You can easily do that with a for loop,
public static void main(String[] args) {
  String aToZ="ABCD.....1234"; // 36 letter.
  String randomStr=generateRandom(aToZ);
}
private static String generateRandom(String aToZ) {
    Random rand=new Random();
    StringBuilder res=new StringBuilder();
    for (int i = 0; i < 17; i++) {
       int randIndex=rand.nextInt(aToZ.length()); 
       res.append(aToZ.charAt(randIndex));            
    }
    return res.toString();
}