Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to com.mongodb.BasicDBObject

Question

I am trying to retrieve one particular field("Versions.id") from mongo using java

Mongo query - db.getCollection('SettlementInstance').find({"_id.timeSlice" : [2018,1,1], "_id.type" : "TRANSMISSION"})

This is the data stored in mongo collection. I want to retrieve "id" : "3d8a3b31-aaa0-4c2e-82b3-5eaaa80d80e1" using java

   {
            "_id" : {
                "timeSlice" : [ 
                    2018, 
                    1, 
                    1
                ],
                "type" : "TRANSMISSION",
                "@objectName" : "SettlementInstance"
            },
            "Versions" : [ 
                {
                    "id" : "3d8a3b31-aaa0-4c2e-82b3-5eaaa80d80e1",
                    "status" : "ACTIVE",
                    "version" : NumberLong(11447)
                }
            ]
        }

The Java program I was trying to retrieve the "id" field under "Versions" object, my code gives an exception.

String feedName = "ServicePointInputAdapter";
            Mongo mongo = new Mongo(host);
            DB db = mongo.getDB(("dbname"));
            DBCollection collection = db.getCollection("SettlementInstance");
            BasicDBObject whereQuery = new BasicDBObject();
            whereQuery.put("_id.type", "TRANSMISSION");
            whereQuery.put("_id.timeSlice", Arrays.asList(2018,1,1));
            DBCursor cursor = collection.find(whereQuery);
            try {
                while (cursor.hasNext()) {
                    DBObject Features = cursor.next();
                    BasicDBList features = (BasicDBList) Features.get("Versions");
                    BasicDBObject[] featuresArr = features.toArray(new BasicDBObject[0]);
                    for (BasicDBObject dbobj : featuresArr) {
        *****                BasicDBObject si_id = (BasicDBObject) dbobj.get("id"); ***** //Getting error in the line }
                }
            } finally {
                cursor.close();
            }
        }
    }

Unfortunately I am getting exception such as -

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to com.mongodb.BasicDBObject.

Could some one help me resolve the issue and retrieve the "Versions.id" field from the collection.

vijayinani · Accepted Answer · 2018-04-18 05:02:11Z

1

As the error clearly says that you are trying to cast a java.lang.String to com.mongodb.BasicDBObject which is throwing java.lang.ClassCastException

Change the line of code as below (as BasicDBObject implements HashMap):

for (BasicDBObject dbobj : featuresArr) {
    String id = (String)((HashMap)dbobj).get("id");
    System.out.println(id);
}

edited Apr 18, 2018 at 5:02

answered Apr 18, 2018 at 4:23

vijayinani

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4 Comments

janopan Over a year ago

I already tried as String and tried to print the si_id - The program printed whole part under Versions such as - { "id" : "3d8a3b31-aaa0-4c2e-82b3-5eaaa80d80e1" , "status" : "ACTIVE" , "version" : 11447} I specifically just want the "id" field to be printed.

vijayinani Over a year ago

Typecast the BasicDBObject to HashMap and get id out of it.

vijayinani Over a year ago

@janopan Thanks for marking the answer as correct. Also, can you please upvote it?

janopan Over a year ago

stackoverflow.com/questions/49699907/… Can you try to answer this on mongoid field (_id). I just want to get/print "_id.employeeId" field only

Juliyanage Silva · Accepted Answer · 2018-04-18 04:27:32Z

0

This get() method only returns the value. So it was a string when you do this

dbobj.get("id")

You should pass the field name as the parameter and you will get the value. So change this line like below

BasicDBObject si_id = (BasicDBObject) dbobj.get("id");

to 
String dbObjectId = dbobj.get("id");

answered Apr 18, 2018 at 4:27

Juliyanage Silva

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2 Comments

janopan Over a year ago

I already tried as String and tried to print the si_id - The program printed whole part under Versions such as - { "id" : "3d8a3b31-aaa0-4c2e-82b3-5eaaa80d80e1" , "status" : "ACTIVE" , "version" : 11447} I specifically just want the "id" field to be printed

Juliyanage Silva Over a year ago

have you tried using only this DBObject Features = cursor.next(); in hasNext() loop?. You can directly get the results there.

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