code: public static String base64ToString(String base64Value) {
          Utils.require(base64Value.length() % 4 == 0,
                        "Invalid base64 value (must be a multiple of 4 chars): " + base64Value);
          byte[] utf8String = DatatypeConverter.parseBase64Binary(base64Value);
          return toString(utf8String);
      }
docs: Decode the given base64 value to binary, then decode the result as a UTF-8 sequence
  and return the resulting String.
  
  @param base64Value   Base64-encoded value of a UTF-8 encoded string.
  @return              Decoded string value.