java.util.stream.Collector.Characteristics Java Examples

@SuppressWarnings("unchecked")
static <K, D, A, M extends Map<K, D>> PartialCollector<Map<K, A>, M> grouping(Supplier<M> mapFactory,
        Collector<?, A, D> downstream) {
    BinaryOperator<A> downstreamMerger = downstream.combiner();
    BiConsumer<Map<K, A>, Map<K, A>> merger = (map1, map2) -> {
        for (Map.Entry<K, A> e : map2.entrySet())
            map1.merge(e.getKey(), e.getValue(), downstreamMerger);
    };

    if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
        return (PartialCollector<Map<K, A>, M>) new PartialCollector<>((Supplier<Map<K, A>>) mapFactory,
                merger, Function.identity(), ID_CHARACTERISTICS);
    }
    Function<A, D> downstreamFinisher = downstream.finisher();
    return new PartialCollector<>((Supplier<Map<K, A>>) mapFactory, merger, map -> {
        map.replaceAll((k, v) -> ((Function<A, A>) downstreamFinisher).apply(v));
        return (M) map;
    }, NO_CHARACTERISTICS);
}
 

private static <T> Collector<T, AtomicLong, Long> countAssertions(Consumer<T> assertion) {
  /**
   * Collects a stream by running the passed-in assertion on all stream items. The collection also
   * asserts that the stream is non-empty, to avoid false positives when accidentally producing
   * empty streams, e.g. by filtering.
   */
  return Collector.of(
      AtomicLong::new,
      (count, t) -> {
        count.incrementAndGet();
        assertion.accept(t);
      },
      (a, b) -> new AtomicLong(a.get() + b.get()),
      count -> {
        long value = count.get();
        if (value == 0) {
          throw new IllegalStateException("Stream was empty, did not assert anything.");
        }
        return value;
      },
      Characteristics.UNORDERED);
}
 

/**
 * Perform a partial mutable reduction using the supplied {@link Collector}
 * on a series of adjacent elements.
 * 
 * <p>
 * This is a <a href="package-summary.html#StreamOps">quasi-intermediate</a>
 * partial reduction operation.
 * 
 * @param <R> the type of the elements in the resulting stream
 * @param <A> the intermediate accumulation type of the {@code Collector}
 * @param collapsible a non-interfering, stateless predicate to apply to the
 *        pair of adjacent elements of the input stream which returns true
 *        for elements which should be collected together.
 * @param collector a {@code Collector} which is used to combine the
 *        adjacent elements.
 * @return the new stream
 * @since 0.3.6
 */
public <R, A> StreamEx<R> collapse(BiPredicate<? super T, ? super T> collapsible,
        Collector<? super T, A, R> collector) {
    Supplier<A> supplier = collector.supplier();
    BiConsumer<A, ? super T> accumulator = collector.accumulator();
    StreamEx<A> stream = collapseInternal(collapsible, t -> {
        A acc = supplier.get();
        accumulator.accept(acc, t);
        return acc;
    }, (acc, t) -> {
        accumulator.accept(acc, t);
        return acc;
    }, collector.combiner());
    if (collector.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {
        @SuppressWarnings("unchecked")
        StreamEx<R> result = (StreamEx<R>) stream;
        return result;
    }
    return stream.map(collector.finisher());
}
 

public static <X> Collector<X, ?, Set<X>> toDuplicateSet(int min) {
    if (min < 2) {
        throw new IllegalArgumentException();
    }
    Supplier<Map<X, Integer>> supplier = HashMap::new;
    BiConsumer<Map<X, Integer>, X> accumulator = (map, key) -> map.merge(key, 1, Integer::sum);
    BinaryOperator<Map<X, Integer>> combiner = Functions.mergeToLeftMap(Integer::sum);
    Function<Map<X, Integer>, Set<X>> finisher = Functions.keySetWhereValues(v -> v >= min);
    return Collector.of(supplier, accumulator, combiner, finisher, Characteristics.UNORDERED);
}
 

public static <X> Collector<X, ?, Stream<X>> toDuplicateStream(int min) {
    if (min < 2) {
        throw new IllegalArgumentException();
    }
    Supplier<Map<X, Integer>> supplier = HashMap::new;
    BiConsumer<Map<X, Integer>, X> accumulator = (map, key) -> map.merge(key, 1, Integer::sum);
    BinaryOperator<Map<X, Integer>> combiner = Functions.mergeToLeftMap(Integer::sum);
    Function<Map<X, Integer>, Stream<X>> finisher = Functions.keyStreamWhereValues(v -> v >= min);
    return Collector.of(supplier, accumulator, combiner, finisher, Characteristics.UNORDERED);
}
 

