通过鉴别器函数对流进行分区

Question

Streams API 中缺少的功能之一是“分区依据”转换，例如Clojure 中定义的。假设我想重现 Hibernate 的 fetch join：我想发出一条 SQL SELECT 语句来从结果中接收这种对象：

class Family {
   String surname;
   List<String> members;
}

我发出：

SELECT f.name, m.name 
FROM Family f JOIN Member m on m.family_id = f.id
ORDER BY f.name

然后我检索(f.name, m.name) 记录的平坦流。现在我需要将其转换为Family 对象流，其中包含其成员列表。假设我已经有一个Stream<ResultRow>；现在我需要将其转换为Stream<List<ResultRow>>，然后通过映射转换将其转换为Stream<Family>。

转换的语义如下：只要提供的鉴别器函数一直返回相同的值，就一直将流收集到List；一旦值发生变化，将List 作为输出流的一个元素发出并开始收集新的List。

希望能写出这样的代码（我已经有了resultStream方法）：

Stream<ResultRow> dbStream = resultStream(queryBuilder.createQuery(
        "SELECT f.name, m.name"
      + " FROM Family f JOIN Member m on m.family_id = f.id"
      + " ORDER BY f.name"));
Stream<List<ResultRow> partitioned = partitionBy(r -> r.string(0), dbStream);
Stream<Family> = partitioned.map(rs -> {
                    Family f = new Family(rs.get(0).string(0));
                    f.members = rs.stream().map(r -> r.string(1)).collect(toList());
                    return f;
                 });

不用说，我希望生成的流保持惰性（非物化），因为我希望能够在不达到任何 O(n) 内存限制的情况下处理任何大小的结果集。如果没有这个关键要求，我会对提供的 groupingBy 收集器感到满意。

Answer 1

解决方案需要我们定义一个自定义的Spliterator，它可以用来构造分区流。我们需要通过它自己的拆分器访问输入流并将其包装到我们的拆分器中。然后从我们的自定义拆分器构造输出流。

下面的 Spliterator 会将任何 Stream<E> 转换为 Stream<List<E>>，前提是 Function<E, ?> 作为鉴别器函数。请注意，必须对输入流进行排序才能使此操作有意义。

import java.util.*;
import java.util.Spliterators.AbstractSpliterator;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

import static java.util.Comparator.naturalOrder;

public class PartitionBySpliterator<E> extends AbstractSpliterator<List<E>> {
    private final Spliterator<E> spliterator;
    private final Function<? super E, ?> partitionBy;
    private HoldingConsumer<E> holder;
    private Comparator<List<E>> comparator;

    public PartitionBySpliterator(
            Spliterator<E> toWrap,
            Function<? super E, ?> partitionBy
    ) {
        super(Long.MAX_VALUE, toWrap.characteristics() & ~SIZED | NONNULL);
        this.spliterator = toWrap;
        this.partitionBy = partitionBy;
    }

    public static <E> Stream<List<E>> partitionBy(
            Function<E, ?> partitionBy, Stream<E> in
    ) {
        return StreamSupport.stream(
                new PartitionBySpliterator<>(in.spliterator(), partitionBy), false);
    }

    @Override
    public boolean tryAdvance(Consumer<? super List<E>> action) {
        final HoldingConsumer<E> h;
        if (holder == null) {
            h = new HoldingConsumer<>();
            if (!spliterator.tryAdvance(h)) {
              return false;
            }
            holder = h;
        } else {
          h = holder;
        }
        final ArrayList<E> partition = new ArrayList<>();
        final Object partitionKey = partitionBy.apply(h.value);
        boolean didAdvance;
        do {
          partition.add(h.value);
        }
        while ((didAdvance = spliterator.tryAdvance(h))
                && Objects.equals(partitionBy.apply(h.value), partitionKey));
        if (!didAdvance) {
          holder = null;
        }
        action.accept(partition);
        return true;
    }

    static final class HoldingConsumer<T> implements Consumer<T> {
        T value;

        @Override
        public void accept(T value) {
            this.value = value;
        }
    }

    @Override
    public Comparator<? super List<E>> getComparator() {
        final Comparator<List<E>> c = this.comparator;
        return c != null ? c : (this.comparator = comparator());
    }

    private Comparator<List<E>> comparator() {
        @SuppressWarnings({"unchecked", "rawtypes"})
        final Comparator<? super E> innerComparator =
                Optional.ofNullable(spliterator.getComparator())
                        .orElse((Comparator) naturalOrder());
        return (left, right) -> {
            final int c = innerComparator.compare(left.get(0), right.get(0));
            return c != 0 ? c : innerComparator.compare(
                    left.get(left.size() - 1), right.get(right.size() - 1));
        };
    }
}

Answer 2

对于那些只想对流进行分区的人，可以使用映射器和收集器。

class Person {

    String surname;
    String forename;

    public Person(String surname, String forename) {
        this.surname = surname;
        this.forename = forename;
    }

    @Override
    public String toString() {
        return forename;
    }

}

class Family {

    String surname;
    List<Person> members;

    public Family(String surname, List<Person> members) {
        this.surname = surname;
        this.members = members;
    }

    @Override
    public String toString() {
        return "Family{" + "surname=" + surname + ", members=" + members + '}';
    }

}

private void test() {
    String[][] data = {
        {"Kray", "Ronald"},
        {"Kray", "Reginald"},
        {"Dors", "Diana"},};
    // Their families.
    Stream<Family> families = Arrays.stream(data)
            // Build people
            .map(a -> new Person(a[0], a[1]))
            // Collect into a Map<String,List<Person>> as families
            .collect(Collectors.groupingBy(p -> p.surname))
            // Convert them to families.
            .entrySet().stream()
            .map(p -> new Family(p.getKey(), p.getValue()));
    families.forEach(f -> System.out.println(f));
}

Answer 3

可以由collapse 和StreamEx 完成

StreamEx.of(queryBuilder.createQuery(
    "SELECT f.name, m.name"
    + " FROM Family f JOIN Member m on m.family_id = f.id"
    + " ORDER BY f.name"))
        .collapse((a, b) -> a.string(0).equals(b.string(0)), Collectors.toList())
        .map(l -> new Family(l.get(0).string(0), StreamEx.of(l).map(r -> r.string(1)).toList())) 
        .forEach(System.out::println);