Java范围和变量的生命周期 [英] Java scope and lifetime of variable

问题描述

我编写了以下程序来显示2到50（含）之间的所有素数。该程序按预期运行，但当我重新检查代码时，我想知道为什么它没有失败。 if 语句可以更改 isprime 变量的值。但是，一旦内部 for 代码块{}被遗忘，这个改变是否会被遗忘？这意味着 isprime 将保持 true 并显示所有数字。

I wrote the following program to display all prime numbers between 2 and 50 (inclusive). The program ran as intended but when I reexamined the code I wondered why it had not failed. The if statement can change the value of the isprime variable. However, is this change not forgotten once the inner for code block {} is left? This would mean that isprime would remain true and all numbers would be displayed.

class Prime {
    public static void main (String args []) {

    int a, b;
    boolean isprime;

    for (a = 2; a < 51; a++) {

        isprime = true;

        for (b = a-1; b > 1; b--) {

            if (a % b == 0) isprime = false;
        }

        if (isprime) System.out.println(a);
    }
}
}

推荐答案

嗯，正如您所见，这不是它的工作原理：变量的范围是声明它的块，包括任何子块。

Well, as you see, that's not how it works: the scope of the variable is the block where it is declared, including any sub block.

修改子块中的变量会修改它的所有范围。每次新块开始时都不会生成变量的副本。

Modifying the variable in a sub block modifies it for all of its scope. A copy of the variable is not made everytime a new block starts.

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