public class RandomUtil {    private static final String ALPHA_NUMBER = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz";    private static final String NUMBER = "0123456789";    private static final Random RANDOM;    static {        long timeInMillis = SystemClock.getTimeInMillis();        RANDOM = new Random(timeInMillis);    }    public static String randomNum(int len) {        return random(NUMBER, len);    }    public static String randomStr(int len) {        return random(ALPHA_NUMBER, len);    }    private static String random(String base, int len) {        StringBuilder sb = new StringBuilder();        for (int i = 0; i < len; i++) {            int number = RANDOM.nextInt(base.length());            sb.append(base.charAt(number));        }        return sb.toString();    }}