出错，为什么我只有Get方法可以运行？其他三个方法都报错

@Test

public void testGetStudents(){

Students s = (Students) session.get(Students.class, 1);

System.out.println(s.getClass().getName());

}

public void testLoadStudents(){

Students s = (Students) session.load(Students.class, 1);

System.out.println(s);

}

public void testUpdateStudents(){

Students s = (Students) session.get(Students.class, 1);

s.setGender("女");

session.update(s);

}

public void testDeleteStudents(){

Students s = (Students) session.get(Students.class, 1);

session.delete(s);

}