在JPA中使用GROUP BY子句时发生ClassCastException

Question

I am working with JPA. while doing GROUP BY clause example it's throwing ClassCastException.

Below Is My code:

public class StudentGrouping 
{
public static void main(String[] args)
{
    EntityManager entityManager = EntityManagerUtil.getEmf()
            .createEntityManager();
    try {
            EntityTransaction entr = entityManager.getTransaction();
            entr.begin();
            Query query = entityManager
                    .createQuery("SELECT student.studentName, SUM(student.studentAge) FROM Student student GROUP BY student.studentName");
            List<?> list = query.getResultList();
            Iterator<?> iterator = list.iterator();
            while (iterator.hasNext())
            {
                System.out.println("entered into loop");
                Student student = (Student) iterator.next();
                System.out.print("Student Name:"+student.getStudentName());
                System.out.print(" Age:"+ student.getStudentAge());
                System.out.println();

        }
        entr.commit();
        System.out.println("success");
    } 
    catch (Exception e)
    {
        e.printStackTrace();
    } finally {
        entityManager.close();
    }

}

}

Below is The Expection:

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to com.demo.entities.Student
at com.demo.action.StudentGrouping.main(StudentGrouping.java:28)

My POJO class Fields are:

@Column(name = "studentName")
private String studentName;
@Column(name = "studentAge")
private int studentAge;

Is my GROUP BY clause query is wrong.

Answer 1

Student student = (Student) iterator.next(); is the problem as you aren't actually pulling back an entire student.

SELECT student.studentName, SUM(student.studentAge) FROM Student student GROUP BY student.studentName

Your query is pulling back these two fields. If you want to map to a student object, you'd have to use the following.

FROM Student student

Then do the calculation by hand on the data. If you wanted to use your original query, you'd have to parse each individual value with, rather than using iterator lets say they were in a resultList.

for (Object[] result : resultList) {
 String studentName = (String) result[0]
 Integer age = (Integer) result[1];
}

Answer 2

This is because I am sure that you Student class does not look like

  String studentName;
  Integer yyyy;

which is what you are getting when you ask for

  student.studentName, SUM(student.studentAge)

I suggest that you either create a class that looks like your result, or just treat the result as an Object[]

As in

Object student[] = (Object[])iterator.next();
System.out.print("Student Name:"+student[0]);
System.out.print("Max Age:"+student[1]);

Answer 3

Unless you have defined the Student class to be a String and a number, you're not getting back a Student class, so you can't cast it to that in this line:

Student student = (Student) iterator.next();

That, and you probably should also define what kind of classes your List and Iterators are handling.

Answer 4

The problem is you are actually not selecting the student instead you are trying to select custom values from student.

try this: First create a constructor inside the entity with two param ie. the name and age.

then use this query:

SELECT NEW com.sample.Student(student.studentName, SUM(student.studentAge)) FROM Student student GROUP BY student.studentName;

You need to specify the full class name (sample:com.sample.Student) in the query