public static <T> Collector<T, ?, T> toSingleResult(Supplier<? extends RuntimeException> exceptionSupplier) {
    Objects.requireNonNull(exceptionSupplier);
    Supplier<List<T>> supplier = LinkedList<T>::new;
    BiConsumer<List<T>, T> accumulator = List<T>::add;
    BinaryOperator<List<T>> combiner = Functions.mergeLeft(List::addAll);
    Function<List<T>, T> finisher = list -> {
        if (list.size() != 1) {
            throw exceptionSupplier.get();
        }
        return list.get(0);
    };
    return Collector.of(supplier, accumulator, combiner, finisher, Characteristics.UNORDERED);
}
 

/** Returns a collector that accumulates the the input NBT tags into a new NBT list. */
public static <T> Collector<T, NBTTagCompound, NBTTagCompound> toCompound(
	Function<T, String> keyMapper, Function<T, NBTBase> tagMapper) {
	return Collector.of(NBTTagCompound::new,
		(compound, element) ->
			compound.setTag(keyMapper.apply(element), tagMapper.apply(element)),
		(left, right) -> {
			for (String key : right.getKeySet())
				left.setTag(key, right.getTag(key));
			return left;
		}, Characteristics.IDENTITY_FINISH);
}
 

/** Returns a collector that accumulates the the input elements into a new NBT list. */
public static <T> Collector<T, NBTTagList, NBTTagList> toList() {
	return Collector.of(NBTTagList::new,
		(list, element) ->
			list.appendTag(createTag(element)),
		(left, right) -> {
			for (NBTBase tag : iterate(right))
				left.appendTag(tag);
			return left;
		}, Characteristics.IDENTITY_FINISH);
}
 

@Test
public void testIfAllMatch() {
    assertThrows(NullPointerException.class, () -> MoreCollectors.ifAllMatch(null, Collectors.toList()));
    assertThrows(NullPointerException.class, () -> MoreCollectors.ifAllMatch(i -> true, null));
    assertFalse(MoreCollectors.ifAllMatch(i -> true, Collectors.toList()).characteristics()
        .contains(Characteristics.UNORDERED));
    assertTrue(MoreCollectors.ifAllMatch(i -> true, Collectors.toSet()).characteristics()
        .contains(Characteristics.UNORDERED));
    Supplier<Stream<Integer>> five = () -> IntStreamEx.range(5).boxed();
    checkShortCircuitCollector("ifAllMatch: all match", Optional.of(asList(0, 1, 2, 3, 4)), 5, five,
            MoreCollectors.ifAllMatch(i -> true, Collectors.toList()));

    Supplier<Stream<Integer>> ints = () -> IntStreamEx.ints().boxed();
    checkShortCircuitCollector("ifAllMatch: shirtCircuit downstream", Optional.of(asList(0, 1, 2)), 3, ints,
            MoreCollectors.ifAllMatch(i -> true, MoreCollectors.head(3)), true);

    checkShortCircuitCollector("ifAllMatch: shirtCircuit downstream", Optional.empty(), 4, ints,
            MoreCollectors.ifAllMatch(i -> i < 3, MoreCollectors.head(5)), true);

    checkShortCircuitCollector("ifAllMatch: some match", Optional.empty(), 11, ints,
            MoreCollectors.ifAllMatch(i -> i < 10, Collectors.toList()), true);

    checkShortCircuitCollector("ifAllMatch: some match", Optional.empty(), 2, five,
            MoreCollectors.ifAllMatch(i -> i % 2 == 0, Collectors.toList()));

    checkShortCircuitCollector("ifAllMatch: empty stream", Optional.of(Collections.emptyList()), 0, Stream::empty,
            MoreCollectors.ifAllMatch(i -> true, Collectors.toList()));
}
 

