java - 在JPA中使用GROUP BY子句时发生ClassCastException

我正在与JPA合作。在执行GROUP BY子句示例时，将引发ClassCastException。

以下是我的代码：

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();
    }

}

}

以下是期望：

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

我的POJO类字段是：

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

是我的GROUP BY子句查询错误。

最佳答案

这是因为我确定您的Student类看起来不像

  String studentName;
  Integer yyyy;

当您要求时得到的是什么

  student.studentName, SUM(student.studentAge)

我建议您要么创建一个看起来像您的结果的类，要么将结果视为Object []

如

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