Question

Why I have to use Object[] to print the list if I select specific columns:

Session session = DaoSF.getSessionFactory().openSession();
List<Object[]> list = new ArrayList<Object[]>(); /* Object[] */

Query criteria = session.createQuery(
  "select test.col1, test.col2, test.col3
  "from Test test " +
  "group by test.col1, test.col2, test.col3");

list = criteria.list();

for (Object[] item : list)
{
  System.out.println(item[1] + ", " + item[2] + ", " + item[3]); 
}

And why is it possible to iterate the same select (select * — not the specific columns) using original Test object?

List<Test> list = new ArrayList<Test>(); /* Test */
Query criteria = session.createQuery(
  "from Test test " +
  "group by test.col1, test.col2, test.col3");

list = criteria.list();

for (Test item : list)
{
  System.out.println(item.getCol1 + ", " + item.getCol2 + ", " + item.getCol3); 
}

Is it possible to “convert” Object[] to Test object?

Answer 1

Try this approach; first create a constructor in your Test class:

public Test(Col1Type col1, Col2Type2 col2, Col3Type col3) {
  this.col1 = col1;
  this.col2 = col2;
  this.col3 = col3;
}

Now in the query, you can say:

select new Test(t.col1, t.col2, t.col3) 
from Test t

This will give Hibernate a so-called row-mapper constructor out of which it can construct the objects of Test. Then you will have a List<Test> from query.list(). This approach has a catch that you should provide different constructors beside the default one for different possible queries.