与“if”中的 == 和 = 混淆陈述

发布于 2024-08-24 02:51:43 字数 290 浏览 7 评论 0原文

我知道我们不能像在其他几种语言中那样在 java 的 if 语句中使用赋值运算符。

那

            int a;

            if(a = 1) {  }

会给出编译错误。

但是下面的代码工作正常，怎么样？

           boolean b;

           if(b = true) {   }

编辑：这是规则不能在 if 语句中使用赋值的例外情况吗？

原文

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

that is

            int a;

            if(a = 1) {  }

will give a compilation error.

but the following code works fine, how?

           boolean b;

           if(b = true) {   }

EDIT : Is this the exception to rule that assignment cant be used in if statement.

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破晓 2024-08-31 02:51:43

因为赋值的“结果”是分配的值...所以在第二种情况下它仍然是一个布尔表达式。 if 表达式要求条件为 boolean 表达式，第二个表达式满足，但第一个表达式不满足。实际上，您的两个片段是：

int a;

a = 1;
if (a) { }

？

boolean b;

b = true;
if (b) { }

从扩展中可以清楚地看出第二个版本将编译但第一个版本不会编译

这是不直接比较真假的原因之一。所以我总是只写 if (b) 而不是 if (b == true) 和 if (!b) 而不是 if (b == false)。诚然，当 b 和 c 是 boolean 变量时，您仍然会遇到 if (b == c) 的问题- 那里的拼写错误可能会导致问题。但我不能说这曾经发生在我身上。

编辑：响应您的编辑 - 各种赋值都可以在 if 语句中使用 - 和 while 循环等，只要整体条件表达式为 boolean< /代码>。例如，您可能会：

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

虽然我通常避免条件中的副作用，但这个特定的习惯用法对于上面显示的示例或使用InputStream.read通常很有用。基本上就是“当我读到的值有用时，就使用它”。

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) { }

and

boolean b;

b = true;
if (b) { }

Is it clear from that expansion that the second version will compile but not the first?

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.

EDIT: 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
}

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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