Java 8在分组时不维护顺序 [英] Java 8 is not maintaining the order while grouping

问题描述

我使用Java 8进行数据分组。但获得的结果不是按顺序形成的。

I m using Java 8 for grouping by data. But results obtained are not in order formed.

Map<GroupingKey, List<Object>> groupedResult = null;

        if (!CollectionUtils.isEmpty(groupByColumns)) {

            Map<String, Object> mapArr[] = new LinkedHashMap[mapList.size()];

            if (!CollectionUtils.isEmpty(mapList)) {
                int count = 0;
                for (LinkedHashMap<String, Object> map : mapList) {
                    mapArr[count++] = map;
                }
            }
            Stream<Map<String, Object>> people = Stream.of(mapArr);
            groupedResult = people
                    .collect(Collectors.groupingBy(p -> new GroupingKey(p, groupByColumns), Collectors.mapping((Map<String, Object> p) -> p, toList())));


public static class GroupingKey 

        public GroupingKey(Map<String, Object> map, List<String> cols) {

            keys = new ArrayList<>();

            for (String col : cols) {
                keys.add(map.get(col));
            }
        }

        // Add appropriate isEqual() ... you IDE should generate this
        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final GroupingKey other = (GroupingKey) obj;
            if (!Objects.equals(this.keys, other.keys)) {
                return false;
            }
            return true;
        }

        @Override
        public int hashCode() {
            int hash = 7;
            hash = 37 * hash + Objects.hashCode(this.keys);
            return hash;
        }

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

        public ArrayList<Object> getKeys() {
            return keys;
        }

        public void setKeys(ArrayList<Object> keys) {
            this.keys = keys;
        }

    }

这里我使用的是我的类groupingKey通过它从ux动态传递。如何以排序的形式获取此groupByColumns？

Here i am using my class groupingKey by which i m dynamically passing from ux. How can get this groupByColumns in sorted form?

推荐答案

不维护订单是地图的属性存储结果。如果您需要特定的 Map 行为，则需要请求特定的地图实施。例如。 LinkedHashMap 维护广告订单：

Not maintaining the order is a property of the Map that stores the result. If you need a specific Map behavior, you need to request a particular Map implementation. E.g. LinkedHashMap maintains the insertion order:

groupedResult = people.collect(Collectors.groupingBy(
    p -> new GroupingKey(p, groupByColumns),
    LinkedHashMap::new,
    Collectors.mapping((Map<String, Object> p) -> p, toList())));

---

顺便说一下，没有理由复制在创建 Stream 之前，将 mapList 的内容放入数组中。您只需调用 mapList.stream（）即可获得相应的流。

---

By the way, there is no reason to copy the contents of mapList into an array before creating the Stream. You may simply call mapList.stream() to get an appropriate Stream.

此外， Collectors.mapping（（Map< String，Object> p） - > p，toList（））已过时。 p-> p 是一个身份映射，所以没有理由要求映射：

Further, Collectors.mapping((Map<String, Object> p) -> p, toList()) is obsolete. p->p is an identity mapping, so there’s no reason to request mapping at all:

groupedResult = mapList.stream().collect(Collectors.groupingBy(
    p -> new GroupingKey(p, groupByColumns), LinkedHashMap::new, toList()));

---

但即使是 GroupingKey 已过时。它基本上包含了一个 List 的值，所以你可以首先使用 List 作为键。 列出的实现 hashCode 和等于相应（但你之后不得修改这些密钥 List 。）

---

But even the GroupingKey is obsolete. It basically wraps a List of values, so you could just use a List as key in the first place. Lists implement hashCode and equals appropriately (but you must not modify these key Lists afterwards).

Map<List<Object>, List<Object>> groupedResult=
  mapList.stream().collect(Collectors.groupingBy(
    p -> groupByColumns.stream().map(p::get).collect(toList()),
    LinkedHashMap::new, toList()));

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