Retrieve array from MongoDB in Java

Question

I try to read out all fields of an array of a document in MongoDB. I tried to adapt a posted solution from Stackoverflow to my use case, but I get the error message:

Exception in thread "main" java.lang.ClassCastException: java.util.ArrayList cannot be cast to com.mongodb.BasicDBList
    at Connector.getTopics(Connector.java:249)

Basically I want to read out the array in "Topics" of the first document in the collection "Topics" and save the values to an arrayList and return it.

public ArrayList<String> getTopics() throws RemoteException {
        // TODO Auto-generated method stub

        MongoClient mongoClient = new MongoClient( "localhost" , 27017 );
        MongoDatabase database = mongoClient.getDatabase("JMS");
        MongoCollection<Document> collection = database.getCollection("Topics"); 

        ArrayList<String> list = new ArrayList<String>();

        Document document = collection.find(eq("_id", 1.0)).first();

                ListIterator<Object> topics = ((BasicDBList) document.get("Topics")).listIterator();


                while(trustedList.hasNext()){

                    Object nextItem = topics.next();



                    list.add((String) nextItem);


                }


        return list;

}

I do not completely understand the posted solution to get all values of the array, so I am a bid confused how to correct the cast error.

I'm not sure about MongoDB, but if the method document.get("Topics")).listIterator() returns an ArrayList why don't you just use it as such, instead of casting it to BasicDBList (which might be deprecated API, just a guess) — xander
– xander, Commented Aug 1, 2017 at 13:02

Harish Barma · Accepted Answer · 2017-08-01 14:07:33Z

0

I think that you need to store topics in a BasicDBList object:

 BasicDBList topics = (BasicDBList) document.get("Topics");

Then you can iterate it and store into your array:

List<String> result = new ArrayList<String>();
for(Object element: topics) {
     result.add((String) element);
}

edited Aug 1, 2017 at 14:07

Harish Barma

6348 silver badges15 bronze badges

answered Aug 1, 2017 at 13:02

Antonio314

616 bronze badges

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3 Comments

Julian Herbold Over a year ago

Im a bid confused, where is topicsMongo actually used?

Antonio314 Over a year ago

I mean that BasicDBList is like a Java List so you don't need get the iterator, you can iterate on it in a "foreach", like an Array: I wrote 'list' when I should wrote 'topicsMongo'. Now the answer is edited, thanks to @the4kman

Julian Herbold Over a year ago

mhh it says: "java.util.ArrayList cannot be cast to com.mongodb.BasicDBList"

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