整数是不可改变的 [英] Is Integer Immutable

问题描述

我知道这可能是非常愚蠢的，但很多地方声称Java中的Integer类是不可变的，但是代码如下：

I know this is probably very stupid, but a lot of places claim that the Integer class in Java is immutable, yet the following code:

Integer a=3;
Integer b=3;
a+=b;
System.out.println(a);

执行（预期）结果时没有任何问题6.因此，a的值已经有效地改变了。这不意味着Integer是可变的吗？
次要问题和一些小问题：不可变类不需要复制构造函数。有人在乎解释原因吗？

Executes without any trouble giving the (expected) result 6. So effectively the value of a has changed. Doesn't that mean Integer is mutable? Secondary question and a little offtopic: "Immutable classes do not need copy constructors". Anyone care to explain why?

推荐答案

不可变并不意味着 a 永远不能等于另一个价值。例如， String 也是不可变的，但我仍然可以这样做：

Immutable does not mean that a can never equal another value. For example, String is immutable too, but I can still do this:

String str = "hello";
// str equals "hello"
str = str + "world";
// now str equals "helloworld"

那么那里发生了什么？由于 String 是不可变的，所以显然 str 没有改变。但它现在等于不同的东西。这是因为 str 现在是一个完全新实例化的对象，就像你的 Integer 一样。所以 a 的值没有变异，但它被一个全新的对象所取代，即 new Integer（6）。

So what happened there? Since String is immutable, clearly str was not changed. But it now equals something different. This is because str is now a completely newly instantiated object, just as your Integer is. So the value of a did not mutate, but it was replaced with a completely new object, i.e. new Integer(6).

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