@Override
    public Double getMonthlyPayment() throws Exception {

        if (fixedMonthlyPayment.equals(INITIAL_PAYMENT_AMT)) {
            calculateMortgage();
        }

        if (fixedMonthlyPayment.equals(INVALID_PAYMENT_AMT)) {
            throw new RuntimeException("Could not calculate monthly payment.");
        }

        return fixedMonthlyPayment;
    }