code: protected <N extends Page & Externalizable> PageFile<N> makePageFile(Class<N> cls) {
      @SuppressWarnings("unchecked")
      final PageFileFactory<N> castFactory = (PageFileFactory<N>) pageFileFactory;
      return castFactory.newPageFile(cls);
    }
docs: Make the page file for this index.
  
  @param <N>
  page type
  @param cls
  Class information
  @return Page file