class Inventory {

    private List<Supply> supplies = new ArrayList<>();

    int getQuantity(Supply supply) { 
        if (supply == null) {
            throw new NullPointerException("supply must not be null"); 
        }

        int quantity = 0;
        for (Supply supplyInStock : supplies) {
            if (supply.equals(supplyInStock)) {
                quantity++;
            } 
        }
        return quantity;

    }
}