code: public INodeHardLinkFile getHardLinkedFile(int i) {
      if (i < this.linkedFiles.size()) {
        return this.linkedFiles.get(i);
      }
      return null;
    }
docs: Return the i th of the hardlinked file
  @param i
  @return the i th of the hardlinked files and return null if i is out of the boundary.