code: public static String join(Object[] array, String separator) {
          if (array == null) {
              return null;
          }
          StringBuilder strBuilder = new StringBuilder();
          for (int i = 0, z = array.length; i < z; i++) {
              if (i > 0) {
                  strBuilder.append(separator);
              }
              strBuilder.append(array[i]);
          }
          return strBuilder.toString();
      }
docs: <p>
  Internal utility method. DO NOT use this method directly.
  </p>
  
  @param array array containing the elements to be joined
  @param separator separator to be used for the joined result
  @return the array elements, converted to String and joined with the separator