Java8的stream之groupingBy()分组排序

朱雀 2024-04-06 10:59 42阅读 0赞

#### Java8的stream之groupingBy()分组排序 ####

*  一、默认升序排序
 *  二、descendingMap()降序排序
 *  三、总结

--------------------

*  groupingBy()是Stream API中最强大的收集器Collector之一，提供与SQL的GROUP BY子句类似的功能。
 *  需要指定一个属性才能使用，通过该属性执行分组。我们通过提供功能接口的实现来实现这一点 - 通常通过传递lambda表达式。

## 一、默认升序排序 ##

stationMap = peopleList.stream().collect(Collectors.groupingBy(People::getJgId));

## 二、descendingMap()降序排序 ##

stationMap = peopleList.stream().collect(Collectors.groupingBy(People::getJgId,TreeMap::new,Collectors.toList())).descendingMap();

## 三、总结 ##

TreeMap默认按照key升序排序,collectPlan.descendingMap()可以进行降序排序

List<People> peopleList = Lists.newArrayList();
            peopleList.add(new People(1, "小王", 5));
            peopleList.add(new People(1, "小李", 4));
            peopleList.add(new People(2, "小张", 3));
            peopleList.add(new People(2, "小皇", 2));
            peopleList.add(new People(2, "小刘", 1));
    
            //分组排序并加序号
            Map<Integer, List<People>> stationMap = new HashMap<Integer, List<People>>();
            // 使用stream流将list转为map，key为指定字段的值
            stationMap = peopleList.stream().collect(Collectors.groupingBy(People::getJgId));
            log.info(stationMap.toString());
    
            stationMap = peopleList.stream().collect(Collectors.groupingBy(People::getJgId,TreeMap::new,Collectors.toList())).descendingMap();
            log.info(stationMap.toString());

![在这里插入图片描述][c49b376048ce46fdb1432b24e1ecefbd.png]

[c49b376048ce46fdb1432b24e1ecefbd.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/06/098d58d0bfed478c8c897e438960df85.png