code: private int indexOf(SQSMessageIdentifier findMessage) {
          int i = 0;
          for (SQSMessageIdentifier sqsMessageIdentifier : unAckMessages) {
              i++;
              if (sqsMessageIdentifier.equals(findMessage)) {
                  return i;
              }
          }
          return -1;
      }
docs: Return the index of message if the message is in queue. Return -1 if
  message does not exist in queue.