Peter,
> EnumSet<Characteristics> intersection =
EnumSet.copyOf(keyCollector.characteristics());
> intersection.retainAll(valCollector.characteristics());
> return intersection;
Please note that `copyOf` should either receive an EnumSet or non-empty
Collection to obtain the enum class. This is not guaranteed by
`characteristics()`
implementation, thus failure is possible.
Testcase:
new BiCollector<>(Collectors.counting(),
Collectors.toSet()).characteristics();
Fails with
Exception in thread "main" java.lang.IllegalArgumentException: Collection
is empty
at java.base/java.util.EnumSet.copyOf(EnumSet.java:173)
at BiCollector.characteristics(BiCollector.java:77)
at Main.main(Main.java:21)
With best regards,
Tagir Valeev.
On Mon, Jun 11, 2018 at 7:40 PM Peter Levart <peter.lev...@gmail.com> wrote:
> Hi,
>
> Have you ever wanted to perform a collection of the same Stream into two
> different targets using two Collectors? Say you wanted to collect
> Map.Entry elements into two parallel lists, each of them containing keys
> and values respectively. Or you wanted to collect elements into groups
> by some key, but also count them at the same time? Currently this is not
> possible to do with a single Stream. You have to create two identical
> streams, so you end up passing Supplier<Stream> to other methods instead
> of bare Stream.
>
> I created a little utility Collector implementation that serves the
> purpose quite well:
>
> /**
> * A {@link Collector} implementation taking two delegate Collector(s)
> and producing result composed
> * of two results produced by delegating collectors, wrapped in {@link
> Map.Entry} object.
> *
> * @param <T> the type of elements collected
> * @param <K> the type of 1st delegate collector collected result
> * @param <V> tye type of 2nd delegate collector collected result
> */
> public class BiCollector<T, K, V> implements Collector<T,
> Map.Entry<Object, Object>, Map.Entry<K, V>> {
> private final Collector<T, Object, K> keyCollector;
> private final Collector<T, Object, V> valCollector;
>
> @SuppressWarnings("unchecked")
> public BiCollector(Collector<T, ?, K> keyCollector, Collector<T, ?,
> V> valCollector) {
> this.keyCollector = (Collector)
> Objects.requireNonNull(keyCollector);
> this.valCollector = (Collector)
> Objects.requireNonNull(valCollector);
> }
>
> @Override
> public Supplier<Map.Entry<Object, Object>> supplier() {
> Supplier<Object> keySupplier = keyCollector.supplier();
> Supplier<Object> valSupplier = valCollector.supplier();
> return () -> new
> AbstractMap.SimpleImmutableEntry<>(keySupplier.get(), valSupplier.get());
> }
>
> @Override
> public BiConsumer<Map.Entry<Object, Object>, T> accumulator() {
> BiConsumer<Object, T> keyAccumulator = keyCollector.accumulator();
> BiConsumer<Object, T> valAccumulator = valCollector.accumulator();
> return (accumulation, t) -> {
> keyAccumulator.accept(accumulation.getKey(), t);
> valAccumulator.accept(accumulation.getValue(), t);
> };
> }
>
> @Override
> public BinaryOperator<Map.Entry<Object, Object>> combiner() {
> BinaryOperator<Object> keyCombiner = keyCollector.combiner();
> BinaryOperator<Object> valCombiner = valCollector.combiner();
> return (accumulation1, accumulation2) -> new
> AbstractMap.SimpleImmutableEntry<>(
> keyCombiner.apply(accumulation1.getKey(),
> accumulation2.getKey()),
> valCombiner.apply(accumulation1.getValue(),
> accumulation2.getValue())
> );
> }
>
> @Override
> public Function<Map.Entry<Object, Object>, Map.Entry<K, V>>
> finisher() {
> Function<Object, K> keyFinisher = keyCollector.finisher();
> Function<Object, V> valFinisher = valCollector.finisher();
> return accumulation -> new AbstractMap.SimpleImmutableEntry<>(
> keyFinisher.apply(accumulation.getKey()),
> valFinisher.apply(accumulation.getValue())
> );
> }
>
> @Override
> public Set<Characteristics> characteristics() {
> EnumSet<Characteristics> intersection =
> EnumSet.copyOf(keyCollector.characteristics());
> intersection.retainAll(valCollector.characteristics());
> return intersection;
> }
> }
>
>
> Do you think this class is general enough to be part of standard
> Collectors repertoire?
>
> For example, accessed via factory method Collectors.toBoth(Collector
> coll1, Collector coll2), bi-collection could then be coded simply as:
>
> Map<String, Integer> map = ...
>
> Map.Entry<List<String>, List<Integer>> keys_values =
> map.entrySet()
> .stream()
> .collect(
> toBoth(
> mapping(Map.Entry::getKey, toList()),
> mapping(Map.Entry::getValue, toList())
> )
> );
>
>
> Map.Entry<Map<Integer, Long>, Long> histogram_count =
> ThreadLocalRandom
> .current()
> .ints(100, 0, 10)
> .boxed()
> .collect(
> toBoth(
> groupingBy(Function.identity(), counting()),
> counting()
> )
> );
>
>
> Regards, Peter
>
>
