class SpliteratorExample {

    private static class GroupSpliterator<T> implements Spliterator<Stream<T>> {

        private final int grouping;
        private final Spliterator<T> spliterator;

        GroupSpliterator(int grouping, Spliterator<T> spliterator) {
            this.grouping = grouping;
            this.spliterator = spliterator;
        }

        @Override
        public boolean tryAdvance(Consumer<? super Stream<T>> action) {
            Stream.Builder<T> builder = Stream.builder();
            boolean finished = false;
            for (int i = 0; i < grouping; i++) {
                if (spliterator.tryAdvance(builder::add)) {
                    finished = true;
                }
            }
            final Stream<T> subStream = builder.build();
            action.accept(subStream);
            return !finished;
        }

        @Override
        public Spliterator<Stream<T>> trySplit() {
            final Spliterator<T> aNewSpliterator = this.spliterator.trySplit();
            return new GroupSpliterator<T>(grouping, aNewSpliterator);
        }

        @Override
        public long estimateSize() {
            return spliterator.estimateSize() / grouping;
        }

        @Override
        public int characteristics() {
            return this.spliterator.characteristics();
        }
    }

    public static void main(String... args) {
        final List<String> data = Arrays.asList("1", "2", "3", "4", "5", "6");
        final GroupSpliterator<String> spliterator = new GroupSpliterator<>(3, data.spliterator());
        final Stream<Stream<String>> stream = StreamSupport.stream(spliterator, false);
        // transform the nested stream into collection
        final List<List<String>> groupedData = stream.collect(Collectors.toList())
                .stream()
                .map(subStream -> subStream.collect(Collectors.toList()))
                .collect(Collectors.toList());
        // print result
        groupedData.forEach(list -> {
            final String joined = list.stream().collect(Collectors.joining(" "));
            System.out.println("[" + joined + "]");
        });
    }

}