You should first make a String that holds all of the letters/numbers that you want.
Then, make a Random. e. g. Random rnd = new Random;
Finally, make something that actually gets a random character from your String containing your alphabet.
For example,
import java.util.Random;
public class randomCharacter {
public static void main(String[] args) {
String alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ?/.,";
Random rnd = new Random();
char char = alphabet.charAt(rnd.nextInt(alphabet.length()));
// do whatever you want with the character
}
}
See this.
It's where I got this info from.