code: public static String join(final Iterable<?> iterable, final char separator) {
          if (iterable == null) {
              return null;
          }
          return join(iterable.iterator(), separator);
      }
docs: <p>Joins the elements of the provided {@code Iterable} into
  a single String containing the provided elements.</p>
  
  <p>No delimiter is added before or after the list. Null objects or empty
  strings within the iteration are represented by empty strings.</p>
  
  <p>See the examples here: {@link #join(Object[],char)}. </p>
  
  @param iterable  the {@code Iterable} providing the values to join together, may be null
  @param separator  the separator character to use
  @return the joined String, {@code null} if null iterator input
  @since 2.3