code: public boolean exists(String filename)
          throws IOException, ServerException {
          if (filename == null) {
              throw new IllegalArgumentException("Required argument missing");
          }
          try {
              Reply reply =
                  controlChannel.exchange(new Command("RNFR", filename));
              if (Reply.isPositiveIntermediate(reply)) {
                  controlChannel.execute(new Command("ABOR"));
                  return true;
              } else {
                  return false;
              }
          } catch (FTPReplyParseException rpe) {
              throw ServerException.embedFTPReplyParseException(rpe);
          } catch (UnexpectedReplyCodeException urce) {
              throw ServerException.embedUnexpectedReplyCodeException(
                                  urce,
                                  "Abort failed");
          }
      }
docs: Checks if given file/directory exists on the server.
  
  @param  filename
  file or directory name
  @return true if the file exists, false otherwise.