stream流方式获取三级菜单

 @Override
    public PageUtils queryPage(Map<String, Object> params) {
        IPage<CategoryEntity> page = this.page(
                new Query<CategoryEntity>().getPage(params),
                new QueryWrapper<CategoryEntity>()
        );

        return new PageUtils(page);
    }

    @Override
    public List<CategoryEntity> listWithTree() {

        //1.查出所有分类
        List<CategoryEntity> categoryEntities = baseMapper.selectList(null);


        //组装成父子树形结构
        //找到所有的一级分类
        List<CategoryEntity> levelMenus = categoryEntities.stream().filter((categoryEntity) -> {
            return categoryEntity.getParentCid() == 0;
        }).map((menu)->{
            //元素类型转换方法，返回经过处理的数据
            menu.setChildren(getChildrens(menu, categoryEntities));
            //返回处理后数据
            return menu;
        }).sorted((menu1,menu2)->{
            //排序
            return (menu1.getSort()==null?0:menu1.getSort()) - (menu2.getSort()==null?0:menu2.getSort());
        }).collect(Collectors.toList());

        return levelMenus;
    }

    //递归查找所有菜单的字菜单
    private List<CategoryEntity> getChildrens(CategoryEntity root,List<CategoryEntity> all){

        List<CategoryEntity> childrenMenu = all.stream().filter((CategoryEntity) -> {
            return CategoryEntity.getParentCid() == root.getCatId();
        }).map((categoryEntity)->{
            //找到子菜单
            categoryEntity.setChildren(getChildrens(categoryEntity, all));
            return categoryEntity;
        }).sorted((menu1,menu2)->{
            //菜单的排序
            return (menu1.getSort()==null?0:menu1.getSort()) - (menu2.getSort()==null?0:menu2.getSort());
        }).collect(Collectors.toList());
        return childrenMenu;
    }
}