将列表拆分为多个列表，在java 8中具有固定数量的元素 [英] Split list into multiple lists with fixed number of elements in java 8

问题描述

我想要一些类似于scala分组函数的东西。基本上，一次挑选2个元素并处理它们。以下是相同的参考：

I want to something which is similar to the scala grouped function. Basically, pick 2 elements at a time and process them. Here is a reference for the same :

将列表拆分为多个具有固定元素数量的列表

Lambdas确实提供了groupingBy和partitioningBy之类的东西，但没有它们似乎与Scala中的分组函数相同。任何指针都会受到赞赏。

Lambdas do provide things like groupingBy and partitioningBy but none of them seem to do the same as the grouped function in Scala. Any pointers would be appreciated.

推荐答案

这听起来像是一个像低级流操作就像 Stream API本身提供的操作一样。 （相对）简单解决方案可能如下所示：

It sounds like a problem that is better handled like a low-level Stream operation just like the ops provided by the Stream API itself. A (relative) simple solution may look like:

public static <T> Stream<List<T>> chunked(Stream<T> s, int chunkSize) {
    if(chunkSize<1) throw new IllegalArgumentException("chunkSize=="+chunkSize);
    if(chunkSize==1) return s.map(Collections::singletonList);
    Spliterator<T> src=s.spliterator();
    long size=src.estimateSize();
    if(size!=Long.MAX_VALUE) size=(size+chunkSize-1)/chunkSize;
    int ch=src.characteristics();
    ch&=Spliterator.SIZED|Spliterator.ORDERED|Spliterator.DISTINCT|Spliterator.IMMUTABLE;
    ch|=Spliterator.NONNULL;
    return StreamSupport.stream(new Spliterators.AbstractSpliterator<List<T>>(size, ch)
    {
        private List<T> current;
        @Override
        public boolean tryAdvance(Consumer<? super List<T>> action) {
            if(current==null) current=new ArrayList<>(chunkSize);
            while(current.size()<chunkSize && src.tryAdvance(current::add));
            if(!current.isEmpty()) {
                action.accept(current);
                current=null;
                return true;
            }
            return false;
        }
    }, s.isParallel());
}

简单测试：

chunked(Stream.of(1, 2, 3, 4, 5, 6, 7), 3)
  .parallel().forEachOrdered(System.out::println);

优点是您不需要为后续流处理提供所有项目的完整集合，例如

The advantage is that you do not need a full collection of all items for subsequent stream processing, e.g.

chunked(
    IntStream.range(0, 1000).mapToObj(i -> {
        System.out.println("processing item "+i);
        return i;
    }), 2).anyMatch(list->list.toString().equals("[6, 7]")));

将打印：

processing item 0
processing item 1
processing item 2
processing item 3
processing item 4
processing item 5
processing item 6
processing item 7
true

而不是而不是处理一千个 IntStream.range（0,1000）项。这也可以使用无限来源 Stream s：

rather than processing a thousand items of IntStream.range(0, 1000). This also enables using infinite source Streams:

chunked(Stream.iterate(0, i->i+1), 2).anyMatch(list->list.toString().equals("[6, 7]")));

---

如果您对完全物化的集合感兴趣比应用后续 Stream 操作，您只需使用以下操作：

---

If you are interested in a fully materialized collection rather than applying subsequent Stream operations, you may simply use the following operation:

List<Integer> list=Arrays.asList(1, 2, 3, 4, 5, 6, 7);
int listSize=list.size(), chunkSize=2;
List<List<Integer>> list2=
    IntStream.range(0, (listSize-1)/chunkSize+1)
             .mapToObj(i->list.subList(i*=chunkSize,
                                       listSize-chunkSize>=i? i+chunkSize: listSize))
             .collect(Collectors.toList());

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