为什么对对象的第二次引用不会改变 java 中的内容？答案

【问题标题】：why a second reference to an object does not alter the content in java?为什么对对象的第二次引用不会改变 java 中的内容？
【发布时间】：2022-01-16 05:53:27
【问题描述】：

在下面的代码中，我创建了 Movie 类的 3 个实例。我已经为这三个对象分配了一些数据。其次，我创建了对对象movie1(movie4) 的第二个引用。我还为movie4的属性赋值。我打印了电影 1 的新属性但内容没有改变。为什么会这样？确切的原因是什么？我知道更新内容的其他方法，但我想知道为什么第二次引用不起作用。
内容是movie1还是一样的。为什么？

class Driver{

    public static void main(String[] args){
        
        Movie movie1 = new Movie("The Shawshank Redemption", 1994, 9.3);
        Movie movie2 = new Movie("The Godfather", 1972, 9.2);
        Movie movie3 = new Movie("The Dark Knight", 2008, 9.0);

        **Movie movie4 = movie1;
        movie4 = new Movie("The Return of the King", 2003, 8.9);**

        movie4.name = "The Return of the King";
        movie4.year_of_release = 2003;
        movie4.rating = 8.9;
        System.out.println("\n*********New Movie 1 Details*************\n");
        movie1.printDetails();

    }
}

【问题讨论】：

因为movie4 不再指向movie1。您没有修改 movie4 指向的对象，只是 what movie4 指向的对象。
你能再解释一下吗？
将movie1 和movie4 视为标签。您刚刚将movie4 标签移动到另一个对象。这不应该改变 movie1 仍然坚持的内容。
理解变量不是对象对于 Java 编程来说至关重要，它是一个一次只能引用一个对象（或持有一个原始值，但那在旁边）这里的重点）。

标签： java object reference instance

【解决方案1】：

这使得movie4 指向movie1：

Movie movie4 = movie1;

但是，那么您立即更新了movie4 以指向一个不同的 对象：

movie4 = new Movie("The Return of the King", 2003, 8.9);

因此，此时movie1 和movie4 不再指向同一个Movie，因此您通过movie4 所做的更改不会影响movie1 指向的对象。

如果您更改对象而不替换movie4：

Movie movie4 = movie1;
movie4.name = "The Return of the King";

...那么无论您查看movie1.name 还是movie4.name，您都会看到对象的变化。

更多细节：

在你这样做之后：

Movie movie1 = new Movie("The Shawshank Redemption", 1994, 9.3);
Movie movie2 = new Movie("The Godfather", 1972, 9.2);
Movie movie3 = new Movie("The Dark Knight", 2008, 9.0);

...你的记忆中有这样的东西（省略了很多细节）：

+−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影1:Ref4516−−−−−−−>| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：“肖申克……” | | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影2:Ref8469−−−−−−−->| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：《教父》| | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影3:Ref4789−−−−−−−->| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：《黑暗骑士》 | | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+

然后你这样做了：

Movie movie4 = movie1;

...使movie4 和movie1 指向同一部电影：

电影1:Ref4516−−−+ | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−>| （电影） | | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影4:Ref4516−−−+ |名称：“肖申克……” | | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影2:Ref8469−−−−−−−->| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：《教父》| | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影3:Ref4789−−−−−−−->| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：《黑暗骑士》 | | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+

到目前为止，一切都很好，但后来你做到了：

movie4 = new Movie("The Return of the King", /*...*/);

...这使得movie4 指向一个新对象：

因此，分配给 movie4.name 只会更改新对象，而不是 movie1 指向的对象。

如果您没有这样做movie4 = new Movie(/*...*/) 位，那么movie1 和movie4 仍然指向同一个对象

...然后分配给movie4.name 将更改movie4 和movie1 都指向的对象：

movie4.name = "The Return of the King";

电影1:Ref4516−−−+ | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−>| （电影） | | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影4:Ref4516−−−+ |名称：“...的回归” | | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影2:Ref8469−−−−−−−->| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：《教父》| | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ 电影3:Ref4789−−−−−−−->| （电影） | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ |名称：《黑暗骑士》 | | ... | +−−−−−−−−−−−−−−−−−−−−−−−−−−−−+

【讨论】：

【解决方案2】：

执行此命令后

Movie movie4 = movie1;

movie4 的值是对movie1 的引用，所以此时对movie4 所做的更改将反映在movie1 中。

movie4 = new Movie("The Return of the King", 2003, 8.9);

但是在这个命令之后，movie4 不再持有对movie1 的引用，它的值是一个全新的引用。因此，所做的任何更改都只会在movie4 指向的新对象上进行，而不是在movie1 上进行

【讨论】：