我真的很喜欢Java 8流和Guava的不可变集合，但是我不知道如何将两者一起使用。
例如，如何实现将流结果收集到Collector中的Java 8 ImmutableMultimap？
奖励积分：我希望能够提供键/值映射器，类似于Collectors.toMap()的工作方式。
.collect(ImmutableSet.toImmutableSet())
.collect(ImmutableMap.toImmutableMap())
.collect(Maps.toImmutableEnumMap())
.collect(Sets.toImmutableEnumSet())
.collect(Tables.toTable())
.collect(ImmutableList.toImmutableList())
.collect(Multimaps.toMultimap(...))
出于历史原因，我将下面的答案留在下面。
[圣＃@！（我明白了！
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); } }main（）输出：
input: {10=[20.0, 25.0], 50=[60.0]} output: {51=[59.0], 11=[24.0, 19.0]}资源
Arjit提供了一个极好的资源，展示了如何为其他Guava集合实现Collector：http://blog.comsysto.com/2014/11/12/java-8-collectors-for-guava-collections/
Stream<T> stream = ...
ImmutableXxx<T> collection = ImmutableXxx.copyOf(stream.iterator());