重复键的Java 8流总和条目 [英] Java 8 stream sum entries for duplicate keys

问题描述

我正在使用Java 8流按某个键对条目列表进行分组，然后按日期对分组进行排序.我还想做的是折叠"组中具有相同日期的任何两个条目并将它们汇总.我有一个这样的课程(出于示例目的而精简)

I am using Java 8 streams to group a list of entries by a certain key and then sorting the groups by date. What I would like to do in addition is to "collapse" any two entries within a group that have the same date and sum them up. I have a class like this (stripped down for example purposes)

class Thing {
    private String key;
    private Date activityDate;
    private float value;
    ...
}

然后我将它们分组如下:

Then I'm grouping them like so:

Map<String, List<Thing>> thingsByKey = thingList.stream().collect(
                Collectors.groupingBy(
                        Thing::getKey,
                        TreeMap::new,
                        Collectors.mapping(Function.identity(), toSortedList())
                ));

private static Collector<Thing,?,List<Thing>> toSortedList() {
        return Collectors.collectingAndThen(toList(),
                l -> l.stream().sorted(Comparator.comparing(Thing::getActivityDate)).collect(toList()));
    }

我想做的是，如果任何两个Thing条目具有完全相同的日期，则将这些值的值相加并合拢，这样，

What I would like to do is, if any two Thing entries have the exact same date, sum up the values for those and collapse them down such that,

事物1 日期= 1/1/2017 值= 10

Thing1 Date=1/1/2017 Value=10

事物2 日期= 1/1/2017 值= 20

Thing2 Date=1/1/2017 Value=20

在2017年1月1日变成30.

Turns into 30 for 1/1/2017.

完成这样的事情的最好方法是什么?

What's the best way to accomplish something like that?

推荐答案

我将您的Thing类稍加更改为使用LocalData，并添加了一个非常简单的toString:

I have slightly change your Thing class to use LocalData and added a very simple toString:

@Override
public String toString() {
   return " value = " + value;
}

如果我理解正确，那么这就是您所需要的:

If I understood correctly, than this is what you need:

Map<String, TreeMap<LocalDate, Thing>> result = Arrays
            .asList(new Thing("a", LocalDate.now().minusDays(1), 12f), new Thing("a", LocalDate.now(), 12f), new Thing("a", LocalDate.now(), 13f))
            .stream()
            .collect(Collectors.groupingBy(Thing::getKey,
                    Collectors.toMap(Thing::getActivityDate, Function.identity(),
                            (Thing left, Thing right) -> new Thing(left.getKey(), left.getActivityDate(), left.getValue() + right.getValue()),
                            TreeMap::new)));


 System.out.println(result); // {a={2017-06-24= value = 12.0, 2017-06-25= value = 25.0}}

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