Java 8 在分组时不保持顺序 [英] Java 8 is not maintaining the order while grouping

问题描述

我正在使用 Java 8 按数据分组.但是得到的结果并不是按顺序形成的.

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;
        }

    }

我在这里使用我的类 groupingKey，我通过它动态地从 ux 传递.如何以排序形式获取此 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

不维护顺序是存储结果的 Map 的一个属性.如果您需要特定的 Map 行为，则需要 请求一个特定的Map实现.例如.LinkedHashMap 维护插入顺序:

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())));

<小时>

顺便说一句，在创建 Stream 之前，没有理由将 mapList 的内容复制到数组中.您可以简单地调用 mapList.stream() 来获得合适的 Stream.

---

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 p) -> p, toList()) 已过时.p->p 是一个身份映射，所以根本没有理由请求 mapping:

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()));

<小时>

但即使是 GroupingKey 也已过时.它基本上包装了一个 List 值，所以你可以首先使用一个 List 作为键.List 适当地实现 hashCode 和 equals(但您不得修改这些键 Lists 之后).

---

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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