java自动生层2到20位的字符串账号

以下是Java代码示例，可以生成2到20位的随机字符串账号：

import java.util.Random;

public class RandomStringGenerator {
    private static final String CHARACTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    public static String generateRandomString(int length) {
        Random random = new Random();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < length; i++) {
            int randomIndex = random.nextInt(CHARACTERS.length());
            char randomChar = CHARACTERS.charAt(randomIndex);
            sb.append(randomChar);
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        for (int i = 2; i <= 20; i++) {
            System.out.println("Random string with length " + i + ": " + generateRandomString(i));
        }
    }
}

该代码中使用了一个包含大小写字母和数字的字符集，根据指定长度生成随机字符串。使用循环语句可以生成2到20位长度的随机字符串账号。