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())
   )
);