重複要素を考慮した Collector クラス

DistinctCountingCollector.java

package pinzolo.util.collectors;

import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
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;

public class DistinctCountingCollector<T> implements Collector<T, Set<T>, Long> {
  @Override
  public Supplier<Set<T>> supplier() {
    return HashSet<T>::new;
  }

  @Override
  public BiConsumer<Set<T>, T> accumulator() {
    return (set, obj) -> set.add(obj);
  }

  @Override
  public BinaryOperator<Set<T>> combiner() {
    return (set1, set2) -> {
      set1.addAll(set2);
      return set1;
    };
  }

  @Override
  public Function<Set<T>, Long> finisher() {
    return set -> Long.valueOf(set.size());
  }

  @Override
  public Set<Collector.Characteristics> characteristics() {
    return Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.UNORDERED));
  }
}