How to collect Stream<Map<K,V>> into Map<K,List<V>> using java 8?

Question

I have a stream of Map<String,Double> that I want to collect into a single Map<String,List<Double>>. Does anybody have a suggestion on how to do this?

Thanks!

Answer 1

First you need to flatten your stream of maps into a stream of map entries. Then, use Collectors.groupingBy along with Collectors.mapping:

Map<String,List<Double>> result = streamOfMaps
    .flatMap(map -> map.entrySet().stream())
    .collect(Collectors.groupingBy(
        Map.Entry::getKey, 
        Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

Answer 2

Say i had:

Stream<Map<String, Double>> mapStream

Then the answer is:

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