Java 8 Stream API中的多个聚合函数 [英] Multiple aggregate functions in Java 8 Stream API

问题描述

我有一个类定义如

public class TimePeriodCalc {
    private double occupancy;
    private double efficiency;
    private String atDate;
}

我想使用Java 8 Stream API执行以下SQL语句。

I would like to perform the following SQL statement using Java 8 Stream API.

SELECT atDate, AVG(occupancy), AVG(efficiency)
FROM TimePeriodCalc
GROUP BY atDate

我试过：

Collection<TimePeriodCalc> collector = result.stream().collect(groupingBy(p -> p.getAtDate(), ....

可以在代码中添加什么来选择多个属性？我正在考虑使用多个收集器，但实际上不知道如何这样做。

What can be put into the code to select multiple attributes ? I'm thinking of using multiple Collectors but really don't know how to do so.

推荐答案

要做到这一点没有自定义的收集器（不再对结果进行流式传输），你可以这样做。它有点脏，因为它首先收集到 Map< String，List< TimePeriodCalc>> 然后流式传输该列表并获得平均加倍。

To do it without a custom Collector (not streaming again on the result), you could do it like this. It's a bit dirty, since it is first collecting to Map<String, List<TimePeriodCalc>> and then streaming that list and get the average double.

由于您需要两个平均值，它们会被收集到持有人或对，这种情况我正在使用 AbstractMap.SimpleEntry

Since you need two averages, they are collected to a Holder or a Pair, in this case I'm using AbstractMap.SimpleEntry

  Map<String, SimpleEntry<Double, Double>> map = Stream.of(new TimePeriodCalc(12d, 10d, "A"), new TimePeriodCalc(2d, 16d, "A"))
            .collect(Collectors.groupingBy(TimePeriodCalc::getAtDate,
                    Collectors.collectingAndThen(Collectors.toList(), list -> {
                        double occupancy = list.stream().collect(
                                Collectors.averagingDouble(TimePeriodCalc::getOccupancy));
                        double efficiency = list.stream().collect(
                                Collectors.averagingDouble(TimePeriodCalc::getEfficiency));
                        return new AbstractMap.SimpleEntry<>(occupancy, efficiency);
                    })));

    System.out.println(map);

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