code: public Reply execute(Command cmd)
          throws
              ServerException,
              IOException,
              FTPReplyParseException,
              UnexpectedReplyCodeException {
  
          Reply reply = exchange(cmd);
          // check for positive reply
          if (!Reply.isPositiveCompletion(reply)) {
              throw new UnexpectedReplyCodeException(reply);
          }
          return reply;
      }
docs: Write the command to the control channel,
  block until reply arrives and check if the command
  completed successfully (reply code 200).
  If so, return the reply, otherwise throw exception.
  Before calling this method make sure that no old replies are
  waiting on the control channel. Otherwise the reply returned
  may not be the reply to this command.
  @throws java.io.IOException on I/O error
  @throws FTPReplyParseException on bad reply format
  @throws UnexpectedReplyCodeException if reply is not a positive
  completion reply (code 200)
  @param cmd FTP command
  @return the first reply that waits in the control channel