@Override
    public List<Catalogue> sort(List<Catalogue> catalogueTree, SortTypeEnum sortTypeEnum) {
        log.debug(
                "sort catalogue tree based on first letter. catalogueTree: {}, sortTypeEnum: {}",
                catalogueTree,
                sortTypeEnum);
        Collator collator = Collator.getInstance(Locale.CHINA);
        return recursionSortCatalogues(catalogueTree, sortTypeEnum, collator);
    }
@Test
    public void sortEmptyTest2() {
        List<Catalogue> catalogueTree = null;
        SortTypeEnum sortTypeEnum = SortTypeEnum.ASC;

        List<Catalogue> resultList = catalogueTreeSortFirstLetterStrategyTest.sort(catalogueTree, sortTypeEnum);
        assertEquals(Lists.newArrayList(), resultList);
    }