Is it possible to .flatMap() or .collect() evenly multiple collections

Question

For instance there are collections [1,2,3,4,5], [6,7,8], [9,0]. Any way to avoid loops with iterators to interleave these collections

Answer 1

I am not sure if there's a simpler way with the Stream API, but you can do this using a stream over the indices of all the lists to consider:

static <T> List<T> interleave(List<List<T>> lists) {
    int maxSize = lists.stream().mapToInt(List::size).max().orElse(0);
    return IntStream.range(0, maxSize)
                    .boxed()
                    .flatMap(i -> lists.stream().filter(l -> i < l.size()).map(l -> l.get(i)))
                    .collect(Collectors.toList());
}

This gets the size of the greatest list in the given lists. For each index, it then flat maps it with a stream formed by the elements of each list at that index (if the element exist).

Then you can use it with

public static void main(String[] args) {
    List<Integer> list1 = Arrays.asList(1,2,3,4,5);
    List<Integer> list2 = Arrays.asList(6,7,8);
    List<Integer> list3 = Arrays.asList(9,0);
    System.out.println(interleave(Arrays.asList(list1, list2, list3))); // [1, 6, 9, 2, 7, 0, 3, 8, 4, 5]
}

---

Using the protonpack library, you can use the method interleave to do just that:

List<Stream<Integer>> lists = Arrays.asList(list1.stream(), list2.stream(), list3.stream());
List<Integer> result = StreamUtils.interleave(Selectors.roundRobin(), lists).collect(Collectors.toList());
System.out.println(result);

Answer 2

I like Tunaki’s answer and have been working a bit more on it:

static <T> List<T> interleave(List<? extends Collection<T>> lists) {
    int maxSize = lists.stream().mapToInt(Collection::size).max().orElse(0);
    List<Iterator<T>> iterators = lists.stream().map(l -> l.stream().iterator()).collect(Collectors.toList());
    return IntStream.range(0, maxSize)
            .boxed()
            .flatMap(i -> iterators.stream().filter(it -> it.hasNext()).map(it -> it.next()))
            .collect(Collectors.toList());
}

I wanted to see if we can treat the initial lists as streams too, it turned out we can. As a side effect, we avoid get(i) on a list (if that was really a very long LinkedList, we would want to avoid it). I didn’t find a way to do without knowledge of a max length of the original lists.

Answer 3

Not a very good solution though. Just a try

    List<String> listA = Arrays.asList("a1","a2","a3","a4");
    List<String> listB = Arrays.asList("b1","b2","b3","b4");
    List<String> listC = Arrays.asList("c1","c2","c3","c4");
    List<String> combined = Stream.of(listA, listB, listC).flatMap(Collection::stream).collect(Collectors.toList());
    AtomicInteger index = new AtomicInteger();
    List<String> list1 = combined
            .stream().filter(x -> index.incrementAndGet() % 4 ==1).collect(Collectors.toList());
    index.set(0);
    List<String> list2 = combined
    .stream().filter(x -> index.incrementAndGet() % 4 ==2).collect(Collectors.toList());
    index.set(0);
    List<String> list3 = combined
    .stream().filter(x -> index.incrementAndGet() % 4 ==3).collect(Collectors.toList());
    index.set(0);
    List<String> list0 = combined
            .stream().filter(x -> index.incrementAndGet() % 4 ==0).collect(Collectors.toList());
    List<String> desiredOutput = Stream.of(list1, list2, list3,list0).flatMap(Collection::stream).collect(Collectors.toList());
    System.out.println(String.join(",", desiredOutput));