如何在Java中的if语句中为变量赋值 [英] How can you assign a variable a value inside a if statement in Java

问题描述

我需要做这样的事情，

if (first_var > second_var)
  int difference = first_var - second_var;
if (first_var < second_var)
  int difference = second_var - first_var;

当我尝试编译它时，会出现错误，说明变量差异可能尚未初始化。使变量差异全局也无济于事。

When I try to compile this, an error comes stating the variable 'difference' may not have been initialized. Making the variable 'difference' global doesn't help either.

推荐答案

您需要了解的问题包括：

The issues that you need to learn are:

• 变量范围


• 块声明以及如何创建新范围


• 为什么你更喜欢使用 {...} 阻止如果陈述


• 如何保证明确分配


• 何时使用 if-else 而不是 if（something）{...} if（！something）{ ...}

• The scope of variables
• Block declaration and how that creates new scopes
• Why you should prefer to use {...} block for if statements
• How to guarantee definite assignment
• When to use if-else instead of if (something) {...} if (!something) {...}

顺便说一下，找到两个值之差的惯用方法是：

By the way, the idiomatic way to find the difference of two values is:

int difference = Math.abs(firstVar - secondVar);

请注意 Math.abs 有一个极端情况：当参数为 Integer.MIN_VALUE 时，返回的值为 Integer.MIN_VALUE 。那是因为 -Integer.MIN_VALUE == Integer.MIN_VALUE 。这里的问题是数字的32位二进制补码表示。

Do note that Math.abs has a corner case: when the argument is Integer.MIN_VALUE, the returned value is Integer.MIN_VALUE. That's because -Integer.MIN_VALUE == Integer.MIN_VALUE. The issue here is 32-bit two's complement representation of numbers.

• JLS 16 Definite Assignment


• JLS 14.4.2范围本地变量声明


• JLS 14.9.2 if-then-else语句


• Math.abs（int）


• Wikipedia / Two's complement （阅读：最多负数）

• JLS 16 Definite Assignment
• JLS 14.4.2 Scope of Local Variable Declarations
• JLS 14.9.2 The if-then-else Statement
• Math.abs(int)
• Wikipedia/Two's complement (read: the most negative number)

这是一次修复片段的尝试，通过在之前声明变量，如果

Here's one attempt to fix the snippet, by declaring the variable before the if

int difference;
if (first_var > second_var) {
  difference = first_var - second_var;
}
if (first_var < second_var) {
  difference = second_var - first_var;
}

// note: difference is not definitely assigned here!
//           (but at least it's in scope!)

我们现在遇到问题明确赋值：如果 first_var == second_var ，变量差值仍然没有赋值。

We now have a problem with definite assignment: if first_var == second_var, the variable difference is still not assigned a value.

这是第二次尝试：

int difference = 0;
if (first_var > second_var) {
  difference = first_var - second_var;
}
if (first_var < second_var) {
  difference = second_var - first_var;
}

// note: difference is in scope here, and definitely assigned

初学者倾向于这样做，但这排除了使差异 最终局部变量的可能性，因为它可能被赋值两次。

Beginners tend to do this, but this precludes the possibility of making difference a final local variable, because it's possibly assigned value twice.

这是一个更好的尝试：

final int difference;
if (first_var > second_var) {
  difference = first_var - second_var;
} else {
  difference = second_var - first_var;
}

// note: difference is in scope, and is definitely assigned here,
//       (and declared final)

仍然有办法改善这一点。

There are still ways to improve on this.

一旦你对语言和编程更加熟悉，你可以使用以下习语：

Once you're more comfortable with the language and programming, you may use the following idiom:

final int difference = (first_var > second_var) ? first_var - second_var
                                                : second_var - first_var;

这使用？：三元/有条件运营商。小心这个操作员;它有一些令人惊讶的行为，它肯定会被滥用。明智地，谨慎地使用。

This uses the ?: ternary/conditional operator. Do be careful with this operator; it has some behaviors that may be surprising, and it definitely can be abused. Use carefully, judiciously, idiomatically.

• JLS 15.25条件运算符？：

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