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
