private List<? extends BudgetItem> processBudgetItems(List<? extends BudgetItem> budgetItemList) throws Exception {
        int index = 0;

        for(BudgetItem item : budgetItemList) {

            if(item.getItemMonetaryAmount() != null) {
                BigDecimal monetaryAmount = multiplyMonetaryValueByTwelve(item.getItemMonetaryAmount());
                budgetItemList.get(index).setItemMonetaryAmount(monetaryAmount);
            } else {
                throw new BudgetItemsMissingException("BudgetItem monetaryAmount is null.");
            }

            index++;
        }

        return budgetItemList;
    }