Question

Streams API中缺少的一个功能是“分区依据”转换，例如Clojure中定义的。假设我想重现Hibernate的获取连接：我想发出一个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

我检索了(f.name, m.name)条记录的平面流。现在我需要将它转换为Family个对象的流，并在其中包含其成员列表。假设我已经有Stream<ResultRow>;现在我需要将其转换为Stream<List<ResultRow>>，然后使用映射转换对其进行处理，将其转换为Stream<Family>。

转换的语义如下：只要提供的鉴别器函数保持返回相同的值，就继续将流收集到List中;一旦值发生变化，将List作为输出流的元素发出，并开始收集新的List。

我希望能够编写这种代码（我已经有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;
                 });

毋庸置疑，我希望得到的流保持延迟（非物化），因为我希望能够处理任何大小的结果集而不会遇到任何O（n）内存限制。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

该解决方案要求我们定义可用于构造分区流的自定义Spliterator。我们需要通过自己的spliterator访问输入流并将其包装到我们的。然后从我们的自定义分裂器构造输出流。

以下Spliterator会将Stream<E>变为Stream<List<E>>，并提供Function<E, ?>作为鉴别器功能。请注意，必须为此操作订购输入流才有意义。

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collapse使用StreamEx

来完成

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);

通过鉴别器功能对流进行分区

3 个答案: