code: public void addFile (File file)
    {
      if (!files_.containsKey (file)) {
        long modifiedTime = file.exists() ? file.lastModified() : -1;
        files_.put (file, new Long (modifiedTime));
      }
    }
docs: Add file to listen for. File may be any java.io.File (including a
  directory) and may well be a non-existing file in the case where the
  creating of the file is to be trepped.
  <p>
  More than one file can be listened for. When the specified file is
  created, modified or deleted, listeners are notified.
  
  @param file  File to listen for.