Collections.sort 이용해서 쉽게 비교하자!

Collections.sort(categories, new Comparator<Category>() {
    @Override
    public int compare(Category c1, Category c2) {
        if (c1.getOrder() > c2.getOrder()) return 1;
        if (c1.getOrder() < c2.getOrder()) return -1;
        return 0;
    }
});