/**
 * Adapts a {@code Collector} accepting elements of type {@code U} to one
 * accepting elements of type {@code T} by applying a flat mapping function
 * to each input element before accumulation. The flat mapping function maps
 * an input element to a {@link Stream stream} covering zero or more output
 * elements that are then accumulated downstream. Each mapped stream is
 * {@link java.util.stream.BaseStream#close() closed} after its contents
 * have been placed downstream. (If a mapped stream is {@code null} an empty
 * stream is used, instead.)
 * 
 * <p>
 * This method is similar to {@code Collectors.flatMapping} method which
 * appears in JDK 9. However when downstream collector is
 * <a href="package-summary.html#ShortCircuitReduction">short-circuiting</a>
 * , this method will also return a short-circuiting collector.
 * 
 * @param <T> the type of the input elements
 * @param <U> type of elements accepted by downstream collector
 * @param <A> intermediate accumulation type of the downstream collector
 * @param <R> result type of collector
 * @param mapper a function to be applied to the input elements, which
 *        returns a stream of results
 * @param downstream a collector which will receive the elements of the
 *        stream returned by mapper
 * @return a collector which applies the mapping function to the input
 *         elements and provides the flat mapped results to the downstream
 *         collector
 * @throws NullPointerException if mapper is null, or downstream is null.
 * @since 0.4.1
 */
public static <T, U, A, R> Collector<T, ?, R> flatMapping(Function<? super T, ? extends Stream<? extends U>> mapper,
        Collector<? super U, A, R> downstream) {
    Objects.requireNonNull(mapper);
    BiConsumer<A, ? super U> downstreamAccumulator = downstream.accumulator();
    Predicate<A> finished = finished(downstream);
    if (finished != null) {
        return new CancellableCollectorImpl<>(downstream.supplier(), (acc, t) -> {
            if (finished.test(acc))
                return;
            try (Stream<? extends U> stream = mapper.apply(t)) {
                if (stream != null) {
                    stream.spliterator().forEachRemaining(u -> {
                        downstreamAccumulator.accept(acc, u);
                        if (finished.test(acc))
                            throw new CancelException();
                    });
                }
            } catch (CancelException ex) {
                // ignore
            }
        }, downstream.combiner(), downstream.finisher(), finished, downstream.characteristics());
    }
    return Collector.of(downstream.supplier(), (acc, t) -> {
        try (Stream<? extends U> stream = mapper.apply(t)) {
            if (stream != null) {
                stream.spliterator().forEachRemaining(u -> downstreamAccumulator.accept(acc, u));
            }
        }
    }, downstream.combiner(), downstream.finisher(), downstream.characteristics().toArray(new Characteristics[0]));
}
 

/**
 * {@inheritDoc}
 *
 * <p>
 * If special <a
 * href="package-summary.html#ShortCircuitReduction">short-circuiting
 * collector</a> is passed, this operation becomes short-circuiting as well.
 */
@Override
public <R, A> R collect(Collector<? super T, A, R> collector) {
    Predicate<A> finished = finished(collector);
    if (finished != null) {
        BiConsumer<A, ? super T> acc = collector.accumulator();
        BinaryOperator<A> combiner = collector.combiner();
        Spliterator<T> spliterator = spliterator();
        if (!isParallel()) {
            A a = collector.supplier().get();
            if (!finished.test(a)) {
                try {
                    // forEachRemaining can be much faster
                    // and take much less memory than tryAdvance for certain
                    // spliterators
                    spliterator.forEachRemaining(e -> {
                        acc.accept(a, e);
                        if (finished.test(a))
                            throw new CancelException();
                    });
                } catch (CancelException ex) {
                    // ignore
                }
            }
            return collector.finisher().apply(a);
        }
        Spliterator<A> spltr;
        if (!spliterator.hasCharacteristics(Spliterator.ORDERED)
            || collector.characteristics().contains(Characteristics.UNORDERED)) {
            spltr = new UnorderedCancellableSpliterator<>(spliterator, collector.supplier(), acc, combiner,
                    finished);
        } else {
            spltr = new OrderedCancellableSpliterator<>(spliterator, collector.supplier(), acc, combiner, finished);
        }
        return collector.finisher().apply(
            new StreamEx<>(StreamSupport.stream(spltr, true), context).findFirst().get());
    }
    return rawCollect(collector);
}
 

BaseCollector(Supplier<A> supplier, BiConsumer<A, A> merger, Function<A, R> finisher,
        Set<Characteristics> characteristics) {
    this.supplier = supplier;
    this.merger = merger;
    this.finisher = finisher;
    this.characteristics = characteristics;
}
 

static <T, E, A, R> Collector<T, A, R> flatMapping(
    Function<? super T, ? extends Stream<? extends E>> mapper, Collector<E, A, R> collector) {
  BiConsumer<A, E> accumulator = collector.accumulator();
  return Collector.of(
      collector.supplier(),
      (a, input) -> mapper.apply(input).forEachOrdered(e -> accumulator.accept(a, e)),
      collector.combiner(),
      collector.finisher(),
      collector.characteristics().toArray(new Characteristics[0]));
}
 

