Retrieving java objects list rather than requested objects list from database

Question

I have a problem while I'm trying to pull data from my DB requesting to be in a specific object type. I've created query, which is fetching objects of Java Object type not the type that I need. Here is my DAO class:

import com.jackowiak.Domain.TurbinesData;
import com.jackowiak.Model.TurbineDataCSVReader;
import com.jackowiak.Utils.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.query.Query;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.persistence.EntityManager;
import javax.persistence.EntityTransaction;
import java.util.List;

public class TurbinesDaoBean {
    private static final Logger LOG = LoggerFactory.getLogger(TurbinesDaoBean.class);

    public List<TurbinesData> getTurbineDataFromDB(String turbineName) {

        LOG.info("Initializating DB connection to get turbine data");
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();

        Query query = session.createQuery("select windSpeed, turbinePower from TurbinesData where turbineName = :turbineName");
        query.setParameter("turbineName", turbineName);

        session.getTransaction().commit();
        List<TurbinesData> results = query.list();

        LOG.debug("Data for turbine " + turbineName + " collected successfully");
        return results;


    }
}

And here is my Entity class:

    @Entity
    @Table(name = "TurbinesData")
    public class TurbinesData {

        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        @Column(name = "ID", unique = true, nullable = false)
        protected long id;

        @Column(nullable = false, length = 50, name = "Nazwa_turbiny")
        protected String turbineName;

        @Column(nullable = false, length = 20, name = "V_wiatru")
        protected Double windSpeed;

        @Column(nullable = false, length = 20, name = "Moc_turbiny")
        protected Double turbinePower;

        public TurbinesData() {
        }

        public TurbinesData(Double windSpeed, Double turbinePower) {
            this.windSpeed = windSpeed;
            this.turbinePower = turbinePower;
        }

        public TurbinesData(String turbineName, Double windSpeed, Double turbinePower) {
            this.turbineName = turbineName;
            this.windSpeed = windSpeed;
            this.turbinePower = turbinePower;
        } 
// getters and setters
}

I would like to receive list of TurbinesData objects after executing query

Please check this question: stackoverflow.com/questions/10829197/… — Ivan
– Ivan, Commented Aug 24, 2018 at 18:01
When I'm using TypedQuery it throws Cannot create TypedQuery for query with more than one return using requested result type [com.jackowiak.Domain.TurbinesData] exception — Jabbasnik
– Jabbasnik, Commented Aug 24, 2018 at 18:07

Minar Mahmud · Accepted Answer · 2018-08-24 18:38:45Z

1

Change the jpql to:

"FROM TurbinesData td WHERE td.turbineName = :turbineName"

And then use TypedQuery

EDIT: According to your comment you want to retrieve only two fields. You need to do:

"SELECT NEW package.to.TurbinesData(td.windSpeed, td.turbinePower) FROM TurbinesData td WHERE td.turbineName = :turbineName"

Note:

Need to have proper constructor defined.
Need to use fully qualified name

edited Aug 24, 2018 at 18:38

answered Aug 24, 2018 at 18:21

Minar Mahmud

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3 Comments

Jabbasnik Over a year ago

I can agree with this solution, I've achieved this earlier, but in this case I'm retrieving all data from DB and I think it's not the most efficient way to do it. What I mean is shouldn't I pull only necessary data in this case objects with 2 fields rather than with 4 fields?

Minar Mahmud Over a year ago

@Jabbasnik Understood. I can't tell how much performance difference will be there in this case, but that can be achieved. I have updated my answer.

Jabbasnik Over a year ago

It's working. I didn't know I can call constructors from hql. Thank You!

suraj singh · Accepted Answer · 2018-08-24 18:03:16Z

0

You can typecast List<Object> to List<custom class>

Like.

return (List<T>) query.list();

Hope that helps.

answered Aug 24, 2018 at 18:03

suraj singh

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2 Comments

Jabbasnik Over a year ago

Unfortunately it's not working - still getting Object[] list

suraj singh Over a year ago

I realised issue in your code. For mapped entity in hibernate, we go with hql queries than complete query, which return results in desired 'List<CustomObj>' Example: 'String hql = "from Product where description like :keyword"; Query query = session.createQuery(hql); query.setParameter("keyword", "P1"); List<Product> listProducts = query ' Try this way.

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