code: public static String base64ToHex(String base64Value) throws IllegalArgumentException {
          byte[] binary = base64ToBinary(base64Value);
          return DatatypeConverter.printHexBinary(binary);
      }
docs: Decode the given Base64-encoded String to binary and then return as a string of hex
  digits.
  
  @param base64Value               Base64-encoded string.
  @return                          Decoded binary value re-encoded as a hex string.
  @throws IllegalArgumentException If the given string is not a valid Base64 value.