Collect a Stream of Map<K,V> to Map<K,List<V>>

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 Map< K, List<V> >. Is there a way to do this? I know the toMap collector has a binary function to basically choose which value is returned, but can it keep track of the converted list?

i.e.

if a is a Stream< Map< String, Int > >

a.flatMap(map -> map.entrySet().stream()).collect(
    Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (val1, val2) -> ??
);

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())
   )
);