Działanie:
int number1 = Integer.parseInt(reader.nextLine());
 String operation = reader.nextLine();
                    if (operation.equals("+")) {
                        int number2 = Integer.parseInt(reader.nextLine());
                        Calculator.Add(number1, number2);

Metoda:
public static double Add (int number1, int number2) {
        double result = (double)number1+number2;
        return result;
    }