Java 8 list to map with stream

I have a List<Item> collection. I need to convert it into Map<Integer, Item> The key of the map must be the index of the item in the collection. I can not figure it out how to do this with streams. Something like:

items.stream().collect(Collectors.toMap(...));

Any help?

As this question is identified as possible duplicate I need to add that my concrete problem was - how to get the position of the item in the list and put it as a key value

Solution 1:

You can create a Stream of the indices using an IntStream and then convert them to a Map :

Map<Integer,Item> map = 
    IntStream.range(0,items.size())
             .boxed()
             .collect(Collectors.toMap (i -> i, i -> items.get(i)));

Solution 2:

One more solution just for completeness is to use custom collector:

public static <T> Collector<T, ?, Map<Integer, T>> toMap() {
    return Collector.of(HashMap::new, (map, t) -> map.put(map.size(), t), 
            (m1, m2) -> {
                int s = m1.size();
                m2.forEach((k, v) -> m1.put(k+s, v));
                return m1;
            });
}

Usage:

Map<Integer, Item> map = items.stream().collect(toMap());

This solution is parallel-friendly and does not depend on the source (you can use list without random access or Files.lines() or whatever).