code: public static String join(Collection collection, String separator) {
        if (collection == null) {
            return null;
        }
        return join(collection.iterator(), separator);
    }
docs: <p>Joins the elements of the provided <code>Collection</code> into
  a single String containing the provided elements.</p>
  
  <p>No delimiter is added before or after the list.
  A <code>null</code> separator is the same as an empty String ("").</p>
  
  <p>See the examples here: {@link #join(Object[],String)}. </p>
  
  @param collection  the <code>Collection</code> of values to join together, may be null
  @param separator  the separator character to use, null treated as ""
  @return the joined String, <code>null</code> if null iterator input
  @since 2.3