public void add() {
        Session session = sessionFactory.openSession();
        session.beginTransaction();
        Set students  =  new HashSet();
         for(int i =0;i<5;i++){
             Students s = new Students();
             s.setAddress(null);
             s.setGender(null);
             s.setName("num"+i);
             s.setClassnum(1);
             s.setClassnum(1);
             students.add(s);
         }
        Classes  c = new Classes();
        c.setClass_name("cs_1");
        c.setStudents(students);
        session.save(c);
        session.getTransaction().commit();
        session.close();
    }