为什么对对象的第二次引用不会改变java中的内容？
在下面的代码中，我创建了电影类的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。我还为电影4的属性赋值。我打印了电影1的新属性，但内容没有改变。为什么会发生这种情况？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，但仍然相同。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();
}
}
共有2个答案
执行此命令后
Movie movie4 = movie1;
movie4的值是对movie1的引用，因此此时对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不再包含对movie1的引用，它的值是一个全新的引用。因此，所做的任何更改仅在movie4指向的新对象上进行，而不是在movie1
这使得movi4指向movi1：
Movie movie4 = movie1;
但是，随后您立即更新了movie4以指向不同的对象：
movie4 = new Movie("The Return of the King", 2003, 8.9);
因此，此时，movie1和movie4不再指向相同的Movie，因此通过movie4进行的更改不会影响movie1指向的对象。
如果您在没有替换movi4的情况下更改了对象：
Movie movie4 = movie1;
movie4.name = "The Return of the King";
...那么无论您查看movie1.name还是movie4.name，您都会看到对象的更改。
更详细一点：
在你这么做之后：
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);
...您的内存中有类似的内容（省略了很多细节）：
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie1:Ref4516−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Shawshank..." |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie2:Ref8469−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Godfather" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie3:Ref4789−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Dark Knight" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
然后你这样做了：
Movie movie4 = movie1;
...制作movie4和movie1指向同一电影：
movie1:Ref4516−−−+
| +−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−>| (Movie) |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie4:Ref4516−−−+ | name: "The Shawshank..." |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie2:Ref8469−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Godfather" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie3:Ref4789−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Dark Knight" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
到目前为止，还不错，但你做到了：
movie4 = new Movie("The Return of the King", /*...*/);
...这使得movie4指向一个新对象：
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie1:Ref4516−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Shawshank..." |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie2:Ref8469−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Godfather" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie3:Ref4789−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Dark Knight" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie4:Ref9546−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Return of..." |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
因此，分配给movie4.name只是更改了新对象，而不是movi1指向的对象。
如果您没有这样做，那么movie4=新电影（/*…*/）bit，那么movie1和movie4仍然指向同一对象
movie1:Ref4516−−−+
| +−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−>| (Movie) |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie4:Ref4516−−−+ | name: "The Shawshank..." |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie2:Ref8469−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Godfather" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie3:Ref4789−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Dark Knight" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
...然后分配给movie4.name将更改movi4和movi1指向的对象：
movie4.name = "The Return of the King";
movie1:Ref4516−−−+
| +−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−>| (Movie) |
| +−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie4:Ref4516−−−+ | name: "The Return of..." |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie2:Ref8469−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Godfather" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
movie3:Ref4789−−−−−−−>| (Movie) |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
| name: "The Dark Knight" |
| ... |
+−−−−−−−−−−−−−−−−−−−−−−−−−−+
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