Collectors.java 文件源码

public static <T, L, K> Collector<T, ?, ImmutableMultimap<K, T>> groupingByValues(Function<? super T, ? extends Iterable<? extends K>> classifier) {
    return ImmutableGenericCollector.<T, LinkedListMultimap<K, T>, ImmutableMultimap<K, T>>builder()
        .supplier(LinkedListMultimap::create)
        .accumulator((map, t) -> {
            classifier.apply(t).forEach(k -> {
                map.put(k, t);
            });
        })
        .combiner((a,b) -> {
            LinkedListMultimap<K, T> ret = LinkedListMultimap.create(a);
            ret.putAll(b);
            return ret;
        })
        .finisher(map -> ImmutableMultimap.copyOf(map))
        .build();
}