Group a map into another mapped based on a property within the map

I have the following code:

class Scratch {

  public static void main(String[] args) {
    Map<String, Id> idDefinitions = Map.of(
      "A", new Id(Group.GROUP_A),
      "B", new Id(Group.GROUP_B),
      "C", new Id(Group.GROUP_A));

    Map<String, Object> ids = Map.of("A", new Object(), "B", new Object(), "C", new Object());
    
    Map<Group, Map<String, Object>> idsByGroup = ?
  }

  private static class Id {
    private Group group;
    public Id(Group group) {
      this.group = group;
    }
  }

  private enum Group {
    GROUP_A,
    GROUP_B
  }
}

Based on the idDefinitions and the ids, I want to create a new Map where all the ids are grouped by their Group. I can solve that iteratively with a for-loop but I’m wondering if there are stream alternatives, using the groupingBy collector.

>Solution :

If you just use Collectors.groupingBy to group by Group. This will give you a Map<Group, List<Map.Entry<String, Id>>>

But we want the values of the map to be another Map, so we should pass a downstream collector to groupingBy, the toMap collector. The downstream is a stream of Map.Entry<String, Id>, so the maps’ keys would be entry.getKey, and the values would be the Objects from ids.

Map<Group, Map<String, Object>> idsByGroup = idDefinitions
    .entrySet().stream()
    .filter(x -> ids.containsKey(x.getKey()))
    .collect(
        Collectors.groupingBy(
            x -> x.getValue().group,
            Collectors.toMap(
                Map.Entry::getKey,
                x -> ids.get(x.getKey())
            )
        )
    );