使用Collectors.groupingby创建一个集合的映射

Question

I know how to create a Map<T, List<U>> , using Collectors.groupingBy :

Map<Key, List<Item>> listMap = items.stream().collect(Collectors.groupingBy(s->s.key));

How would I modify that code to create Map<Key, Set<Item>> ? Or can I not do it using stream and so have to create it manually using a for loop etc.?

Answer 1

Use Collectors.toSet() as a downstream in groupingBy :

Map<Key, Set<Item>> map = items.stream()
            .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));

Answer 2

You have to use a downstream collector like this:

Map<Key, Set<Item>> listMap = items.stream()
    .collect(Collectors.groupingBy(s -> s.key, Collectors.toSet()));

Answer 3

I also like the non-stream solution sometimes:

 Map<Key, Set<Item>> yourMap = new HashMap<>();
 items.forEach(x -> yourMap.computeIfAbsent(x.getKey(), ignoreMe -> new HashSet<>()).add(x));

If you really wanted you could exercise to do the same via compute/merge methods too