Question

When I run this the merge sort is not happening and I can’t figure out where the problem is. int brelem and SortThread task arguments are there only because I need them to Override a method of the parent class properly so they do not play any role in this case.

class MergeSort extends Sort {

@Override
ArrayList sort(ArrayList<Integer> a, int brelem, SortThread task) {
    if (a.size() > 1) {

        ArrayList<Integer> firstHalf = new ArrayList<>(a.subList(0, a.size() / 2));

        firstHalf.subList(0, a.size() / 2);
        firstHalf=sort(firstHalf, a.size() / 2, task);

        ArrayList<Integer> secondHalf = new ArrayList<>(a.subList(a.size()/2, a.size()));

        secondHalf=sort(secondHalf, secondHalf.size(), task);

        ArrayList<Integer> temp = merge(firstHalf, secondHalf);
        a.subList(0, temp.size());
    }
    return a;
}

private static ArrayList merge(ArrayList<Integer> list1, ArrayList<Integer> list2) {
    ArrayList<Integer> temp = new ArrayList<>();

    int current1 = 0;
    int current2 = 0;
    int current3 = 0;

    while (current1 < list1.size() && current2 < list2.size()) {
        if (list1.get(current1) < list2.get(current2)) {
            temp.set((current3++), list1.get(current1++));
        } else {
            temp.set((current3++), list2.get(current2++));
        }
    }

    while (current1 < list1.size()) {
        temp.set((current3++), list1.get(current1++));
    }

    while (current2 < list2.size()) {
        temp.set((current3++), list2.get(current2++));
    }

    return temp;
}

}

Answer 1

Among other things, you are returning a, not temp. As far as I can tell, you never modify a.

Perhaps you are confused about what List.subList() does. It returns a view of the list, so whatever you’re doing with it in sort‘s penultimate statement will have no effect since you’re disregarding the return value.