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  经过10000次测试写的工具类的随机字符串重复率为0.0

下面是生成指定长度的随机字符串和测试随机字符串函数的重复率以及测试代码：

import java.util.Random;

public class RandomNum {
    private static char ch[] = { ‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘, ‘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‘, ‘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‘ };//最后又重复两个0和1，因为需要凑足数组长度为64

    private static Random random = new Random();

    //生成指定长度的随机字符串
    public static synchronized String createRandomString(int length) {
        if (length > 0) {
            int index = 0;
            char[] temp = new char[length];
            int num = random.nextInt();
            for (int i = 0; i < length % 5; i++) {
                temp[index++] = ch[num & 63];//取后面六位，记得对应的二进制是以补码形式存在的。
                num >>= 6;//63的二进制为:111111
                // 为什么要右移6位？因为数组里面一共有64个有效字符。为什么要除5取余？因为一个int型要用4个字节表示，也就是32位。
            }
            for (int i = 0; i < length / 5; i++) {
                num = random.nextInt();
                for (int j = 0; j < 5; j++) {
                    temp[index++] = ch[num & 63];
                    num >>= 6;
                }
            }
            return new String(temp, 0, length);
        }
        else if (length == 0) {
            return "";
        }
        else {
            throw new IllegalArgumentException();
        }
    }

    //根据指定个数，测试随机字符串函数的重复率
    public static double rateOfRepeat(int number) {
        int repeat = 0;
        String[] str = new String[number];
        for (int i = 0; i < number; i++) {//生成指定个数的字符串
            str[i] = RandomNum.createRandomString(10);
        }
        for (int i = 0; i < number; i++) {//查找是否有相同的字符串
            for (int j = i + 1; j < number - 1; j++) {
                if (str[i].equals(str[j]))
                    repeat++;
            }
        }
        return ((double) repeat) / number;
    }

    public static void main(String[] args) {
        System.out.println(RandomNum.createRandomString(14));
        double rate = RandomNum.rateOfRepeat(10000);//测试10000次的重复率
        System.out.println("重复率:" + rate);
    }
}

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