Java 8嵌套（多级）分组依据

Question

我下面有几节课

class Pojo {

    List<Item> items;

}

class Item {

    T key1;

    List<SubItem> subItems;

}

class SubItem {

    V key2;

    Object otherAttribute1;

}

我想基于key1每个聚合来聚合项目，对于子项目，应按key2以下方式聚合：

Map<T, Map<V, List<Subitem>>

Java 8 Collectors.groupingBy嵌套怎么可能？

我正在尝试某事，并在

pojo.getItems()

    .stream()

    .collect(

        Collectors.groupingBy(Item::getKey1, /* How to group by here SubItem::getKey2*/)

    );

注意：这与级联不同，级联groupingBy基于此处讨论的同一对象中的字段进行多级聚合

Answer 1

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会出现在多个组中。在这种情况下，您想要执行一种flatMap操作。实现此目的的一种方法是Stream.flatMap与临时对一起使用，以容纳和的组合，Item然后SubItem再进行收集。由于不存在标准对类型，因此典型的解决方案是Map.Entry：Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()&nbsp; &nbsp; .flatMap(item -> item.subItems.stream()&nbsp; &nbsp; &nbsp; &nbsp; .map(sub -> new AbstractMap.SimpleImmutableEntry<>(item, sub)))&nbsp; &nbsp; .collect(Collectors.groupingBy(e -> e.getKey().getKey1(),&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Collectors.mapping(Map.Entry::getValue,&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Collectors.groupingBy(SubItem::getKey2))));不需要这些临时对象的替代方法是flatMap直接在收集器中执行操作，但是不幸的flatMapping是，直到Java 9才存在。这样，解决方案看起来像Map<T, Map<V, List<SubItem>>> result = pojo.getItems().stream()&nbsp; &nbsp; .collect(Collectors.groupingBy(Item::getKey1,&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Collectors.flatMapping(item -> item.getSubItems().stream(),&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Collectors.groupingBy(SubItem::getKey2))));如果我们不想等待Java 9，可以在代码库中添加一个类似的收集器，因为实现起来并不难：static <T,U,A,R> Collector<T,?,R> flatMapping(&nbsp; &nbsp; Function<? super T,? extends Stream<? extends U>> mapper,&nbsp; &nbsp; Collector<? super U,A,R> downstream) {&nbsp; &nbsp; BiConsumer<A, ? super U> acc = downstream.accumulator();&nbsp; &nbsp; return Collector.of(downstream.supplier(),&nbsp; &nbsp; &nbsp; &nbsp; (a, t) -> { try(Stream<? extends U> s=mapper.apply(t)) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if(s!=null) s.forEachOrdered(u -> acc.accept(a, u));&nbsp; &nbsp; &nbsp; &nbsp; }},&nbsp; &nbsp; &nbsp; &nbsp; downstream.combiner(), downstream.finisher(),&nbsp; &nbsp; &nbsp; &nbsp; downstream.characteristics().toArray(new Collector.Characteristics[0]));}

Answer 2

如果将分组为Map<T, List<…>>，则T键是唯一的，但是当将多个分组为时Map<T, Map<V, List<…>>>，V键仅在其子图中是唯一的。但是，根据您的输入数据结构和实际目标，这可能（可能）不是问题。