class Calculator {
  public int add(int x, int y) {
    return x + y;
  }

  public int sub(int x, int y) {
    return x - y; // could be negative, but that's ok
  }

  public int mul(int x, int y) {
    return x * y;
  }

  public int div(int x, int y) {
    // there's something wrong here.
    try {
      return x / y;
    }
    catch (Exception err) {
      System.out.println(err);
      System.out.println("You cannot divide by zero!");
      System.exit(1);
    }
    return -1; // this will never be reached
  }

  public void print_menu() {
    String menu = "0) Exit\n1) Add\n2) Sub\n3) Mul\n4) Div\n5) Factorial";
    System.out.println(menu);
  }
}