如何将集合转换为按嵌套集合属性的元素分组的 Guava Multimap?

2022-01-22 00:00:00 java-8 java java-stream guava

我有一个 List<Foo> 并且想要一个 Guava Multimap<String, Foo> 我们将 Foo 的每个标签分组集合<字符串>getTags() 函数.

I have a List<Foo> and want a Guava Multimap<String, Foo> where we've grouped the Foos by each tag of their Collection<String> getTags() function.

我使用的是 Java 8，因此 lambda 和方法引用很好/值得鼓励.

I am using Java 8, so lambdas and method references are fine/encouraged.

例如，如果我有:

foo1, tags=a,b,c
foo2, tags=c,d
foo3, tags=a,c,e

我会得到一个 Multimap<String, Foo> :

a -> foo1, foo3
b -> foo1
c -> foo1, foo2, foo3
d -> foo2
e -> foo3

推荐答案

您可以为此使用自定义收集器:

You can use custom collector for this:

Multimap<String, Foo> map = list.stream().collect(
    ImmutableMultimap::builder,
    (builder, value) -> value.getTags().forEach(tag -> builder.put(tag, value)),
    (builder1, builder2) -> builder1.putAll(builder2.build())
).build();

这不会导致额外的副作用(请参阅 here on this)，是并发且更惯用的.

This does not cause extra side effects (see here on this), is concurrent and more idiomatic.

您还可以将这些临时 lambda 提取到成熟的收集器中，如下所示:

You can also extract these ad-hoc lambdas into a full-fledged collector, something like this:

public static <T, K> Collector<T, ?, Multimap<K, T>> toMultimapByKey(Function<? super T, ? extends Iterable<? extends K>> keysMapper) {
    return new MultimapCollector<>(keysMapper);
}

private static class MultimapCollector<T, K> implements Collector<T, ImmutableMultimap.Builder<K, T>, Multimap<K, T>> {
    private final Function<? super T, ? extends Iterable<? extends K>> keysMapper;

    private MultimapCollector(Function<? super T, ? extends Iterable<? extends K>> keysMapper) {
        this.keysMapper = keysMapper;
    }

    @Override
    public Supplier<ImmutableMultimap.Builder<K, T>> supplier() {
        return ImmutableMultimap::builder;
    }

    @Override
    public BiConsumer<ImmutableMultimap.Builder<K, T>, T> accumulator() {
        return (builder, value) -> keysMapper.apply(value).forEach(k -> builder.put(k, value));
    }

    @Override
    public BinaryOperator<ImmutableMultimap.Builder<K, T>> combiner() {
        return (b1, b2) -> b1.putAll(b2.build());
    }

    @Override
    public Function<ImmutableMultimap.Builder<K, T>, Multimap<K, T>> finisher() {
        return ImmutableMultimap.Builder<K, T>::build;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return Collections.emptySet();
    }
}

那么集合将如下所示:

Multimap<String, Foo> map = list.stream().collect(toMultimapByKey(Foo::getTags));

如果顺序对您不重要，您也可以从 characteristics() 方法返回 EnumSet.of(Characteristics.UNORDERED).这可以使内部收集机制更有效地发挥作用，尤其是在并行归约的情况下.

You can also return EnumSet.of(Characteristics.UNORDERED) from characteristics() method if the order is not important for you. This can make internal collection machinery act more efficiently, especially in case of parallel reduction.

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