CancellableCollectorImpl(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner,
                         Function<A, R> finisher, Predicate<A> finished,
                         Set<java.util.stream.Collector.Characteristics> characteristics) {
    this.supplier = supplier;
    this.accumulator = accumulator;
    this.combiner = combiner;
    this.finisher = finisher;
    this.finished = finished;
    this.characteristics = characteristics;
}
 

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

DoubleCollectorImpl(Supplier<A> supplier, ObjDoubleConsumer<A> doubleAccumulator,
                    BiConsumer<A, A> merger, Function<A, R> finisher, Set<Characteristics> characteristics) {
    super(supplier, merger, finisher, characteristics);
    this.doubleAccumulator = doubleAccumulator;
}
 

LongCollectorImpl(Supplier<A> supplier, ObjLongConsumer<A> longAccumulator, BiConsumer<A, A> merger,
                  Function<A, R> finisher, Set<Characteristics> characteristics) {
    super(supplier, merger, finisher, characteristics);
    this.longAccumulator = longAccumulator;
}
 

IntCollectorImpl(Supplier<A> supplier, ObjIntConsumer<A> intAccumulator, BiConsumer<A, A> merger,
                 Function<A, R> finisher, Set<Characteristics> characteristics) {
    super(supplier, merger, finisher, characteristics);
    this.intAccumulator = intAccumulator;
}
 

@Test
void testCharacteristics()
{
    Set<Characteristics> characteristics = cookieStoreCollector.characteristics();
    assertEquals(characteristics, EnumSet.of(Characteristics.UNORDERED, Characteristics.IDENTITY_FINISH));
}
 

<T> Collector<T, A, R> asRef(BiConsumer<A, T> accumulator) {
    return Collector.of(supplier, accumulator, combiner(), finisher, characteristics
            .toArray(new Characteristics[0]));
}
 

PartialCollector(Supplier<A> supplier, BiConsumer<A, A> merger, Function<A, R> finisher,
        Set<Characteristics> characteristics) {
    super(supplier, merger, finisher, characteristics);
}
 

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

private static <T, A, R> Characteristics[] getCharacteristics(Collector<T, A, R> downstream) {
    return downstream.characteristics().stream().toArray(Characteristics[]::new);
}
 

/**
 * Returns a {@code Collector} which just ignores the input and calls the
 * provided supplier once to return the output.
 * 
 * @param <T> the type of input elements
 * @param <U> the type of output
 * @param supplier the supplier of the output
 * @return a {@code Collector} which just ignores the input and calls the
 *         provided supplier once to return the output.
 */
private static <T, U> Collector<T, ?, U> empty(Supplier<U> supplier) {
    return new CancellableCollectorImpl<>(() -> NONE, (acc, t) -> {
        // empty
    }, selectFirst(), acc -> supplier.get(), alwaysTrue(), EnumSet.of(Characteristics.UNORDERED,
        Characteristics.CONCURRENT));
}
 

/**
 * Adapts a {@code Collector} to perform an additional finishing
 * transformation.
 * 
 * <p>
 * Unlike {@link Collectors#collectingAndThen(Collector, Function)} this
 * method returns a
 * <a href="package-summary.html#ShortCircuitReduction">short-circuiting
 * collector</a> if the downstream collector is short-circuiting.
 *
 * @param <T> the type of the input elements
 * @param <A> intermediate accumulation type of the downstream collector
 * @param <R> result type of the downstream collector
 * @param <RR> result type of the resulting collector
 * @param downstream a collector
 * @param finisher a function to be applied to the final result of the
 *        downstream collector
 * @return a collector which performs the action of the downstream
 *         collector, followed by an additional finishing step
 * @throws NullPointerException if downstream is null, or finisher is null.
 * @see Collectors#collectingAndThen(Collector, Function)
 * @since 0.4.0
 */
public static <T, A, R, RR> Collector<T, A, RR> collectingAndThen(Collector<T, A, R> downstream,
        Function<R, RR> finisher) {
    Predicate<A> finished = finished(downstream);
    if (finished != null) {
        return new CancellableCollectorImpl<>(downstream.supplier(), downstream.accumulator(), downstream
                .combiner(), downstream.finisher().andThen(finisher), finished, downstream.characteristics()
                        .contains(Characteristics.UNORDERED) ? UNORDERED_CHARACTERISTICS : NO_CHARACTERISTICS);
    }
    return Collectors.collectingAndThen(downstream, finisher);
}
 

