code: public String toFtpCmdArgument() {
          if (this.sporCommandParam == null && this.vector != null) {
              StringBuffer cmd = new StringBuffer();
              for (int i = 0; i < this.vector.size(); i ++) {
                  HostPort hp = (HostPort)this.vector.get(i);
                  if (i != 0) {
                      cmd.append(' ');
                  }
                  cmd.append(hp.toFtpCmdArgument());
              }
              this.sporCommandParam = cmd.toString();
          }
          return this.sporCommandParam;
      }
docs: Returns the host-port infromation in the
  format used by SPOR command.
  
  @return host-port information in SPOR command parameter
  representation.