public class RandomString
{
    private static final Random RANDOM_GEN = new Random();
    private static final char[] ALPHABETS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();

    public static void main(String[] args)
    {
        for (int i = 0; i < 10; i++)
        {
            System.out.println(genRandomString2());
        }
    }

    /**
     * Generates one of the possible 10^16 strings. Each string is equally likely.
     */
    public static String genRandomString()
    {
        StringBuilder sb = new StringBuilder();
        // we iterate for 4 times, each time appending a 4-character string to sb
        for (int i = 0; i < 4; i++)
        {
            // generate a number between 0-9999
            String s = Integer.toString(RANDOM_GEN.nextInt(10000));

            // if the number has less than 4 digits, pad the leading digits with 0s
            for (int j = 0, zeros = 4 - s.length(); j < zeros; j++)
            {
                sb.append('0');
            }
            sb.append(s);
        }
        return sb.toString();
    }

    /**
     * Generates one of the possible 36^16 strings. Each string is equally likely.
     */
    public static String genRandomString2()
    {
        int nAlphabets = ALPHABETS.length;

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 16; i++)
        {
            // pick a character from the ALPHABETS array at random
            sb.append(ALPHABETS[RANDOM_GEN.nextInt(nAlphabets)]);
        }
        return sb.toString();
    }
}