autoboxing是否调用valueOf（）？ [英] Does autoboxing call valueOf()?

问题描述

我正在尝试确定以下陈述是否保证为真：

I'm trying to determine whether the following statements are guaranteed to be true:

((Boolean)true) == Boolean.TRUE
((Boolean)true) == Boolean.valueOf(true)
((Integer)1) == Integer.valueOf(1)

我一直认为自动装箱相当于在相应类型上调用 valueOf（） 。我见过的每一个讨论 主题似乎支持我的假设。但我在JLS中找到的只有以下内容（§5.1.7）：

I've always assumed that autoboxing was equivalent to calling valueOf() on the corresponding type. Every discussion that I've seen on the topic seems to support my assumption. But all I could find in the JLS was the following (§5.1.7):

如果值 p 被装箱是 -128 和 int 类型的整数字面值> 127 包含（§3.10.1），或布尔文字 true 或 false （§3.10.3），或'\ u0000'和'\ u007f'之间的字符文字（§3.10.4），然后让 a 和 b 成为<$ c的任意两次拳击转换的结果$ C> p 。总是这样的情况 a == b 。

If the value p being boxed is an integer literal of type int between -128 and 127 inclusive (§3.10.1), or the boolean literal true or false (§3.10.3), or a character literal between '\u0000' and '\u007f' inclusive (§3.10.4), then let a and b be the results of any two boxing conversions of p. It is always the case that a == b.

这描述了行为与 valueOf（）相似* 。但似乎没有任何保证实际调用 valueOf（），这意味着理论上可以有一个实现为自动装箱的值保留一个单独的专用缓存。在这种情况下，缓存的自动装箱值和常规缓存的装箱值之间可能没有相同的身份。

That describes behavior identical similar* to that of valueOf(). But there doesn't seem to be any guarantee that valueOf() is actually invoked, meaning there could theoretically be an implementation that keeps a separate, dedicated cache for autoboxed values. In such a case, there might not be identity equality between cached autoboxed values and regular cached boxed values.

Oracle的自动装箱教程事实上说明了 li.add（i）编译为 li.add（Integer.valueOf（i）），其中 i 是 INT 。但我不知道该教程是否应被视为权威来源。

Oracle's autoboxing tutorial states matter-of-factly that li.add(i) is compiled to li.add(Integer.valueOf(i)), where i is an int. But I don't know whether the tutorial should be considered an authoritative source.

_{*略微弱一点保证比 valueOf（），因为它只引用文字值。}

_{*It's a slightly weaker guarantee than valueOf(), as it only refers to literal values.}

推荐答案

我首先考虑你的问题是什么编译器为自动装箱生成的代码是什么？

I first tought your question was a dupe of What code does the compiler generate for autoboxing?

然而，在您对@ElliottFrisch发表评论后，我发现它有所不同：

However, after your comment on @ElliottFrisch I realized it was different :

我知道编译器的行为方式。我试图弄清楚
是否保证这种行为。

I know the compiler behaves that way. I'm trying to figure out whether that behavior is guaranteed.

对于其他读者，假设表现得那样 表示使用 valueOf 。

For other readers, assume that "behaves that way" means using valueOf.

请记住，Java有多个编译器。为了合法，他们必须遵守 JLS 。因此，只要这里的所有规则都得到尊重，就无法保证内部如何实现自动装箱。

Remember that there are multiples compilers for Java. To be "legal" they must follow the contract given in the JLS. Therefore, as long as all the rules here are respected, there is no guarantee of how autoboxing is internally implemented.

但我认为没有理由不使用 valueOf ，特别是它使用缓存的值，并且是这篇文章由Joseph D. Darcy撰写。

But I don't see any reason to not use valueOf, specially that it uses the cached values and is the recommended way as per this article by Joseph D. Darcy.

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