Java8的stream之groupingBy()分组排序

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• 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));
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二、descendingMap()降序排序

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