Java 8 Nested (Multi level) group by
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Java 8 Nested (Multi level) group by
提问by Sandesh Kumar
I have few classes like below
我有几个像下面这样的课程
class Pojo {
List<Item> items;
}
class Item {
T key1;
List<SubItem> subItems;
}
class SubItem {
V key2;
Object otherAttribute1;
}
I want to aggregate the items based on key1and for each aggregation, subitems should be aggregated by key2in following way:
我想根据key1每个聚合来聚合项目，子项目应该通过key2以下方式聚合：
Map<T, Map<V, List<Subitem>>
How is this possible with Java 8 Collectors.groupingBynesting?
Java 8Collectors.groupingBy嵌套怎么可能做到这一点？
I was trying something and stuck halfway at
我正在尝试一些东西并卡在了一半
pojo.getItems()
.stream()
.collect(
Collectors.groupingBy(Item::getKey1, /* How to group by here SubItem::getKey2*/)
);
Note: This not same as cascaded groupingBywhich does multilevel aggregation based on fields in the same object as discussed here
注：这不一样的级联groupingBy，其不基于在同一个对象的字段多聚集的讨论在这里
回答by Holger
You can't group a single item by multiple keys, unless you accept the item to potentially appear in multiple groups. In that case, you want to perform a kind of flatMapoperation.
您不能通过多个键对单个项目进行分组，除非您接受该项目可能出现在多个组中。在这种情况下，您想要执行某种flatMap操作。
One way to achieve this, is to use Stream.flatMapwith a temporary pair holding the combinations of Itemand SubItembefore collecting. Due to the absence of a standard pair type, a typical solution is to use Map.Entryfor that:
实现此目的的一种方法是使用Stream.flatMap一个临时对来保存Item和SubItem收集之前的组合。由于没有标准对类型，典型的解决方案是使用Map.Entry：
Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()
.flatMap(item -> item.subItems.stream()
.map(sub -> new AbstractMap.SimpleImmutableEntry<>(item, sub)))
.collect(Collectors.groupingBy(e -> e.getKey().getKey1(),
Collectors.mapping(Map.Entry::getValue,
Collectors.groupingBy(SubItem::getKey2))));
An alternative not requiring these temporary objects would be performing the flatMapoperation right in the collector, but unfortunately, flatMappingwon't be there until Java?9.
另一种不需要这些临时对象的方法是flatMap在收集器中执行操作，但不幸的flatMapping是，直到 Java?9 才会出现。
With that, the solution would look like
有了这个，解决方案看起来像
Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()
.collect(Collectors.groupingBy(Item::getKey1,
Collectors.flatMapping(item -> item.getSubItems().stream(),
Collectors.groupingBy(SubItem::getKey2))));
and if we don't want to wait for Java?9 for that, we may add a similar collector to our code base, as it's not so hard to implement:
如果我们不想等待 Java?9，我们可以在我们的代码库中添加一个类似的收集器，因为它并不难实现：
static <T,U,A,R> Collector<T,?,R> flatMapping(
Function<? super T,? extends Stream<? extends U>> mapper,
Collector<? super U,A,R> downstream) {
BiConsumer<A, ? super U> acc = downstream.accumulator();
return Collector.of(downstream.supplier(),
(a, t) -> { try(Stream<? extends U> s=mapper.apply(t)) {
if(s!=null) s.forEachOrdered(u -> acc.accept(a, u));
}},
downstream.combiner(), downstream.finisher(),
downstream.characteristics().toArray(new Collector.Characteristics[0]));
}
