流收集并根据独立谓词收集成多个结果


问题内容

我盯着一些命令性代码,试图将其转换为纯函数式样式。基本上有一个迭代的for循环,inputSet在该循环中,我检查3个谓词,并outputSets根据匹配的谓词填充3个谓词。输出集可以重叠。如何使用Java
8 Streams / map / filter /等以纯功能方式实现此目的?


问题答案:

最简单的解决方案(除了将所有内容保留为更容易之外)是创建三个单独的流:

Set<MyObj> set1 = inputSet.stream().filter(pred1).collect(Collectors.toSet());
Set<MyObj> set2 = inputSet.stream().filter(pred2).collect(Collectors.toSet());
Set<MyObj> set3 = inputSet.stream().filter(pred3).collect(Collectors.toSet());

如果有谓词列表,则可以创建相应的集合列表:

List<Predicate<MyObj>> predicates = Arrays.asList(pred1, pred2, pred3);
List<Set<MyObj>> result = predicates.stream()
        .map(pred -> inputSet.stream().filter(pred).collect(Collectors.toSet()))
        .collect(Collectors.toList());

在此,结果列表中的第一个集合对应于第一个谓词,依此类推。

如果您真的想单次处理输入(无论出于何种原因),则可以为此编写一个特殊的收集器。这是很普遍的一种:

public static <T, A, R> Collector<T, ?, List<R>> multiClassify(
        List<Predicate<T>> predicates, Collector<? super T, A, R> downstream) {
    Supplier<A> dsSupplier = downstream.supplier();
    BiConsumer<A, ? super T> dsAccumulator = downstream.accumulator();
    BinaryOperator<A> dsCombiner = downstream.combiner();

    Supplier<List<A>> supplier = () -> Stream.generate(dsSupplier)
            .limit(predicates.size()).collect(Collectors.toList());

    BiConsumer<List<A>, T> accumulator = (list, t) -> IntStream
            .range(0, predicates.size()).filter(i -> predicates.get(i).test(t))
            .forEach(i -> dsAccumulator.accept(list.get(i), t));

    BinaryOperator<List<A>> combiner = (l1, l2) -> IntStream.range(0, predicates.size())
            .mapToObj(i -> dsCombiner.apply(l1.get(i), l2.get(i)))
            .collect(Collectors.toList());

    Characteristics[] dsCharacteristics = downstream.characteristics().toArray(
            new Characteristics[0]);
    if (downstream.characteristics().contains(Characteristics.IDENTITY_FINISH)) {
        @SuppressWarnings("unchecked")
        Collector<T, ?, List<R>> result = (Collector<T, ?, List<R>>) (Collector<T, ?, ?>) 
            Collector.of(supplier, accumulator, combiner, dsCharacteristics);
        return result;
    }
    Function<A, R> dsFinisher = downstream.finisher();
    Function<List<A>, List<R>> finisher = l -> l.stream().map(dsFinisher)
           .collect(Collectors.toList());
    return Collector.of(supplier, accumulator, combiner, finisher, dsCharacteristics);
}

它获取谓词列表,并返回每个谓词的下游收集器结果列表。用法示例:

List<String> input = asList("abc", "ade", "bcd", "cc", "cdac");

List<Predicate<String>> preds = asList(
        s -> s.length() == 3, 
        s -> s.startsWith("a"), 
        s -> s.endsWith("c"));
List<Set<String>> result = input.stream().collect(multiClassify(preds, Collectors.toSet()));
// [[bcd, abc, ade], [abc, ade], [cc, abc, cdac]]