List对象转成Map，List对象转成Set，List对象转成List＜参数＞，List对象转成List＜String＞，JDK1.8流式处理

约定不等于承诺〃 2022-11-10 05:36 368阅读 0赞

# 目的： #

# JDK1.8操作，使用lambada表达式处理List #

**第一步： 数据库获取数据**  
List<TbmNewsPopularFeelings> tbmNewsPopularFeelingsList = tbmNewsPopFeeMapper.selectList(wrapper);  
if (!CollectionUtils.isEmpty(tbmNewsPopularFeelingsList)) \{

**第二步，List<对象>  转成Map格式，List对象转成Map**  
    Map<Long, TbmNewsPopularFeelings> resultMap = tbmNewsPopularFeelingsList.stream().collect(Collectors.toMap(TbmNewsPopularFeelings::getNewsId, v -> v, (k1, k2) -> k1));

**第三步 List对象转成List＜参数＞**  
    List<Long> newsIdLsit = tbmNewsPopularFeelingsList.stream().map(TbmNewsPopularFeelings::getNewsId).collect(Collectors.toList());  
    Set newsIdSet = new HashSet();  
    newsIdSet.addAll(newsIdLsit);  
    if (newsIdSet.size() == 1 && newsIdSet.contains(newsId)) \{  
        return null;  
    \} else \{  
        newsIdSet.remove(newsId);  
        return newsIdSet.toString();  
    \}  
\}  
return null;

**第四步，List可以通过group By来改造成一个Map**

//Map<公司id，标签列表>  根据公司id分组
    Map<String, List<TbmEventTags>> businessIdMap = tbmEventTagsList.stream().collect(Collectors.groupingBy(TbmEventTags::getBusinessId));

List<TbmNewsPopularFeelings> tbmNewsPopularFeelingsList = tbmNewsPopFeeMapper.selectList(wrapper);
            if (!CollectionUtils.isEmpty(tbmNewsPopularFeelingsList)) {
                Map<Long, TbmNewsPopularFeelings> resultMap = tbmNewsPopularFeelingsList.stream().collect(Collectors.toMap(TbmNewsPopularFeelings::getNewsId, v -> v, (k1, k2) -> k1));
                List<Long> newsIdLsit = tbmNewsPopularFeelingsList.stream().map(TbmNewsPopularFeelings::getNewsId).collect(Collectors.toList());
                Set newsIdSet = new HashSet();
                newsIdSet.addAll(newsIdLsit);
                if (newsIdSet.size() == 1 && newsIdSet.contains(newsId)) {
                    return null;
                } else {
                    newsIdSet.remove(newsId);
                    return newsIdSet.toString();
                }
            }
            return null;