null == foo与foo == null [英] null == foo versus foo == null

问题描述

这可能只是一个样式问题，但我正在阅读一本Java编码书（'Programming Android'）并且编写者在变量方法之前首先声明为null，这是我不熟悉的实践。例如：

This may just be a style question, but I'm reading a Java coding book ('Programming Android') and the writer all declares null first before a variable method, a practice I am not familiar with. For example:

if (null == foo) {
    //code here
}

或

if (null != foo) {
    //code here
}

而不是

if (foo == null) {
    //code here
}

我看不出顺序如何在语义上/语法上产生差异，或者我错在这里？真的很好奇。

I can't see how the order would make a difference semantically/syntactically, or am I wrong here? Really just curious.

推荐答案

这可能是C / C ++遗留下来的习惯。在C中，你会把常量放在左边，因为如果你错误输入 = 而不是 == 那么会有一个错误，因为您无法将某些内容分配给常量。在Java中，这是不必要的，因为 if（foo = null）也会产生错误，该错误表示对象引用不是布尔值。

It's probably a habit left over from C/C++. In C, you would put constants on the left, because if you mistyped = instead of == there would be an error because you can't assign something to a constant. In Java, this is unnecessary because if (foo = null) also gives an error, which says that an object reference isn't a boolean.

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