Java变量赋值: why is box1 not updated to box3?

Question

我有一个关于变量赋值的 java 基础知识的初学者问题。

在我的示例代码中，我有 3 个框(对象)。我按如下方式分配框:

    Box box1 = new Box("Furniture", 1);
    Box box2 = new Box("Games", 2);
    Box box3 = new Box("Cloths", 3);

    box1 = box2;
    box2 = box3;

    System.out.println(box1.toString());
    System.out.println(box2.toString());

现在，我希望 box1 也指向 box3。但事实证明，它仍然指向 box2，尽管我也将 box2 的引用更改为 box3。为什么会这样？

Answer 1

这是你的初始状态:

     +-----------------+             +----------------+
     |  box1 ( ref )   +------------>|  box1 ( obj )  |
     +-----------------+             +----------------+

     +-----------------+             +----------------+
     |  box2 ( ref )   +------------>|  box2 ( obj )  |
     +-----------------+             +----------------+

     +------------------+            +----------------+
     |  box3 ( ref )    +----------->|  box3 ( obj )  |
     +------------------+            +----------------+

这是 box1 = box2 之后发生的事情:

     +-----------------+             +----------------+
     |  box1 ( ref )   +----+        |  box1 ( obj )  |
     +-----------------+    |        +----------------+
                            |
     +-----------------+    +------> +----------------+
     |  box2 ( ref )   +------------>|  box2 ( obj )  |
     +-----------------+             +----------------+

     +------------------+            +----------------+
     |  box3 ( ref )    +----------->|  box3 ( obj )  |
     +------------------+            +----------------+

这是 box2 = box3 之后发生的事情

     +-----------------+             +----------------+
     |  box1 ( ref )   +----+        |  box1 ( obj )  |
     +-----------------+    |        +----------------+
                            |
     +-----------------+    +------> +----------------+
     |  box2 ( ref )   +----+        |  box2 ( obj )  |
     +-----------------+    |        +----------------+
                            |
     +------------------+   +------->+----------------+
     |  box3 ( ref )    +----------->|  box3 ( obj )  |
     +------------------+            +----------------+

现在您应该能够弄清楚为什么输出是这样的。 :)