Question

import java.util.Random;
import java.util.Scanner;

public class PassGen {
public static void main(String[] args) {
    String[] characters = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","0","1","2","3","4","5","6","7","8","9"};
    StringBuilder b = null;

    Scanner scan = new Scanner(System.in);
    System.out.println("Enter password length.");
    int length = scan.nextInt();
    while (length > 20 || length < 6) {
        System.out.println("Password must be between 6 and 20 characters long.");
        length = scan.nextInt();
    }

    Random rand = new Random();

    for (int i = 0; i <= length; i++) {
        int x = rand.nextInt(characters.length) + 1;
        b = new StringBuilder(length + 1);
        String s = characters[x];
        b.append(s);
    }

    System.out.println("Your password is: " + b.toString());
}
}

出于某种原因，当我运行这个程序时，它只会在显示一个随机字符之前运行一次for循环，而不管输入的长度如何。

Answer 1

不，循环正在运行多次迭代 - 但在每次迭代时，您正在创建一个新的StringBuilder：

// This is inside the loop, but should be outside.
b = new StringBuilder(length + 1);

注意有时，我希望循环抛出一个异常 - 如果没有，你最终会得到一个比你想要的更长的字符串。基本上，你有三个一个一个错误...你应该有：

b = new StringBuilder(length);
for (int i = 0; i < length; i++) {
    int x = rand.nextInt(characters.length);
    b.append(characters[x]);
}

另请注意，如果您只使用字符串而不是字符串数组并使用charAt，那会更简单：

String characters = "ABCDE...9";
...

int x = rand.nextInt(characters.length());
b.append(characters.charAt(x));

Answer 2

    b = new StringBuilder(length + 1);

需要在for循环之外。您拥有它的方式，每次运行for循环时都会创建b。

Answer 3

您总是在循环中重新创建b对象。它将始终只包含一个字符。

For循环仅在完成程序之前运行一次

3 个答案: