.getTree");
      if (!JinxUtils.isNullOrEmpty(collectionId)) {
        params.put("collection_id", collectionId);
      }
      if (!JinxUtils.isNullOrEmpty(userId)) {
        params.put("user_id", userId);
      }
      return jinx.flickrGet(params, CollectionTree.class);
    }
docs: Returns a tree (or sub tree) of collections belonging to a given user.
  <br>
  This method does not require authentication.
  
  @param collectionId Optional. The ID of the collection to fetch a tree for, or zero to fetch the root collection. Defaults to zero.
  @param userId       Optional. The ID of the account to fetch the collection tree for. Deafults to the calling user.
  @return nested tree of collections, and the collections and sets they contain.
  @throws JinxException if there are any errors.
  @see <a href="https://www.flickr.com/services/api/flickr.collections.getTree.html">flickr.collections.getTree</a>