Integer den1 = (Integer) getChildItem(Fraction.DENOMINATOR);
Integer den1 = (Integer) getChild(Fraction.DENOMINATOR);
    public void multiplyWith(Fraction f) throws Exception {/*??        if (f != null) {            Integer num1 = (Integer) getChildItem(Fraction.NUMERATOR);                        if (num1 != null) {                Integer den1 = (Integer) getChildItem(Fraction.DENOMINATOR);                if (den1 != null) {                    setChildItem(Fraction.NUMERATOR, num1.multiplyWith((Integer) f.getChildItem(Fraction.NUMERATOR)));                    setChildItem(Fraction.DENOMINATOR, den1.multiplyWith((Integer) f.getChildItem(Fraction.DENOMINATOR)));                    reduct();                } else {                        throw new Exception("Could not multiply fraction. The denominator 1 is null.");                }            } else {                throw new Exception("Could not multiply fraction. The numerator 1 is null.");            }        } else {                    throw new Exception("Could not multiply fraction. The fraction is null.");        }*/    }