Java qsort

public static List<Wall> qsort(List<Wall> input) {

        if (input.size() <= 1) {
            return input;
        }

        int middle = (int) Math.ceil((double) input.size() / 2);
        Wall pivot = input.get(middle);

        List<Wall> less = new ArrayList<Wall>();
        List<Wall> greater = new ArrayList<Wall>();

        for (int i = 0; i < input.size(); i++) {
            if (input.get(i).effeciency >= pivot.effeciency) {
                if (i == middle) {
                    continue;
                }
                less.add(input.get(i));
            } else {
                greater.add(input.get(i));
            }
        }

        return concatenate(qsort(less), pivot, qsort(greater));
    }

    private static List<Wall> concatenate(List<Wall> less, Wall pivot, List<Wall> greater) {

        List<Wall> list = new ArrayList<>();

        for (int i = 0; i < less.size(); i++) {
            list.add(less.get(i));
        }

        list.add(pivot);

        for (int i = 0; i < greater.size(); i++) {
            list.add(greater.get(i));
        }

        return list;
    }