protected FilePreferences dumpFile(){
synchronized (preferencesFile) {
    try {
      mapper.writeValue(preferencesFile,rootNode);
      return this;
    }
 catch (    IOException e) {
      throw new RuntimeException(e);
    }
  }
}

protected FilePreferences dumpFile ( ) { synchronized ( preferencesFile ) { try { mapper . writeValue ( preferencesFile , rootNode ) ; return this ; } catch ( IOException e ) { throw new RuntimeException ( e ) ; } } }
dump the in - memory cache into the file .
