Collectors.grouping由两列组合不起作用

Question

我想用两列分组Collectors.groupingBy.

我写了以下内容:

public class TestGroupKey 
{
    public static void main(String... args) {
        List<Item> list = Arrays.asList(
            new Item(1, 1, 1),
            new Item(1, 1, 2),
            new Item(1, 1, 3),
            new Item(1, 2, 1),
            new Item(1, 2, 2),
            new Item(2, 1, 1),
            new Item(2, 2, 1),
            new Item(2, 3, 1),
            new Item(2, 4, 1),
            new Item(2, 4, 2),
            new Item(2, 4, 3),
            new Item(2, 4, 4),
            new Item(3, 1, 1),
            new Item(3, 1, 2),
            new Item(3, 1, 3),
            new Item(3, 1, 4),
            new Item(3, 2, 1),
            new Item(3, 2, 2)
        );

        Map<CustomKey, List<Item>> tupleGrouping = 
            list.stream().collect(Collectors.groupingBy(item -> item.customKey));
        tupleGrouping.entrySet().stream()
                .forEach(entry -> {
                    System.out.println(entry.getKey());
                });
    }

    public static class Item {
        public int topLevelId;
        public int secondLevelId;
        public int itemId;
        public CustomKey customKey;

        public Item(int topLevelId, int secondLevelId, int itemId) {
            this.topLevelId = topLevelId;
            this.secondLevelId = secondLevelId;
            this.itemId = itemId;
            this.customKey = new CustomKey(topLevelId, secondLevelId);
        }

        @Override
        public String toString() {
            return String.format("%d%d%d", this.topLevelId, this.secondLevelId, this.itemId);
        }
    }

    public static class CustomKey {
        public int topLevelId;
        public int secondLevelId;

        public CustomKey(int topLevelId, int secondLevelId) {
            this.topLevelId = topLevelId;
            this.secondLevelId = secondLevelId;
        }

        @Override
        public String toString() {
            return String.format("%d%d", this.topLevelId, this.secondLevelId);
        }
    }
}

预期的结果是

11 
12 
21 
22 
23 
24 
31 
32

但实际结果是

24
31
23
22
12
21
24
31
31
24
12
32
32
11
11
11
31

我觉得groupingBy不行.

我使用CustomKey班级有什么问题？

此外,嵌套的映射键正在工作:

Map<Integer, Map<Integer, List<Item>>> entryGrouping = 
    list.stream()
        .collect(Collectors.groupingBy(atta1 -> atta1.topLevelId, 
                                       Collectors.groupingBy(atta2 -> atta2.secondLevelId)));

Answer 1

您的CustomKey类没有重载Object的equals方法,所以groupingBy考虑两个CustomKeys到等于仅当它们是同一个对象(这是从来没有在你的例子真的,因为你创建一个新的CustomKey实例每次Item).

public static class CustomKey {   
    ...
    @Override
    public int hashCode ()
    {
        // if you override equals, you should also override hashCode
    }

    @Override
    public boolean equals (Object other)
    {
        if (other == this)
            return true;
        if (!(other instanceof CustomKey))
            return false;
        CustomKey okey = (CustomKey) other;
        return this.topLevelId == okey.topLevelId && this.secondLevelId == okey.secondLevelId;
    }
    ...
}