java 生成一个64位字符串不重复的随机字符串的算法

要一个64位不重复的随机字符串，可以使用以下算法：

public String generateRandomString() {
    String model = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    StringBuilder sb = new StringBuilder();
    Random random = new Random();
    while (sb.length() < 64) {
        int index = random.nextInt(model.length());
        char c = model.charAt(index);
        if (sb.indexOf(String.valueOf(c)) == -1) {
            sb.append(c);
        }
    }
    return sb.toString();
}