为什么 == 与 Integer.valueOf(String) 的比较对 127 和 128 给出不同的结果? [英] Why do == comparisons with Integer.valueOf(String) give different results for 127 and 128?

问题描述

我不知道为什么这些代码行返回不同的值:

I have no idea why these lines of code return different values:

System.out.println(Integer.valueOf("127")==Integer.valueOf("127"));
System.out.println(Integer.valueOf("128")==Integer.valueOf("128"));
System.out.println(Integer.parseInt("128")==Integer.valueOf("128"));

输出是:

true
false
true

为什么第一个返回true，第二个返回false?127 和 128 之间有什么我不知道的不同之处吗?(当然我知道127 < 128.)

Why does the first one return true and the second one return false? Is there something different that I don't know between 127 and 128? (Of course I know that 127 < 128.)

还有，为什么第三个返回true?

Also, why does the third one return true?

我已阅读这个问题的答案，但我还是没搞懂怎么返回true，为什么第二行的代码返回false.

I have read the answer of this question, but I still didn't get how it can return true, and why the code in second line returns false.

推荐答案

这里有一个显着的区别.

There's a striking difference here.

valueOf 正在返回一个 Integer 对象，该对象的值可能缓存在 -128 和 127 之间.这就是第一个值返回 true - 它已缓存 - 第二个值返回 false - 128 不是缓存值，因此您将获得两个单独的 Integer 实例.

valueOf is returning an Integer object, which may have its values cached between -128 and 127. This is why the first value returns true - it's cached - and the second value returns false - 128 isn't a cached value, so you're getting two separate Integer instances.

请务必注意，您将引用与 Integer#valueOf 进行比较，如果您比较的值大于缓存支持的值，它将not 评估为 true，即使解析的值是等价的(例如:Integer.valueOf(128) == Integer.valueOf(128)).您必须改用 equals().

It is important to note that you are comparing references with Integer#valueOf, and if you are comparing a value that is larger than what the cache supports, it will not evaluate to true, even if the parsed values are equivalent (case in point: Integer.valueOf(128) == Integer.valueOf(128)). You must use equals() instead.

parseInt 正在返回一个原始 int.这就是为什么第三个值返回 true - 128 == 128 被评估，当然是 true.

parseInt is returning a primitive int. This is why the third value returns true - 128 == 128 is evaluated, and is of course, true.

现在，恰好使第三个结果 true:

Now, a fair bit happens to make that third result true:

• 一个拆箱转换发生在 您正在使用的等价运算符和您拥有的数据类型 - 即 int 和 Integer.当然，您会从右侧的 valueOf 获得一个 Integer.

• An unboxing conversion occurs with respect to the equivalence operator you're using and the datatypes you have - namely, int and Integer. You're getting an Integer from valueOf on the right hand side, of course.

转换后，您将比较两个原始 int 值.比较会按照您对原语的预期进行，因此您最终会比较 128 和 128.

After the conversion, you're comparing two primitive int values. Comparison happens just as you would expect it to with respect to primitives, so you wind up comparing 128 and 128.

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