code: public static String join(final long[] array, final char separator) {
          if (array == null) {
              return null;
          }
          return join(array, separator, 0, array.length);
      }
docs: <p>
  Joins the elements of the provided array into a single String containing the provided list of elements.
  </p>
  
  <p>
  No delimiter is added before or after the list. Null objects or empty strings within the array are represented
  by empty strings.
  </p>
  
  <pre>
  StringUtils.join(null, *)               = null
  StringUtils.join([], *)                 = ""
  StringUtils.join([null], *)             = ""
  StringUtils.join([1, 2, 3], ';')  = "1;2;3"
  StringUtils.join([1, 2, 3], null) = "123"
  </pre>
  
  @param array
  the array of values to join together, may be null
  @param separator
  the separator character to use
  @return the joined String, {@code null} if null array input
  @since 3.2