I want to update object inside array in mongodb with java programm

"cmd" : [
{
"a" : 1
},
{
"b" : 2
},
{
"c" : 3
}

]
}
i need to update object inside a array{"d":4}..

<pre>package hellomongo;


import javax.lang.model.element.Element;

import org.bson.Document;

import com.mongodb.BasicDBObject;
import com.mongodb.DBCollection;
import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
 

public class Mongoh{
public static void main(String args[]) {
		
			MongoClient mongoClient = new MongoClient("localhost", 27017);
			 
			MongoDatabase db = mongoClient.getDatabase("admin");
			MongoCollection<Document> coll = db.getCollection("command");

			BasicDBObject lurl = new BasicDBObject();
		    BasicDBObject lurl2 = new BasicDBObject();
		    BasicDBObject lkey = new BasicDBObject();
		    for(Element e : links){
		        DBCollection colls = db.getCollection(e.text());
		        lurl.put("d", e.attr("4"));
		        lurl2.put("$addToSet", new BasicDBObject(lurl));
		        lkey.put("a", e.text());
		        colls.update(lkey, lurl2);
	}
}
	
}