@Override
    public long numberOfPackagesForOwner(String owner) {
        List<Package> ownerPackages = new ArrayList<>();
        if (owner == null) {
            throw new IllegalArgumentException();
        }
        for (Package pack : postOfficeInventory){
            if(pack.getOwner().equals(owner)){
                ownerPackages.add(pack);
            }
        }
        return ownerPackages.size();
    }