Accumulators dosn't work as they should, they just replace the value given instead to process it.

Example:

Document document = new Document("firstname", "Pedro")
.append("lastname", "Doe")
.append("age", 14)
.append("country", "Tanzania")
.append("index", 1);

collection.insertOne(document);

List<Bson> query= asList(Aggregates.match(Filters.eq("firstname", "Pedro")),Aggregates.group("$_id", Accumulators.avg("age", 100), Accumulators.sum("index", 100)));
AggregateIterable<Document> resuls t= collection.aggregate(query);
for(Document d : results)

{ System.out.println(d.toJson()); }

-Result:
{ "_id" :

{ "$oid" : "59dbb669dfbc63214831e70c" }

, "age" : 100.0, "index" : 100 }

-expected:
{ "_id" :

{ "$oid" : "59dbb669dfbc63214831e70c" }

, "age" : 57.0, "index" : 101}