RandomizedMaxCollector.java

public class RandomizedMaxCollector<elementType> implements Collector<elementType, List<elementType>, elementType> {

        private final Comparator<elementType> comparator;
        private final SplittableRandom random;

        public RandomizedMaxCollector(Comparator<elementType> comparator, SplittableRandom random) {
            this.comparator = comparator;
            this.random = random;
        }

        @Override
        public Supplier<List<elementType>> supplier() {
            return ArrayList::new;
        }

        @Override
        public BiConsumer<List<elementType>, elementType> accumulator() {
            return (list, elementType) -> {
                int compared = comparator.compare(list.get(0), elementType);
                switch (compared) {
                    case (0):
                        list.add(elementType);
                        break;
                    case (1):
                        break; // leaving list as is
                    case (-1): {
                        list = supplier().get();
                        list.add(elementType);
                        break;
                    }
                    default:
                        throw new IllegalStateException("Comparator returns [" + compared + "] which is not defined");
                }
            };
        }

        @Override
        public BinaryOperator<List<elementType>> combiner() {
            return (firstList, secondList) -> {
                int compared = comparator.compare(firstList.get(0), secondList.get(0));
                switch (compared) {
                    case(0):
                        firstList.addAll(secondList);
                        return firstList;
                    case(1):
                        return firstList;
                    case(-1):
                        return secondList;
                    default:
                        throw new IllegalStateException("Comparator returns [" + compared + "] which is not defined");
                }
            };
        }

        @Override
        public Function<List<elementType>, elementType> finisher() {
            return list -> list.get(random.nextInt(list.size()));
        }

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