在“if”中与==和=混淆。声明 [英] Getting confused with == and = in "if" statement

问题描述

我知道我们不能在java中的if语句中使用赋值运算符，因为我们在其他几种语言中使用。

I know that we cant use assignment operator in if statements in java as we use in any other few languages.

这是

            int a;

            if(a = 1) {  } 

会出现编译错误。

但以下代码可以正常工作？

but the following code works fine, how?

           boolean b;

           if(b = true) {   }

推荐答案

因为赋值的结果是赋值的值。 ..所以在第二种情况下它仍然是一个 boolean 表达式。 如果表达式要求条件为 boolean 表达式，则由第二个但不满足第一个。实际上，您的两个片段是：

Because the "result" of an assignment is the value assigned... so it's still a boolean expression in the second case. if expressions require the condition to be a boolean expression, which is satisfied by the second but not the first. Effectively, your two snippets are:

int a;

a = 1;
if (a) { }

和

boolean b;

b = true;
if (b) { }

编译但不是第一个？

这是不直接与true和false进行比较的一个原因。所以我总是只写 if（b）而不是 if（b == true）和 if（！b）而不是 if（b == false）。当 b 和 c 时，仍然遇到 if / code>是 boolean 变量，不可否认 - 一个拼写错误可能会导致问题。

This is one reason not to do comparisons with true and false directly. So I would always just write if (b) instead of if (b == true) and if (!b) instead of if (b == false). You still get into problems with if (b == c) when b and c are boolean variables, admittedly - a typo there can cause an issue. I can't say it's ever happened to me though.

编辑：响应您的编辑 - 所有类型的作业可以在中使用if 语句和 while 循环等，只要整体条件表达式 boolean 。例如，您可能有：

Responding to your edit - assignments of all kinds can be used in if statements - and while loops etc, so long as the overall condition expression is boolean. For example, you might have:

String line;
while ((line = reader.readLine()) != null)
{
    // Do something with a line
}

虽然我通常在条件中避免副作用，但这个特定的习语常常对上面的例子有用， InputStream.read 。基本上是虽然我读的价值是有用的，使用它。

While I usually avoid side-effects in conditions, this particular idiom is often useful for the example shown above, or using InputStream.read. Basically it's "while the value I read is useful, use it."

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