code: void addFileName(final String fileName) {
        writeLock.lock();
        try {
           final FileListCacheValue fileList = getFileList();
           boolean done = fileList.add(fileName);
           if (done) {
              updateFileList(fileList);
              if (trace)
                 log.trace("Updated file listing: added " + fileName);
           }
        } finally {
           writeLock.unlock();
        }
     }
docs: Adds a new fileName in the list of files making up this index
  @param fileName