Java 8 收集器用于番石榴不可变集合？

从版本 21 开始，您可以

.collect(ImmutableSet.toImmutableSet())
.collect(ImmutableMap.toImmutableMap())
.collect(Maps.toImmutableEnumMap())
.collect(Sets.toImmutableEnumSet())
.collect(Tables.toTable())
.collect(ImmutableList.toImmutableList())
.collect(Multimaps.toMultimap(...))

更新：我发现一个似乎涵盖了 https://github.com/yanaga/guava-stream 所有番石榴馆藏的实现，并试图在我自己的图书馆中对其进行改进，https://bitbucket.org/cowwoc/guava-jdk8/

出于历史原因，我将前面的答案留在下面。

神圣的#@！（我明白了！

此实现适用于任何 Multimap（可变或不可变），而 shmosel 的解决方案侧重于不可变实现。也就是说，对于不可变的情况，后者可能更有效（我不使用构建器）。

import com.google.common.collect.Multimap;
import java.util.EnumSet;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Collector.Characteristics;
import org.bitbucket.cowwoc.preconditions.Preconditions;

/**
 * A Stream collector that returns a Multimap.
 * <p>
 * @author Gili Tzabari
 * @param <T> the type of the input elements
 * @param <K> the type of keys stored in the map
 * @param <V> the type of values stored in the map
 * @param <R> the output type of the collector
 */
public final class MultimapCollector<T, K, V, R extends Multimap<K, V>>
    implements Collector<T, Multimap<K, V>, R>
{
    private final Supplier<Multimap<K, V>> mapSupplier;
    private final Function<? super T, ? extends K> keyMapper;
    private final Function<? super T, ? extends V> valueMapper;
    private final Function<Multimap<K, V>, R> resultMapper;

    /**
     * Creates a new MultimapCollector.
     * <p>
     * @param mapSupplier  a function which returns a new, empty {@code Multimap} into which intermediate results will be
     *                     inserted
     * @param keyMapper    a function that transforms the map keys
     * @param valueMapper  a function that transforms the map values
     * @param resultMapper a function that transforms the intermediate {@code Multimap} into the final result
     * @throws NullPointerException if any of the arguments are null
     */
    public MultimapCollector(Supplier<Multimap<K, V>> mapSupplier,
        Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends V> valueMapper,
        Function<Multimap<K, V>, R> resultMapper)
    {
        Preconditions.requireThat(mapSupplier, "mapSupplier").isNotNull();
        Preconditions.requireThat(keyMapper, "keyMapper").isNotNull();
        Preconditions.requireThat(valueMapper, "valueMapper").isNotNull();
        Preconditions.requireThat(resultMapper, "resultMapper").isNotNull();

        this.mapSupplier = mapSupplier;
        this.keyMapper = keyMapper;
        this.valueMapper = valueMapper;
        this.resultMapper = resultMapper;
    }

    @Override
    public Supplier<Multimap<K, V>> supplier()
    {
        return mapSupplier;
    }

    @Override
    public BiConsumer<Multimap<K, V>, T> accumulator()
    {
        return (map, entry) ->
        {
            K key = keyMapper.apply(entry);
            if (key == null)
                throw new IllegalArgumentException("keyMapper(" + entry + ") returned null");
            V value = valueMapper.apply(entry);
            if (value == null)
                throw new IllegalArgumentException("keyMapper(" + entry + ") returned null");
            map.put(key, value);
        };
    }

    @Override
    public BinaryOperator<Multimap<K, V>> combiner()
    {
        return (left, right) ->
        {
            left.putAll(right);
            return left;
        };
    }

    @Override
    public Function<Multimap<K, V>, R> finisher()
    {
        return resultMapper;
    }

    @Override
    public Set<Characteristics> characteristics()
    {
        return EnumSet.noneOf(Characteristics.class);
    }
}

[...]

import com.google.common.collect.HashMultimap;
import com.google.common.collect.ImmutableMultimap;
import com.google.common.collect.Multimap;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

/**
 * Stream collectors for Guava collections.
 * <p>
 * @author Gili Tzabari
 */
public final class GuavaCollectors
{
    /**
     * Returns a {@code Collector} that accumulates elements into a {@code Multimap}.
     * <p>
     * @param <T>          the type of the input elements
     * @param <K>          the type of the map keys
     * @param <V>          the type of the map values
     * @param <R>          the output type of the collector
     * @param mapSupplier  a function which returns a new, empty {@code Multimap} into which intermediate results will be
     *                     inserted
     * @param keyMapper    a function that transforms the map keys
     * @param valueMapper  a function that transforms the map values
     * @param resultMapper a function that transforms the intermediate {@code Multimap} into the final result
     * @return a {@code Collector} which collects elements into a {@code Multimap} whose keys and values are the result of
     *         applying mapping functions to the input elements
     */
    public static <T, K, V, R extends Multimap<K, V>> Collector<T, ?, R> toMultimap(
        Supplier<Multimap<K, V>> mapSupplier,
        Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends V> valueMapper,
        Function<Multimap<K, V>, R> resultMapper)
    {
        return new MultimapCollector<>(mapSupplier, keyMapper, valueMapper, resultMapper);
    }

    public static void main(String[] args)
    {
        Multimap<Integer, Double> input = HashMultimap.create();
        input.put(10, 20.0);
        input.put(10, 25.0);
        input.put(50, 60.0);
        System.out.println("input: " + input);
        ImmutableMultimap<Integer, Double> output = input.entries().stream().collect(
            GuavaCollectors.toMultimap(HashMultimap::create,
                entry -> entry.getKey() + 1, entry -> entry.getValue() - 1,
                ImmutableMultimap::copyOf));
        System.out.println("output: " + output);
    }
}

主（） 输出：

input: {10=[20.0, 25.0], 50=[60.0]}
output: {51=[59.0], 11=[24.0, 19.0]}

资源

• Arjit提供了一个很好的资源，展示了如何为其他番石榴系列实施收藏家：http://blog.comsysto.com/2014/11/12/java-8-collectors-for-guava-collections/