title: in Java, how to create n number of Strings with certain properties
body: <p>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</p>
  
  <p>and so on, I have the following, it is working, any enhancements could be applied?
  Also, can we have them as unique strings?</p>
  
  <pre><code>import java.util.Random;
  
  public class RandomNumAndChar {
  
  public static void main(String[] args) {
  
      for (int j = 0; j &lt; 10; j++) {
          Random r = new Random();
          Integer[] rn = new Integer[4];
          String numbers = new String();
          for (int i = 0; i &lt; 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 &lt; 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);
  }
  </code></pre>
  
  <p>}</p>