/**
 * Returns a {@code Collector} that accumulates the input elements into a
 * new {@code Set}. There are no guarantees on the type, mutability,
 * serializability, or thread-safety of the {@code Set} returned; if more
 * control over the returned {@code Set} is required, use
 * {@link Collectors#toCollection(Supplier)}.
 *
 * <p>
 * This is an {@link Collector.Characteristics#UNORDERED unordered}
 * Collector.
 *
 * @param <T>
 *            the type of the input elements
 * @return a {@code Collector} which collects all the input elements into a
 *         {@code Set}
 */
@SuppressWarnings("unchecked")
public static <T> Collector<T, ?, Set<T>> toLinkedHashSet() {
	return Collector.of(LinkedHashSet::new, Set::add, (s, rs) -> {
		s.add((T) rs);
		return s;
	}, Characteristics.IDENTITY_FINISH);
}
 

/**
 * Returns a {@code Collector} which performs downstream reduction if all
 * elements satisfy the {@code Predicate}. The result is described as an
 * {@code Optional<R>}.
 * 
 * <p>
 * The resulting collector returns an empty optional if at least one input
 * element does not satisfy the predicate. Otherwise it returns an optional
 * which contains the result of the downstream collector.
 * 
 * <p>
 * This method returns a
 * <a href="package-summary.html#ShortCircuitReduction">short-circuiting
 * collector</a>: it may not process all the elements if some of items don't
 * satisfy the predicate or if downstream collector is a short-circuiting
 * collector.
 * 
 * <p>
 * It's guaranteed that the downstream collector is not called for elements
 * which don't satisfy the predicate.
 *
 * @param <T> the type of input elements
 * @param <A> intermediate accumulation type of the downstream collector
 * @param <R> result type of the downstream collector
 * @param predicate a non-interfering, stateless predicate to checks whether
 *        collector should proceed with element
 * @param downstream a {@code Collector} implementing the downstream
 *        reduction
 * @return a {@code Collector} witch performs downstream reduction if all
 *         elements satisfy the predicate
 * @throws NullPointerException if mapper is null.
 * @see Stream#allMatch(Predicate)
 * @see AbstractStreamEx#dropWhile(Predicate)
 * @see AbstractStreamEx#takeWhile(Predicate)
 * @since 0.6.3
 */
public static <T, A, R> Collector<T, ?, Optional<R>> ifAllMatch(Predicate<T> predicate,
        Collector<T, A, R> downstream) {
    Objects.requireNonNull(predicate);
    Predicate<A> finished = finished(downstream);
    Supplier<A> supplier = downstream.supplier();
    BiConsumer<A, T> accumulator = downstream.accumulator();
    BinaryOperator<A> combiner = downstream.combiner();
    return new CancellableCollectorImpl<>(
            () -> new PairBox<>(supplier.get(), Boolean.TRUE),
            (acc, t) -> {
                if (acc.b && predicate.test(t)) {
                    accumulator.accept(acc.a, t);
                } else {
                    acc.b = Boolean.FALSE;
                }
            },
            (acc1, acc2) -> {
                if (acc1.b && acc2.b) {
                    acc1.a = combiner.apply(acc1.a, acc2.a);
                } else {
                    acc1.b = Boolean.FALSE;
                }
                return acc1;
            },
            acc -> acc.b ? Optional.of(downstream.finisher().apply(acc.a)) : Optional.empty(),
            finished == null ? acc -> !acc.b : acc -> !acc.b || finished.test(acc.a),
            downstream.characteristics().contains(Characteristics.UNORDERED) ? UNORDERED_CHARACTERISTICS
                    : NO_CHARACTERISTICS);
}
 

