Integers

If you expect someone to type an integer value you can turn the String you get from IO.readln into an int using Integer.parseInt.

void main() {
    String ageString = IO.readln("How old are you? ");
    int age = Integer.parseInt(ageString);
    IO.println("You are " + age + " years old!");
}

So long as they type something which can be interpreted as an int (like 123) you will get a value for the int variable. Otherwise the program will crash.