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

在学习Java的OCA/OCP时，我发现了这一令人惊讶的事情.

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

下面是if(测试条件)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，此输出令人惊讶地为"true".

Now the output of this surprisingly is "true".

我了解到必须有一个关系条件，该条件必须像if (a > b)或if (a != b)一样返回true或false.

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如何返回true.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.但是

= is assignment operator, == is comparison operator. But

x = y

不仅将y的 值分配给x，还 返回 该值.因此，我们可以执行x=(y=1)之类的操作(甚至可以在此处删除括号)，该操作会将1分配给y，然后返回将1分配给x.

not only assign value of y to x, it also returns that value. Thanks to that we can do things like x=(y=1) (we can even drop parenthesis here) which will assign 1 to y, then return that 1 will be assigned to x.

在您的情况下，首先在if (b = true)中将true分配给b，然后将其返回，因此您以if(true)结尾，因此它将始终从该布尔值的分支执行代码.

In your case in if (b = true) first true will be assigned to b then it will be returned, so you end up with if(true) so it will always execute code from branch for that boolean value.

这通常是印刷错误的结果，因为在大多数情况下，我们希望使用==(等号运算符)，而不是=(赋值运算符).

This is often result of typographic error since in most cases we want == (equality operator), instead of = (assignment operator).

为避免此错误，我们可以编写类似

To avoid this mistake we can write code like

作为if (b){..}的
• -由于b == true始终为b，因此我们跳过== true部分. if可以使用b的值来代替对b == true的求值.当我们想使用否定代替==false时，我们可以编写if(!b){..}
• 使用尤达条件 if(true == b){..}-如果由于错误而使用了=的==会得到编译错误，它会通知我们，因为我们不能像true那样将 assign 分配给 value ，我们只能将值赋给变量.

• as if (b){..} - since b == true is always b we skip == true part. if can use value of b instead of evaluating b == true. When we want to use negation instead of ==false we can write if(!b){..}
• using Yoda conditions if(true == b){..} - if by mistake we will use = instead of == we will get compilation error which will inform us about it, because we can't assign anything to value like true, we can only assign values to variables.

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