Collector to flatten collections in Java8

flatten-collector

    /* Flatten list to list */
    public static final <T, R> Collector<T, List<R>, List<R>> flattenCollector(Function<T,List<R>> mapper) {
        return Collector.of(
                () -> new ArrayList<>(),
                (l, a) -> l.addAll(mapper.apply(a)),
                (l, r) -> { l.addAll(r); return l; },
                Collector.Characteristics.UNORDERED
        );
    }
  
    /* Flatten any collection type to List */
    public static final <T, R, C extends Collection<? extends R>> Collector<T, List<R>, List<R>> flattenCollector2(Function<T, C> mapper) {
        return Collector.of(
                () -> new ArrayList<R>(),
                (l, a) -> l.addAll(mapper.apply(a)),
                (l, r) -> { l.addAll(r); return l; },
                Collector.Characteristics.UNORDERED
        );
    }

    /* Trying to preserve type of collection, but does not work. Suspect because of Generics limitation of Java.
       contra-variant/covariant problem. List<String> != Collection<String> in Java.
       */
    public static final <T, R, C extends Collection<R>> Collector<T, C, C> flattenCollector3(Supplier<C> resultSupplier, Function<T, C> mapper) {
        return Collector.of(
                resultSupplier,
                (l, a) -> l.addAll(mapper.apply(a)),
                (l, r) -> { l.addAll(r); return l; },
                Collector.Characteristics.UNORDERED
        );
    }

    public static void main(String[] args) {

        Function<String, Collection<Integer>> toStringCharsColl = s -> s.chars().boxed().collect(Collectors.toList());
        Collection<Integer> result1 = Stream.of("een", "twee", "drie")
                .collect(flattenCollector2(toStringCharsColl));
        Collection<Integer> result1a = Stream.of("een", "twee", "drie")
                .collect(flattenCollector3(() -> new ArrayList(), toStringCharsColl));

        Function<String, List<Integer>> toStringCharsList = s -> s.chars().boxed().collect(Collectors.toList());
        List<Integer> result2 = Stream.of("een", "twee", "drie")
                .collect(flattenCollector2(toStringCharsList));
        List<Integer> result2a = Stream.of("een", "twee", "drie")
                .collect(flattenCollector3(() -> new ArrayList(), toStringCharsList));

        Function<String, Set<Integer>> toStringCharsSet = s -> s.chars().boxed().collect(Collectors.toSet());
        List<Integer> result3 = Stream.of("een", "twee", "drie")
                .collect(flattenCollector2(toStringCharsSet));
        List<Integer> result3a = Stream.of("een", "twee", "drie")
                .collect(flattenCollector3(() -> new HashSet(), toStringCharsSet));

        System.out.println("ok");
    }