Nested Streams java 8 [duplicate]

Question

Trying to use Java 8 Streams for this

 HashMap<String,Games> groupGames = new HashMap<String,Games> ();
 for(Groups group: groups){
    for(int groupId : group.getGroupId){
          groupGames.put(groupId, group.getGame());
    }
 }

This works fine but I want to use java 8 stream to achieve this same functionality.

This is what I have for stream

public void toStream(List<Group> group){
    group.stream().map(groups -> groups.getGroupIds())
            .flatMap(group -> groups.stream()).collect(Collectors.toList());
}

I'm having hard time putting each of the groupId with the game in the hashmap...
I'm able to flatten out the list of groupIds

Answer 1

 groups.stream()
       .flatMap(x -> x.getGroupId().stream()
                    .map(y -> new AbstractMap.SimpleEntry<>(y, x.getGame())))
       .collect(Collectors.toMap(Entry::getKey, Entry::getValue, (x, y) -> y));

If you know there will be no duplicates, you can drop the last argument in (x,y) -> y; or you can provide a merge function that you think would be appropriate.