Collectors.grouping由两列组合不起作用

Question

我想使用Collectors.groupingBy按两列分组。

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

我认为groupingBy无效。

使用CustomKey班级有什么问题？

此外，嵌套地图键正在运行：

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

Answer 1

您的 public void gatherResults(View view){ // Perform action on click switch(view.getId()) { case R.id.button_id_one: //DO what you want for specific id button_id_one break; case R.id.button_id_two: //DO what you want specific id button_id_two break; } } 课程并未覆盖CustomKey的{​​{1}}方法，因此Object认为两个equals相等只有当它们是同一个对象时（在您的示例中永远不会出现这种情况，因为您为每个groupingBy创建了一个新的CustomKey实例）。

CustomKey