code: public static String join(String separator, long... array) {
      checkNotNull(separator);
      if (array.length == 0) {
        return "";
      }
  
      // For pre-sizing a builder, just get the right order of magnitude
      StringBuilder builder = new StringBuilder(array.length * 10);
      builder.append(array[0]);
      for (int i = 1; i < array.length; i++) {
        builder.append(separator).append(array[i]);
      }
      return builder.toString();
    }
docs: Returns a string containing the supplied {@code long} values separated by {@code separator}.
  For example, {@code join("-", 1L, 2L, 3L)} returns the string {@code "1-2-3"}.
  
  @param separator the text that should appear between consecutive values in the resulting string
  (but not at the start or end)
  @param array an array of {@code long} values, possibly empty