Java 8 - 用 GroupBy 总和替换每一行

Question

我有 class 数据，它有 4 个字段并有值列表，然后我想用 java 8 和流替换每一行按年份分组并求和同一年份的值。请看下面的代码和输出。

class Data {
    int id;
    int year;
    String name;
    double value;
}

List<Data> list = new ArrayList<>();
list.add(new Data(101, 2018, "AAA", 20));
list.add(new Data(102, 2019, "BBB", 30));
list.add(new Data(103, 2020, "CCC", 10));
list.add(new Data(104, 2019, "DDD", 50));
list.add(new Data(105, 2020, "EEE", 40));
list.add(new Data(106, 2020, "FFF", 60));

第一个代码尝试：

list.stream().collect(Collectors.groupingBy(Data::getYear, Collectors
    .summingDouble(Data::getValue)));

第二个代码尝试：

list.stream().collect(Collectors.toMap(value -> value.getYear(), Function.identity(),
            (a, b) -> new Data(a.getId(), a.getYear(), a.getName(), a.getValue() + b.getValue()))).values()
            .forEach(value -> System.out.println(value));

输出：

id    year   name   value
101   2018   "AAA"   20
102   2019   "BBB"   80
103   2020   "CCC"   110

预期输出：

id    year   name   value
101   2018   "AAA"   20
102   2019   "BBB"   80
103   2020   "CCC"   110
104   2019   "DDD"   80
105   2020   "EEE"   110
106   2020   "FFF"   110

Answer 1

地图由您的代码中的第二个流创建，每年 preserves only a single entry。

重映射函数 (a, b) -> new Data(a.getId(), a.getYear(), a.getName(), a.getValue() + b.getValue())) 处理重复项，以便每个数据对象的值将是所有值的总和。

为了 retain all 给定年份的 Data 和 change the value of every object to be a total value，您必须采取几个步骤：

创建地图：year --> total value per year（已正确完成）；
创建地图：year --> list of Data 个对象；
将 total value per year 应用于每个 Data 对象。

    public static void main(String[] args) {
        List<Data> dataList = List.of(
                new Data(101, 2018, "AAA", 20),
                new Data(102, 2019, "BBB", 30),
                new Data(103, 2020, "CCC", 10),
                new Data(104, 2019, "DDD", 50),
                new Data(105, 2020, "EEE", 40),
                new Data(106, 2020, "FFF", 60)
        );

        Map<Integer, Double> yearToTotalVal = getTotalValueMap(dataList);

        Map<Integer, List<Data>> yearToData = getYearToDataListMap(dataList);

        appllyTotalValue(yearToTotalVal, yearToData);

        for (Map.Entry<Integer, List<Data>> entry: yearToData.entrySet()) {
            System.out.println(entry);
        }
    }

    private static Map<Integer, Double> getTotalValueMap(List<Data> dataList) {
        return dataList.stream()
                .collect(Collectors.groupingBy(Data::getYear,
                                    Collectors.summingDouble(Data::getValue)));
    }

    private static Map<Integer, List<Data>> getYearToDataListMap(List<Data> dataList) {
        return dataList.stream()
                .collect(Collectors.groupingBy(Data::getYear));
    }

    private static void appllyTotalValue(Map<Integer, Double> yearToTotalVal, 
                                         Map<Integer, List<Data>> yearToData) {
        for (Integer year: yearToTotalVal.keySet()) {
            yearToData.get(year)
                    .replaceAll(data -> new Data(data.getId(),
                                                data.getYear(),
                                                data.getName(),
                                                yearToTotalVal.get(year)));
        }
    }

输出 - 每年的数据：

2018=[Data{id=101, year=2018, name='AAA', value=20.0}]
2019=[Data{id=102, year=2019, name='BBB', value=80.0}, Data{id=104, year=2019, name='DDD', value=80.0}]
2020=[Data{id=103, year=2020, name='CCC', value=110.0}, Data{id=105, year=2020, name='EEE', value=110.0}, Data{id=106, year=2020, name='FFF', value=110.0}]

旁注：

我还建议你制作 Data class immutable

    public class Data {
        private final int id;
        private final int year;
        private final String name;
        private final double value;

        // Constructor, getters, etc
    }

Java 8 - 用 GroupBy 总和替换每一行

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