Collect a Stream of Map to Map>

Question

I have a Stream< Map< K, V > > and I\'m trying to merge those maps together, but preserve duplicate values in a list, so the final type would be

Answer 1

Use groupingBy: see the javadoc, but in your case it should be something like that:

a.flatMap(map -> map.entrySet().stream())
 .collect(
   Collectors.groupingBy(
     Map.Entry::getKey, 
     HashMap::new, 
     Collectors.mapping(Map.Entry::getValue, toList())
   )
);

Or:

a.map(Map::entrySet).flatMap(Set::stream)
 .collect(Collectors.groupingBy(
     Map.Entry::getKey, 
     Collectors.mapping(Map.Entry::getValue, toList())
   )
);

Answer 2

This is a bit wordier than the groupingBy solution, but I just wanted to point out that it is also possible to use toMap (as you initially set out to do) by providing the merge function:

    a.flatMap(map -> map.entrySet().stream()).collect(
        Collectors.toMap(Map.Entry::getKey,
                entry -> { 
                    List<Integer> list = new ArrayList<>();
                    list.add(entry.getValue());
                    return list;
                },
                (list1, list2) -> {
                    list1.addAll(list2);
                    return list1;
                }));