How to add two fields in mongoDB using java driver - mongodb-java

Question

db.student.aggregate([{$project:{rollno:1,per:{$divide:[{$add:["marks1","$marks2","$marks3"]},3]}}}])
how to write this query in java????? here,student is collection with fields rollno,name and marks and i have to find the percentage of the students according to their roll numbers . I am not able to write code for adding their marks as add operator does not support multiple value's for addition.

Answer 1

This seems to work. There are other builder pattern/conveniences but this exposes all the workings. And leaves room for rich dynamic construction.
DBCollection coll = db.getCollection("student");
List<DBObject> pipe = new ArrayList<DBObject>();
/**
* Clearly, lots of room for dynamic behavior here.
* Different sets of marks, etc. And the divisor is
* the length of these, etc.
*/
String[] marks = new String[]{"$marks1","$marks2","$marks3"};
DBObject add = new BasicDBObject("$add", marks);
List l2 = new ArrayList();
l2.add(add);
l2.add(marks.length); // 3
DBObject divide = new BasicDBObject("$divide", l2);
DBObject prjflds = new BasicDBObject();
prjflds.put("rollno", 1);
prjflds.put("per", divide);
DBObject project = new BasicDBObject();
project.put("$project", prjflds);
pipe.add(project);
AggregationOutput agg = coll.aggregate(pipe);
for (DBObject result : agg.results()) {
System.out.println(result);
}