ClassCastException while using list in JPA

Question

I am trying to execute a JPA program in eclipse. I have imported all necessary packages.

This is my POJO(@entity) class

@Entity
public class Student
{
    int id,marks;
    String name;
    Student(int i, int j, String k)
    {
        id=i;marks=j;name=k;
    }
    public int getMarks()
    {
        return marks;
    }
    public void setMarks(int i)
    {
        marks = i;
    }
}

This is a portion of my 'main' class

    EntityManagerFactory emf = Persistence.createEntityManagerFactory("database/database.odb");
        EntityManager em = emf.createEntityManager();
        Student s1;
        em.getTransaction().begin();
        s1 = new Student(1,100,"abcd");
        em.persist(s1);
        s1 = new Student(2,200,"efgh");
        em.persist(s1);
        s1 = new Student(3,300,"ijkl");
        em.persist(s1);
        em.getTransaction().commit();
        Query q1 = em.createQuery("select marks from Student");
        List<Student> result =  q1.getResultList();
        em.getTransaction().begin();
        for(Student s : result)
        {
            if(s.getMarks()%200==0)
                em.remove(s);
            else
                s.setMarks(s.getMarks()*2);
        }
        em.getTransaction().commit();
        q1 = em.createQuery("select marks from Student");
        System.out.println("Details of students are : " + q1.getResultList());

The output i expect is : [200, 600] because the marks in the 1st and 3rd record should get multiplied by 2 and the the 2nd record should get deleted.

But I get an error :

Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to assg6.Student at assg6.Main.main(Main.java:52)

(My project's package name is assg6)

I tried this

List<Student> result =  (List<Student>)q1.getResultList();

and

for(Student s : (List<Student>)result)

Why am I not getting the expected output and how do i get it?

How is this: [java.lang.Integer cannot be cast to assg6.Student] not a very clear description of what the problem is? — Stultuske
– Stultuske, Commented Apr 19, 2018 at 6:46
what is the meaning of java.lang.Integer here? result is not an integer — Amey Waze
– Amey Waze, Commented Apr 19, 2018 at 6:47
the result is actually the value of mark, not the entire information of the student, and the mark is an integer — Stultuske
– Stultuske, Commented Apr 19, 2018 at 6:52

Timir · Accepted Answer · 2018-04-19 08:15:32Z

1

Apparently, q1.getResultList() is returning a list of Integers. Not surprising, since you're saying Query q1 = em.createQuery("select marks from Student");

You probably wanted to write "select s from Student s" instead.

edited Apr 19, 2018 at 8:15

answered Apr 19, 2018 at 6:50

Timir

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6 Comments

Timir Over a year ago

@AmeyWaze Please include line numbers with your code. If not sure how, which is line #52?

Amey Waze Over a year ago

really sorry @Timir the for loop is line 52

Amey Waze Over a year ago

what is s in select s from Student ? @Timit

Timir Over a year ago

Sorry, the inline query should've read "select s from Student s"

Amey Waze Over a year ago

yeah but still, what is s in select s from Student s , the first s i mean

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