Hibernate:POJO中的HashSet对象在会话后更改为PersistentSet.saveOrUpdate（）

我有一个简单的hibernate1到M代码。保存Employee对象时，Employee POJO中的子对象在保存之前是java.util.HashSet，在session.saveOrUpdate之后变成org.hibernate.collection.PersistentSet

private static void addEmployee() {
    System.out.println("Entering addEmployee **********");
    Session session = HibernateSessionUtil.getSession("test/hibernate.cfg.xml");
    Employee1toM employee1toM = new Employee1toM();
    employee1toM.setEmployeeFirstName("Jitendra");
    employee1toM.setEmployeeLastName("Patil");

    EmployeeAddress1toM employeeAddress1toM = new EmployeeAddress1toM();
    employeeAddress1toM.setEmployeeArea("ABC");
    employeeAddress1toM.setEmployeeCity("DEF");
    employeeAddress1toM.setEmployeeState("IJK");

    employeeAddress1toM.setEmployee1toM(employee1toM);
    employee1toM.getEmployeeAddress1toM().add(employeeAddress1toM);

    System.out.println(employee1toM.getEmployeeAddress1toM().getClass().getName());
    // prints java.util.HashSet

    Transaction txn = session.beginTransaction();
    session.saveOrUpdate(employee1toM);
    txn.commit();

    System.out.println(employee1toM.getEmployeeAddress1toM().getClass().getName());
    // prints org.hibernate.collection.PersistentSet

    System.out.println("Leaving addEmployee **********");

}

OutPut:
Entering addEmployee **********
java.util.HashSet
Hibernate: insert into Employee1toM (EMPLOYEE_FIRSTNAME, EMPLOYEE_LASTNAME) values (?, ?)
Hibernate: insert into EmployeeAddress1toM (EMPLOYEE_AREA, EMPLOYEE_CITY, EMPLOYEE_STATE, EMPLOYEE_ID) values (?, ?, ?, ?)
org.hibernate.collection.PersistentSet
Leaving addEmployee **********

配置和POJO都很简单，所以我不分享它。请帮助我理解如何停止HashSet到PersistentSet类型的自动转换