当我们使用赋值而不是比较时，如何评估 if/while 条件? [英] How is if/while condition evaluated when we use assignments instead of comparison?

问题描述

我在学习 OCA/OCP for Java 时发现了这个令人惊讶的事情.

I discovered this surprising thing while learning OCA/OCP for Java.

下面是 if(test condition) 部分的第一段代码令我惊讶.

Below is the first piece of code of which the if(test condition) part surprises me.

public class BooleanIf {
public static void main(String[] args) {
    boolean b = false;
    System.out.println(Boolean.valueOf(b = true));
    if (b = true)
        System.out.println("true");
    else
        System.out.println("false");
}

现在这个令人惊讶的输出是true".

Now the output of this surprisingly is "true".

我了解到必须有一个返回 true 或 false 的关系条件，如 if (a > b) 或 if (a != b) 类似.

I learnt that there has to be a relational condition that returns true or false like if (a > b) or if (a != b) likewise.

我想知道在这种情况下它是如何返回 true 的.它会调用 Boolean.valueOf() 吗?

I want to know how it is returning true for this case. Does it call Boolean.valueOf()?

推荐答案

= 是赋值运算符，== 是比较运算符.

= is assignment operator, == is comparison operator.

但是

x = y

不仅将值从y赋值给变量x，它还返回 那个值.

not only assigns value from y to variable x, but it also returns that value.

多亏了这一点，我们可以做诸如 x=(y=1)(我们甚至可以在这里去掉括号)之类的事情，它将 1 分配给变量 y，然后将返回该 1 并将其分配给 x.

Thanks to that we can do things like x=(y=1) (we can even drop parenthesis here) which will assign 1 to variable y, then will return that 1 and also assign it to x.

因此也可以编写像 if (b = true) 这样的代码，其中 true 将被分配给 b 然后由 if(..) 返回和使用.换句话说，你最终会得到类似 if(true){b=true; 的东西...} 所以它总是从 true 分支执行代码.

Because of that it is also possible to write code like if (b = true) where true will be assigned to b and then be returned and used by if(..). In other words you end up with something like if(true){b=true; ..} so it will always execute code from true branch.

• 省略 ==true 和 ==false 部分.

• 在 if(b==true) 的情况下，我们可以写 if(b) 因为 (b == true) 总是给出与 b 中存储的 aleady 相同的结果.
• 在if(b==false)的情况下，我们可以写if(!b).

• In case of if(b==true) we can write if(b) since (b == true) will always give same result as aleady stored in b.
• In case of if(b==false) we can write if(!b).

在==左侧使用值代替变量.
换句话说，使用 Yoda conditions if(true == b){..}.即使我们错误地写了 = 而不是 == 我们最终会得到 true=b 这将导致编译错误，因为我们不能像 true 那样分配任何东西给 value.我们只能给变量赋值.

use value instead of variable on left side of ==.
In other words use Yoda conditions if(true == b){..}. Even if by mistake we will write = instead of == we will end up with true=b which will end up as compilation error since we can't assign anything to value like true. We can only assign values to variables.

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