Custom Java 8 Collector

public static class MostPopular<T> implements Collector<T, Map<T, Integer>, Optional<T>> {
    @Override
    public Supplier<Map<T, Integer>> supplier() {
        return HashMap::new;
    }

    @Override
    public BiConsumer<Map<T, Integer>, T> accumulator() {
        return (acc, elem) -> {
            Integer value = acc.get(elem);
            if (value == null) {
                value = 1;
            } else {
                value += 1;
            }
            acc.put(elem, value);
        };
    }

    @Override
    public BinaryOperator<Map<T, Integer>> combiner() {
        return (acc1, acc2) -> {
            throw new UnsupportedOperationException();
        };
    }

    @Override
    public Function<Map<T, Integer>, Optional<T>> finisher() {
        return (acc) -> acc.entrySet().stream()
                .reduce((a, b) -> a.getValue() > b.getValue() ? a : b)
                .map(Map.Entry::getKey);
    }

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