T.java

/*
what you need is something that randomly picks one of these: 0-9, 65-90, 95
if number is < 10 => digit, if above it's letter which means you'll cast it to char
means we have 38 possibilities, or 38 + 26 if we wanna include lower case letters
 */
List<Integer> lst = new ArrayList<>(38);
lst.addAll(0, IntStream.rangeClosed(0, 9).boxed().collect(Collectors.toList()));
lst.addAll(10, IntStream.rangeClosed(65, 90).boxed().collect(Collectors.toList()));
lst.add(95);

int random = ThreadLocalRandom.current().nextInt(0, 38);

char result;
if (random < 10) {
    result = Character.forDigit(random, 10);
} else {
    result = (char)((int)lst.get(random));
}

System.out.println(result);