@Test
    void factorialIterShouldThrowInputCanNotBeConvertedToIntegerException() {
        try {
            Constructor<FactorialIter> c = FactorialIter.class.getDeclaredConstructor();
            c.setAccessible(true);
            FactorialIter u = c.newInstance();
            Class[] cArg = new Class[1];
            cArg[0] = String.class;
            Method method = FactorialIter.class.getDeclaredMethod("convertStringToInteger", cArg);
            method.setAccessible(true);
            Object out = method.invoke(u, "some input");
            System.out.println(out);
        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException | InstantiationException e) {
            e.printStackTrace();
        }
    }