Extract specific fields from a json data

Question

I want to extract the fields "2008" and "2009" in the field "total" from this json data. I tried but it shows null pointer exception..i don't know what went wrong. Below is the code which I had tried.

    /*insertion of data*/

        DB preDB = mongo.getDB("database");
            DBCollection coll = preDB.getCollection("agrinindstry");
            BasicDBObject doc1 = new BasicDBObject();
            doc1.put("test", str);
            coll.insert(doc1);
            System.out.println(doc1);


      /***retrieving data***/
            DBCursor cursor = coll.find();

            JSONArray mylist = new JSONArray();
            String result = "";
            while (cursor.hasNext()) 
            {
                result = cursor.next().get("2008").toString();
                mylist.add(result);
                System.out.println(mylist);

            }

this is my json data

{"maindata":[
{

"title":"industry",
"2008":37,
"2009":44,
"2010":42


 },
 {

"title":"agriculture",
"2008":4,
"2009":0,
"2010":6

 }
 ],
"total":{
"title:"sum",
"2008":41,
"2009":44,
 "2010":48

 }
}

Answer 1

((DBObject)cursor.next().get("total")).get("2008").toString();

Answer 2

Your task you can simplify by using any JAVA tootkit for JSON, take look into this example :

Check it out...

For more understanding on JSON and how to parse it, check this:

If you read the javadoc of JSONObject#get(String) which is actually HashMap.get(String), it states

Returns: the value to which the specified key is mapped, or null if this map contains no mapping for the key

Your JSON does not contain a mapping for the key time.

Edit:

Consider this to parse your JSON data:

     {"schedule":{"service":{"type":"radio","key":"radio1","title":"BBC Radio 1",...

You need to first get schedule as a JSONObject, then service as a JSONObject, and then title as a normal String value. Apply this differently depending on the type of JSON value.

See this link

Answer 3

Try this:

     //get the data from mongoDB which documents has "total" key.

     DBObject dbObject = new BasicDBObject().append("total", new BasicDBObject("$exists", true));
     DBCursor cursor = collection.find(dbObject);

        JSONArray mylist = new JSONArray();
        String result = "";
        while (cursor.hasNext()) 
        {
            result = ((DBObject)cursor.next().get("total")).get("2008").toString();
            mylist.add(result);
            System.out.println(mylist);

        }