java - 使用流从列表中获取总和和最大值

Question

嗨，我有一个列表，其中的数据如下所示

[{"month":"April","day":"Friday","count":5},

{"month":"April","day":"Monday","count":6},

{"month":"April","day":"Saturday","count":2},

{"month":"April","day":"Sunday","count":1},

{"month":"April","day":"Thursday","count":7},

{"month":"April","day":"Tuesday","count":8},

{"month":"April","day":"Wednesday","count":10},

{"month":"March","day":"Friday","count":3},

{"month":"March","day":"Monday","count":2},

{"month":"March","day":"Saturday","count":15},

{"month":"March","day":"Sunday","count":11},

{"month":"March","day":"Thursday","count":4},

{"month":"March","day":"Tuesday","count":20},

{"month":"March","day":"Wednesday","count":7},

{"month":"May","day":"Friday","count":2},

{"month":"May","day":"Monday","count":0},

{"month":"May","day":"Saturday","count":7},

{"month":"May","day":"Sunday","count":4},

{"month":"May","day":"Thursday","count":8},

{"month":"May","day":"Tuesday","count":3},

{"month":"May","day":"Wednesday","count":6}]

我的对象类是

String month;
String day;
Integer count;

我想通过使用流获得的是按月分组的计数总和以及该月最大计数的日期。

所以最终结果看起来像这样

四月，星期三，39 三月，星期二，62 五月，星期四，30

我一直在尝试使用流和分组方式，但没有成功。任何帮助表示赞赏。谢谢

编辑

 Map<String, Integer> totalMap = transactions.stream().collect(Collectors.groupingBy(MonthlyTransaction::getMonth, Collectors.summingInt(MonthlyTransaction::getCount)));
     Map<String, String> maxMap = transactions.stream().collect(Collectors.groupingBy(MonthlyTransaction::getMonth)).values().stream().toMap(Object::getDay, Collextions.max(Object::getCount);

显然maxMap方法是错误的，但我不知道如何写。

Answer 1

如果您想在一次遍历中找到每月计数总和以及每月最大计数的日期，我认为您需要一个自定义收集器。

首先，让我们创建一个 Holder 类来存储结果:

public class Statistics {

    private final String dayWithMaxCount;

    private final long totalCount;

    public Statistics(String dayWithMaxCount, long totalCount) {
        this.dayWithMaxCount = dayWithMaxCount;
        this.totalCount = totalCount;
    }

    // TODO getters and toString
}

然后，创建此方法，该方法返回一个收集器，该收集器累积计数总和和最大计数，以及找到该最大值的日期:

public static Collector<MonthlyTransaction, ?, Statistics> withStatistics() {
    class Acc {
        long sum = 0;
        long maxCount = Long.MIN_VALUE;
        String dayWithMaxCount;

        void accumulate(MonthlyTransaction transaction) {
            sum += transaction.getCount();
            if (transaction.getCount() > maxCount) {
                maxCount = transaction.getCount();
                dayWithMaxCount = transaction.getDay();
            }
        }

        Acc merge(Acc another) {
            sum += another.sum;
            if (another.maxCount > maxCount) {
                maxCount = another.maxCount;
                dayWithMaxCount = another.dayWithMaxCount;
            }
            return this;
        }

        Statistics finish() {
            return new Statistics(dayWithMaxCount, sum);
        }
    }
    return Collector.of(Acc::new, Acc::accumulate, Acc::merge, Acc::finish);
}

这使用本地类Acc来累积和合并部分结果。 finish 方法返回 Statistics 类的实例，该类保存最终结果。最后，我使用 Collector.of基于 Acc 类的方法创建收集器。

最后，您可以按如下方式使用上面定义的方法和类:

Map<String, Statistics> statisticsByMonth = transactions.stream()
    .collect(Collectors.groupingBy(MonthlyTransaction::getMonth, withStatistics()));