How to generate a random alpha-numeric string?

Question

I've been looking for a simple Java algorithm to generate a pseudo-random alpha-numeric string. In my situation it would be used as a unique session/key identifier that would "likely" be unique over 500K+ generation (my needs don't really require anything much more sophisticated). Ideally, I would be able to specify a length depending on my uniqueness needs. For example, a generated string of length 12 might look something like "AEYGF7K0DM1X".

Answer 1

static final String AB = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
static SecureRandom rnd = new SecureRandom();

String randomString( int len ){
   StringBuilder sb = new StringBuilder( len );
   for( int i = 0; i < len; i++ ) 
      sb.append( AB.charAt( rnd.nextInt(AB.length()) ) );
   return sb.toString();
}

Answer 2

You can use the UUID class with its getLeastSignificantBits() message to get 64bit of Random data, then convert it to a radix 36 number (i.e. a string consisting of 0-9,A-Z):

Long.toString(Math.abs( UUID.randomUUID().getLeastSignificantBits(), 36));

This yields a String up to 13 characters long. We use Math.abs() to make sure there isn't a minus sign sneaking in.

Answer 3

Java supplies a way of doing this directly. If you don't want the dashes, they are easy to strip out. Just use uuid.replace("-", "")

import java.util.UUID;

public class randomStringGenerator {
    public static void main(String[] args) {
        System.out.println(generateString());
    }

    public static String generateString() {
        String uuid = UUID.randomUUID().toString();
        return "uuid = " + uuid;
    }
}

Output:

uuid = 2d7428a6-b58c-4008-8575-f05549f16316

Answer 4

You mention "simple", but just in case anyone else is looking for something that meets more stringent security requirements, you might want to take a look at jpwgen. jpwgen is modeled after pwgen in Unix, and is very configurable.

Answer 5

You can use Apache library for this: RandomStringUtils

RandomStringUtils.randomAlphanumeric(20).toUpperCase();

Answer 6

import java.util.Date;
import java.util.Random;

public class RandomGenerator {

  private static Random random = new Random((new Date()).getTime());

    public static String generateRandomString(int length) {
      char[] values = {'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','2','3',
               '4','5','6','7','8','9'};

      String out = "";

      for (int i=0;i<length;i++) {
          int idx=random.nextInt(values.length);
          out += values[idx];
      }
      return out;
    }
}