java – 最简单的方式来索引一个集合的属性,该属性本身就是一个

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();

这不会引起额外的副作用(参见here),是并发的,比较惯用的.

您还可以将这些特别的羔羊提取成一个完整的收藏家,如下所示：

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>,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> 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>> finisher() {
        return ImmutableMultimap.Builder<K,T>::build;
    }

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

那么收藏将如下所示：

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

如果您的订单不重要,您还可以从特性()方法返回EnumSet.of(Characteristics.UNORDERED).这可以使内部收集机械更有效地起作用,特别是在并行还原的情况下.