Problem after adding a conventer between DAO and Service class to implement CRUD in MongoDB

I follow this old post to convert POJO to a JSON object using JSP to show a list of users to the browser. However, there is some conflict between the DBCollection and the MongoCollection that working to store user input to the cloud database. If it can be solved, I can use it to do other CRUD while learning how to design my web project.

Having said that, is it someway I can use DAO and Service to do CRUD operation?

Type Exception Report

Message com.mongodb.client.internal.MongoCollectionImpl cannot be cast to com.mongodb.DBCollection

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception
java.lang.ClassCastException: com.mongodb.client.internal.MongoCollectionImpl cannot be cast to com.mongodb.DBCollection
    com.smartcard.service.UserService.<init>(UserService.java:26)
    com.smartcard.controller.admin.ListUsersServlet.doGet(ListUsersServlet.java:27)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:626)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

The Converter

public class UserConverter {

    public static DBObject toDBOjbect(User u) {

        BasicDBObjectBuilder builder = BasicDBObjectBuilder.start().append("email", u.getEmail())
                .append("Full Name", u.getFulleName()).append("password", u.getPassword());

        if (u.getId() != null)
            builder = builder.append("_id", u.getId());

        return builder.get();
    }

    public static User toUser(DBObject userDoc) {
        User u = new User();
        u.setEmail((String) userDoc.get("email"));
        u.setFulleName((String) userDoc.get("fullname"));
        u.setPassword((String) userDoc.get("password"));
        ObjectId userId = (ObjectId) userDoc.get("_id");
        u.setId(u.toString());

        return u;

    }
}

The Service class to list all user

public class UserService {

    // private UserDAO userDAO;
    MongoDatabase mdb = dBUtils.getMongoDB();

    private DBCollection col;

    public UserService() {

        this.col = (DBCollection) mdb.getCollection("User");
    }

    public List<User> listUser() {

        List<User> userList = new ArrayList<User>();
        DBCursor cursor = col.find();
        while (cursor.hasNext()) {
            DBObject doc = cursor.next();
            User u = UserConverter.toUser(doc);
            userList.add(u);
        }
        return userList;
    }
}