Java - 对非静态方法进行静态引用

Question

对于一个项目，我试图创建一个类，在其中我可以使用 hibernate 从数据库中提取数据。

在本类(class)中，我还尝试使用通过 DAO 从数据库中提取的数据创建对象列表。但是当我尝试循环遍历列表时，Eclipse 提醒我我正在尝试对非静态方法进行静态引用。

我的印象是，如果您使用非静态的对象实例(在本例中为运算符 op)，并且它不引用静态方法，那么它就可以工作。为什么这不起作用以及如何修复它？

特别让我困惑的是，老师给出的示例代码做了同样的事情，但确实有效。

package Client.DAO;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;

public class mainTest {

private static EntityManagerFactory entityManagerFactory;

public static void main(String[] args) {

    EntityManager em = null;
    try {
        entityManagerFactory = HibernateUtil.getEntityManagerFactory();
        em = entityManagerFactory.createEntityManager();
    } catch (Throwable ex) {
        System.err.println("Failed to create sessionFactory object." + ex);
        throw new ExceptionInInitializerError(ex);
    }

    operatorDAO dao = new operatorDAOImpl(em);
    List<operator> operators = dao.findAll();
    for (operator op : operators) {
        System.out.print("Id: " + operator.getID());// <-- This doesnt work
        System.out.print(", First Name: " + operator.getName()); // <-- This doesnt work

    }
}
}

供引用，工作示例代码。警告:长

package nl.hu.ict;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;

import nl.hu.ict.dao.HibernateUtil;
import nl.hu.ict.dao.LecturerDao;
import nl.hu.ict.dao.LecturerDaoHibImpl;
import nl.hu.ict.model.Lecturer;

public class MyHibernateTest {

private static EntityManagerFactory entityManagerFactory;

public static void main(String[] args) {
    // get the Hibernate - JPA entityManager
    EntityManager em = null;
    try {
        entityManagerFactory = HibernateUtil.getEntityManagerFactory();
        em = entityManagerFactory.createEntityManager();
    } catch (Throwable ex) {
        System.err.println("Failed to create sessionFactory object." + ex);
        throw new ExceptionInInitializerError(ex);
    }

    // get the program (not necessary)
    // MyHibernateTest test = new MyHibernateTest();

    LecturerDao lectDao = new LecturerDaoHibImpl(em);
    Integer id;

    // handle the database actions

    // list all lecturers
    List<Lecturer> lecturers = lectDao.findAll();
    for (Lecturer lecturer : lecturers) {
        System.out.print("Id: " + lecturer.getID());
        System.out.print(", First Name: " + lecturer.getFirstName());
        System.out.println(", Last Name: " + lecturer.getLastName());
        System.out.println(", Last Name: " + lecturer.getLastName());
    }

    // add an lecturer
    Lecturer lect = new Lecturer();
    lect.setFirstName("Bill");
    lect.setLastName("Pianoman");
    lectDao.persist(lect);
    // get the id of the added lecturer
    id = lect.getID();
    // list to view changes
    lecturers = lectDao.findAll();
    for (Lecturer lecturer : lecturers) {
        System.out.print(lecturer);
    }

    // change a lecturer
    // find find it again
    lect = lectDao.findById(id);
    // change it
    lect.setFirstName("Billy");
    lectDao.update(lect);
    // list to view changes
    lecturers = lectDao.findAll();
    for (Lecturer lecturer : lecturers) {
        System.out.print(lecturer);
    }

    // delete a lecturer
    // find it again
    lect = lectDao.findById(id);
    // delete it
    lectDao.delete(lect);
    // list to view changes
    lecturers = lectDao.findAll();
    for (Lecturer lecturer : lecturers) {
        System.out.print(lecturer);
    }
    em.close();
    System.out.println("-- einde --");
}
}

Answer 1

这是命名困惑；在这部分代码中:

operatorDAO dao = new operatorDAOImpl(em);
List<operator> operators = dao.findAll();
for (operator op : operators) {
    System.out.print("Id: " + operator.getID());// <-- This doesnt work
    System.out.print(", First Name: " + operator.getName()); // <-- This doesnt work
}

您创建一个名为operators 的局部变量来引用从调用DAO 的findAll 方法返回的列表。在 for 循环体内有一个局部变量 op。局部变量op的类型是operator。但是您使用“运算符”就好像它是变量的名称一样。将 operator.getID() 更改为 op.getID()。编译器认为您正在运算符类上调用实例方法 getID，这就是您收到“对非静态方法的静态引用”错误的原因。

您可以通过遵循命名约定来最大限度地减少此类错误。类型(类和接口(interface))应以大写字母开头。实例变量和局部变量应以小写字母开头。按照约定，此代码将变为:

OperatorDAO dao = new OperatorDAOImpl(em);
List<Operator> operators = dao.findAll();
for (Operator operator : operators) {
    System.out.print("Id: " + operator.getID());
    System.out.print(", First Name: " + operator.getName());
}