我作FTP服务端，遇到难题！

Question

//mode
            if(ftpCmd.equalsIgnoreCase("mode")){
                //case "mode": {
                 try {
                   String arg = ftpCmdtok.nextToken();
                   if (arg.equals("s"))
                     showMessage("200 MODE S.");
                   else
                     showMessage("501 MODE S not found.");
                 }
                 catch (Exception e) {
                   showMessage("500 MODE Syntax.");
                  }
                return;
                }
               //stru
            if(ftpCmd.equalsIgnoreCase("stru")){
              // case "stru": {
                 try {
                   String arg = ftpCmdtok.nextToken();
                   if (arg.equals("f"))
                     showMessage("200 STRU F.");
                   else
                     showMessage("501 STRU F not found.");
                 }
                 catch (Exception e) {
                   showMessage("500 STRU Syntax.");
                 }
                  return;
             }               //list
            if(ftpCmd.equalsIgnoreCase("list")) {
               //case "list": {
                 String arg;
                 if (ftpCmdtok.hasMoreTokens())
                   arg = ftpCmdtok.nextToken();
                 else
                   arg = ".";                 ftpList(arg);
                return;
               }
               //nlst;
            if(ftpCmd.equalsIgnoreCase("nlst")) {
               //case "nlst": {
                 String arg;
                 if (ftpCmdtok.hasMoreTokens())
                   arg = ftpCmdtok.nextToken();
                 else
                   arg = ".";                 ftpList(arg);
                 return;
               }
               //    else  if(ftpCmd=="cdup")
            if(ftpCmd.equalsIgnoreCase("cdup")){
                 changeDirectory(currentPath.substring(0,
                                  currentPath.lastIndexOf("\\")));
                  return;
            }
           if(ftpCmd.equalsIgnoreCase("cwd")) {
               //case  "cwd":{
                  try{
                        String arg= ftpCmdtok.nextToken();
                        if(arg.equals("..")) {
                          changeDirectory(currentPath.substring(0,
                                  currentPath.lastIndexOf("\\")));
                        }
                        else   changeDirectory(arg);
                     }
                     catch(Exception e){
                        showMessage("500 CWD syntax.");
                     }
                  return;
              }
              //xpwd: current directory;
            if(ftpCmd.equalsIgnoreCase("xpwd")){
                //case  "xpwd":
                      showMessage("200 PWD "+currentPath);
                      return;
              }
               //stor :put
              if (ftpCmd.equalsIgnoreCase("stor")) {
                //case "stor":{
                   try{
                       String arg=ftpCmdtok.nextToken();
                     String path=currentPath+"\\"+arg;
                       //if (!(path.getCanonicalPath().startsWith(
                        //     rootPath.getCanonicalPath())))
                   if(!path.startsWith(rootPath))
                        throw new SecurityException();                       if(blnBinary) putLocalBinaryFile(path,arg);
                         else putLocalASCIIFile(path,arg);
                        }
                    catch(Exception ex) {
                       showMessage("500 STOR syntax.");
                      }
                     return;
                    }              //retr: get
           if(ftpCmd.equalsIgnoreCase("retr")) {
              //case "retr":{
                 try{
                       String arg=ftpCmdtok.nextToken();
                       File path=new File(currentPath,arg);                       if(!path.getCanonicalPath().startsWith(
                             rootPath.getCanonicalPath()))
                         throw new SecurityException();
                       getRemoteFile(path,arg);
                    }
                   catch(Exception ex) {
                       showMessage("500 RETR syntax.");
                   }
                 return;
                 }
              //help:
           if(ftpCmd.equalsIgnoreCase("help")) {
                //case  "help":{
                     String strhelp=
                           "Java FTP Server - <version 1.0> - ready."+linesep+"Help Message.";
                    showMessage("200 Remotehelp "+strhelp);
                   return;
                  }
              //noop;
           if(ftpCmd.equalsIgnoreCase("noop")){
                showMessage("200 OK.");
                 return ;
             }
                //quit
            if(ftpCmd.equalsIgnoreCase("quit")) {
               //case "quit":{
                      showMessage(
                       "221 Service closing control connection .Goodbye.");
                      resetDefault();
                    return;
              }
            showMessage("502 "+ftpCmd+" not found");
          }
        catch(Exception e){
               showMessage("500 "+ftpCmd+" Syntax.");
         }
     }