Java流处理映射列表的总和值

我想确定行或更好的列的数量：构建一个映射列表的总和，比如list>rows

是否有可能对每个不同列的所有值求和？函数应返回一个映射，列为键，所有值之和为值

summMap.get("columname")

假设我有以下地图列表：

List<Map<String, Long>> mapList = new ArrayList();
Map<String, Object> map1 = new HashMap<>();
Map<String, Object> map2 = new HashMap<>();
Map<String, Object> map3 = new HashMap<>();
map1.put("col1", 90);
map1.put("col2", 50);
map1.put("col3", 10);
map2.put("col1", 90);
map2.put("col2", 50);
map2.put("col3", 10);
map3.put("col1", 90);
map3.put("col2", 50);
map3.put("col3", 10);
mapList.add(map1);
mapList.add(map2);
mapList.add(map3);
Map<String, Long> sum = mapList.stream().distinct().sum() // Example
// result i'm awaiting/expecting
Long sumVal1 = sum.get("col1"); // 270
Long sumVal2 = sum.get("col2"); // 150
Long sumVal3 = sum.get("col3"); // 30

---

Long sumVal=sum.getcol1

这是一个简单的解决方案，它将根据您的要求给出结果：

List<Map<String, Long>> mapList = new ArrayList();
Map<String, Long>       map1    = new HashMap<>();
Map<String, Long>       map2    = new HashMap<>();
Map<String, Long>       map3    = new HashMap<>();
map1.put("col1", 90L);
map1.put("col2", 50L);
map1.put("col3", 10L);
map2.put("col1", 90L);
map2.put("col2", 50L);
map2.put("col3", 10L);
map3.put("col1", 90L);
map3.put("col2", 50L);
map3.put("col3", 10L);
mapList.add(map1);
mapList.add(map2);
mapList.add(map3);

Map<String, Long> sum = new HashMap<>();
mapList.forEach(map -> map.keySet().forEach(
                s -> {
                    mapList.stream()
                           .collect(Collectors.groupingBy(foo -> s,
                                Collectors.summingLong(foo -> map.get(s)))).forEach(
                                    (id, sumTargetCost) ->
                                            sum.put(s, sumTargetCost)
                    );
                }

));

Long sumVal1 = sum.get("col1"); // 270
Long sumVal2 = sum.get("col2"); // 150
Long sumVal3 = sum.get("col3"); // 30

System.out.println("SumVal1: " + sumVal1 + ", SumVal2: " + sumVal2 + ", SumVal3: " + sumVal3);

---

这不支持并行执行，但可以通过修改reduce中的最后一个参数来实现：

通过测试：

final List<Map<String, Long>> maps = new ArrayList<>();

Map<String, Long> map1 = new HashMap<>();
Map<String, Long> map2 = new HashMap<>();

map1.put("col1", 90L);
map1.put("col2", 50L);

map2.put("col1", 90L);
map2.put("col2", 50L);

map2.put("col3", 100L);

maps.add(map1);
maps.add(map2);

final Map<String, Long> sums = reduceLongs(maps);

assertEquals(180L, sums.get("col1").longValue());
assertEquals(100L, sums.get("col2").longValue());
assertEquals(100L, sums.get("col3").longValue());

这很简单

Map<String, Long> sum = mapList.stream()
    .flatMap(m -> m.entrySet().stream())
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, Long::sum));

---

Holger已经提供了一个干净的解决方案，但我认为您也可以尝试flatMap和groupingBy作为：

你问题的整体解决方案是：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import static java.util.stream.Collectors.*;

public class ListMapSum {
    public static void main(String... args) {
        List<Map<String, Long>> mapList = new ArrayList();
        Map<String, Long> map1 = new HashMap<>();
        Map<String, Long> map2 = new HashMap<>();
        Map<String, Long> map3 = new HashMap<>();
        map1.put("col1", 90L);
        map1.put("col2", 50L);
        map1.put("col3", 10L);
        map2.put("col1", 90L);
        map2.put("col2", 50L);
        map2.put("col3", 10L);
        map3.put("col1", 90L);
        map3.put("col2", 50L);
        map3.put("col3", 10L);
        mapList.add(map1);
        mapList.add(map2);
        mapList.add(map3);
        Map<String, Long> sum = mapList.stream().flatMap(map -> map.entrySet().stream())
                .collect(groupingBy(Map.Entry::getKey, summingLong(Map.Entry::getValue)));
        Long sumVal1 = sum.get("col1"); // 270
        Long sumVal2 = sum.get("col2"); // 150
        Long sumVal3 = sum.get("col3"); // 30
    }
}

---

显示您获得的代码。您的问题不清楚-请重新表述您的问题。因此，您有一个映射列表作为columnname、value，并且您希望每个唯一columnname的[value]之和。对吗？我为每个独特的列添加了一个示例Yes。

Map<String, Long> sum = mapList.stream().flatMap(map -> map.entrySet().stream())
                .collect(groupingBy(Map.Entry::getKey, summingLong(Map.Entry::getValue)));

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import static java.util.stream.Collectors.*;

public class ListMapSum {
    public static void main(String... args) {
        List<Map<String, Long>> mapList = new ArrayList();
        Map<String, Long> map1 = new HashMap<>();
        Map<String, Long> map2 = new HashMap<>();
        Map<String, Long> map3 = new HashMap<>();
        map1.put("col1", 90L);
        map1.put("col2", 50L);
        map1.put("col3", 10L);
        map2.put("col1", 90L);
        map2.put("col2", 50L);
        map2.put("col3", 10L);
        map3.put("col1", 90L);
        map3.put("col2", 50L);
        map3.put("col3", 10L);
        mapList.add(map1);
        mapList.add(map2);
        mapList.add(map3);
        Map<String, Long> sum = mapList.stream().flatMap(map -> map.entrySet().stream())
                .collect(groupingBy(Map.Entry::getKey, summingLong(Map.Entry::getValue)));
        Long sumVal1 = sum.get("col1"); // 270
        Long sumVal2 = sum.get("col2"); // 150
        Long sumVal3 = sum.get("col3"); // 30
    }
}