Answer by Chamith Malinda for How to generate a random alpha-numeric string

With come modifications to gurantee to have a number in the returned string.

    private static String generateRandomCouponMethod3(int lengthOfString){    final String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";    final String numbers = "0123456789";    SecureRandom secureRandom = new SecureRandom();    StringBuilder sb = new StringBuilder(lengthOfString);    for(int i = 0 ; i < lengthOfString ; ++i){        if(i%2 == 0){            sb.append(alphabet.charAt(secureRandom.nextInt(alphabet.length())));        }else{            sb.append(numbers.charAt(secureRandom.nextInt(numbers.length())));        }    }    return sb.toString();}