in Java, how to create n number of Strings with certain properties

What I would like to do is produce n number of Strings and each string should have 2 char and 4 numbers randomly shuffled, for ex: 2w3e45 34rt56 qw5498 0126fd w5487t

and so on, I have the following, it is working, any enhancements could be applied? Also, can we have them as unique strings?

import java.util.Random;

public class RandomNumAndChar {

public static void main(String[] args) {

    for (int j = 0; j < 10; j++) {
        Random r = new Random();
        Integer[] rn = new Integer[4];
        String numbers = new String();
        for (int i = 0; i < rn.length; i++) {
            rn[i] = r.nextInt(10);
            numbers = numbers + rn[i].toString();
        }
        char c = (char) (r.nextInt(26) + 'a');
        char c2 = (char) (r.nextInt(26) + 'a');
        String chars = "" + c + c2;
        String fin = numbers.concat(chars);
        System.out.println(RandomNumAndChar.shuffle(fin));


    }


}

public static String shuffle(String s) {
    char[] characters = s.toCharArray();
    for (int i = 0; i < characters.length; i++) {
        int randomIndex = (int)(Math.random() * characters.length);
        char temp = characters[i];
        characters[i] = characters[randomIndex];
        characters[randomIndex] = temp;
    }
    return new String(characters);
}

}