java8新特性stream过滤list集合

public R selectAnswerQuestionTopic(@RequestParam Long topicId){
        Map map = new HashMap();
        List list = answerQuestionTopicService.selectAnswerQuestionTopic(topicId);
        List result = list.stream().filter(answerQuestionTopic->
                answerQuestionTopic.getQuestionParentId() == 0//获取1级疑问
        ).map((menu)->{
            menu.setChild(getChildren(menu,list));
            return menu;
        }).sorted((menu1,menu2)->{//按照时间的先后进行排序
            return (int) (menu2.getCreateDate().getTime()-menu1.getCreateDate().getTime());
        }).collect(Collectors.toList());
        map.put("QuestionTree",result);
        return R.ok().data(map);
    }


//==========================工具方法==================================
    //递归查找所以疑问的子疑问
    private List getChildren(AnswerQuestionTopic root,List all){
        List children = all.stream().filter(answerQuestionTopic -> {
           return answerQuestionTopic.getQuestionParentId() == root.getId();
        }).map(menu->{
            menu.setChild(getChildren(menu,all));
           return menu;
        }).sorted((menu1,menu2)->{
            return (int) (menu2.getCreateDate().getTime()-menu1.getCreateDate().getTime());
        }).collect(Collectors.toList());
        return children;
    }