Java中的“范围"是什么? [英] What is 'scope' in Java?

问题描述

我刚开始编码.我想对同一个变量使用两次 switch 语句，我被告知要执行此操作，变量必须在范围内".

I just started coding. I want to use a switch statement twice for the same variable, and I was told that to do this the variable would have to be 'in scope'.

作为初学者，我不知道这意味着什么.那么在范围内是什么意思呢?而且，如果一个变量不在范围内，我该如何使它在范围内?

Being a beginner, I have no idea what that means. So what does being in scope mean? And, if a variable isn't in scope, how do I make it in scope?

推荐答案

局部变量¹在范围内"，如果代码可以访问它，如果不能，则超出范围.在 Java 中，变量的作用域是它们在其中声明的块 ({}).所以:

A local variable¹ is "in scope" if code can access it and out of scope if it can't. In Java, variables are scoped to the block ({}) they're declared in. So:

void foo() {
    int a = 42;

    if (/*some condition*/) {
        String q = "Life, the Universe, and Everything";

        // 1. Both `a` and `q` are in scope here
        System.out.println(a);
        System.out.println(q);
        if (/*another condition*/) {
            // 2. Both `a` and `q` are in scope here, too
            System.out.println(a);
            System.out.println(q);
        }
    }

    // 3. Only `a` is in scope here
    System.out.println(a);
    System.out.println(q); // ERROR, `q` is not in scope
}

注意上面的 (1)、(2) 和 (3):

Note (1), (2), and (3) above:

1. 代码可以访问q，因为q和代码在同一个块中声明；tt可以访问a，因为它是在包含块中声明的.

1. The code can access q because q is declared in the same block as the code; tt can access a because it's declared in the containing block.

代码可以访问q，因为它是在包含块中声明的；它可以访问 a 因为它在下一个块中.

The code can access q because it's declared in the containing block; it can access a because it's in the next block out.

代码可以访问a，但不能访问q，因为q没有在块或任何包含它的块(或其他一些东西).

The code can access a, but not q, because q isn't declared in the block or any of the blocks (or a couple of other things) containing it.

当弄清楚什么是不合格的标识符时(如上面的 a 或 q，而不是 this.foo 中的 foo 或 q.toLowerCase 中的 toLowerCase，它们是 qualified) 是，Java 编译器会在每个地方查找，一个接一个，直到找到匹配项:

When figuring out what an unqualified identifier (like a or q above, as opposed to the foo in this.foo or the toLowerCase in q.toLowerCase, which are qualified) is, the Java compiler will look in each of these places, one after the other, until it finds a match:

• 对于最内层块中具有该名称的变量
• 对于下一个块中具有该名称的变量，依此类推
• 对于当前类中具有该名称的字段²或方法(通常:member)
• 对于来自已导入包的具有该名称的类
• 对于具有该名称的包

该列表中还有其他一些(我不打算与初学者讨论静态导入).

There are a few others for that list (I'm not going to get into static imports with a beginner).

范围还有很多，我建议您阅读一些教程和/或一本 Java 入门书以了解更多信息.

There's a lot more to scope, I suggest working through some tutorials and/or a beginning Java book for more.

¹局部变量"与变量"——Java 语言规范 以比大多数人在普通语言中更通用的方式使用变量".当我在这个答案中说变量"时，我指的是 JLS 所说的local 变量".

¹ "local variable" vs. "variable" - The Java Language Specification uses "variable" in a more general way than most people do in common speech. When I say "variable" in this answer, I mean what the JLS calls a "local variable".

² "field" - JLS 在某些地方将字段称为变量"(在其他地方称为字段")，因此是上面的 (1).:-)

² "field" - The JLS calls fields "variables" in some places (and "fields" in other places), hence (1) above. :-)

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