/**
 * Returns a {@link Map} where elements of this stream with the same key are
 * grouped together. The resulting {@code Map} keys are the keys of this
 * stream entries and the corresponding values are combined using the
 * provided downstream collector.
 *
 * <p>
 * There are no guarantees on the type, mutability, serializability, or
 * thread-safety of the {@code Map} object returned. If more control over
 * the returned {@code Map} is required, use
 * {@link #grouping(Supplier, Collector)}.
 *
 * <p>
 * This is a <a href="package-summary.html#StreamOps">terminal</a>
 * operation.
 *
 * @param <A> the intermediate accumulation type of the downstream collector
 * @param <D> the result type of the downstream reduction
 * @param downstream a {@code Collector} implementing the downstream
 *        reduction
 * @return a {@code Map} containing the elements of this stream
 * @see Collectors#groupingBy(Function, Collector)
 */
public <A, D> Map<K, D> grouping(Collector<? super V, A, D> downstream) {
    Function<Entry<K, V>, K> keyMapper = Entry::getKey;
    Collector<Entry<K, V>, ?, D> mapping = Collectors.mapping(Entry::getValue, downstream);
    if (isParallel() && downstream.characteristics().contains(Characteristics.UNORDERED)) {
        return collect(Collectors.groupingByConcurrent(keyMapper, mapping));
    }
    return collect(Collectors.groupingBy(keyMapper, mapping));
}
 

/**
 * Returns a {@link Map} where elements of this stream with the same key are
 * grouped together. The resulting {@code Map} keys are the keys of this
 * stream entries and the corresponding values are combined using the
 * provided downstream collector. The {@code Map} is created using
 * the provided supplier function.
 *
 * <p>
 * This is a <a href="package-summary.html#StreamOps">terminal</a>
 * operation.
 *
 * @param <A> the intermediate accumulation type of the downstream collector
 * @param <D> the result type of the downstream reduction
 * @param <M> the type of the resulting {@code Map}
 * @param mapSupplier a function which returns a new, empty {@code Map} into
 *        which the results will be inserted
 * @param downstream a {@code Collector} implementing the downstream
 *        reduction
 * @return a {@code Map} containing the elements of this stream
 * @see Collectors#groupingBy(Function, Supplier, Collector)
 */
@SuppressWarnings("unchecked")
public <A, D, M extends Map<K, D>> M grouping(Supplier<M> mapSupplier, Collector<? super V, A, D> downstream) {
    Function<Entry<K, V>, K> keyMapper = Entry::getKey;
    Collector<Entry<K, V>, ?, D> mapping = Collectors.mapping(Entry::getValue, downstream);
    if (isParallel() && downstream.characteristics().contains(Characteristics.UNORDERED)
        && mapSupplier.get() instanceof ConcurrentMap) {
        return (M) collect(Collectors.groupingByConcurrent(keyMapper,
            (Supplier<? extends ConcurrentMap<K, D>>) mapSupplier, mapping));
    }
    return collect(Collectors.groupingBy(keyMapper, mapSupplier, mapping));
}
 

/**
 * Returns a {@code Map} whose keys are the values resulting from applying
 * the classification function to the input elements, and whose
 * corresponding values are the result of reduction of the input elements
 * which map to the associated key under the classification function.
 *
 * <p>
 * The {@code Map} will be created using the provided factory function.
 * 
 * <p>
 * This is a <a href="package-summary.html#StreamOps">terminal</a>
 * operation.
 * 
 * @param <K> the type of the keys
 * @param <D> the result type of the downstream reduction
 * @param <M> the type of the resulting {@code Map}
 * @param classifier the classifier function mapping input elements to keys
 * @param mapFactory a function which, when called, produces a new empty
 *        {@code Map} of the desired type
 * @param downstream a {@code Collector} implementing the downstream
 *        reduction
 * @return a {@code Map} containing the results of the group-by operation
 *
 * @see #groupingBy(Function)
 * @see Collectors#groupingBy(Function, Supplier, Collector)
 * @see Collectors#groupingByConcurrent(Function, Supplier, Collector)
 */
@SuppressWarnings("unchecked")
public <K, D, M extends Map<K, D>> M groupingBy(Function<? super T, ? extends K> classifier,
        Supplier<M> mapFactory, Collector<? super T, ?, D> downstream) {
    if (isParallel() && downstream.characteristics().contains(Characteristics.UNORDERED)
        && mapFactory.get() instanceof ConcurrentMap)
        return (M) rawCollect(Collectors.groupingByConcurrent(classifier,
            (Supplier<ConcurrentMap<K, D>>) mapFactory, downstream));
    return rawCollect(Collectors.groupingBy(classifier, mapFactory, downstream));
}