computeIfAbsent

java 7

 Map<Integer, Set<JsonNode>> laneMap = new HashMap<>(laneJsons.size());
   laneJsons.fields().forEachRemaining(e -> {
   JsonNode roadNode = e.getValue();
   int roadId = roadNode.get(AD_Lane.ROAD_ID.name()).asInt();
   if(laneMap.get(roadId) == null){
    Set<JsonNode> set = new HashSet<>();
    set.add(roadNode);
   }else{
     Set<JsonNode> set = laneMap.get(roadId);
     set.add(roadNode);
   }
  
   laneMap.put(roadId, set);    
});

java 8

 Map<Integer, Set<JsonNode>> laneMap = new HashMap<>();
   laneJsons.fields().forEachRemaining(e -> {
   JsonNode roadNode = e.getValue();
   int roadId = roadNode.get(AD_Lane.ROAD_ID.name()).asInt();
   laneMap.computeIfAbsent(roadId, f -> new HashSet<>()).add(roadNode);
  });