Question

I’m writing a piece of code for a school asignment, we’re implementing sorting algorithms but the algorithm works fine, I’m just having a little thought about why a line of code doens’t work as I expect it to.

The calling code looks like this:

List<Integer> l = new LinkedList<Integer>();
l.add(new Integer(10));
l.add(new Integer(28));
l.add(new Integer(4));
l.add(new Integer(35));
l.add(new Integer(9));

ArraySort.sort(l);
System.out.println(l);

Collections.sort(l);
System.out.println(l);

I will put the code for our sort later on but the question is: How come Collections.sort overwrites the list with the new sorted list while ours does not?

The sort method works fine but simply doens’t update the values of the list l in the calling class. I’m just curious as to how Collections does this, the solution would be to simply return a List from ArraySort.sort and do l = ArraySort.sort but that doesn’t look as good!

Here is the code for the actual sorting:

public static void sort(List l) {
    mergeSort(l);
}

private static List mergeSort(List l) {
    if (l.size() <= 1) {
        return l;
    }
    List left = new LinkedList();
    List right = new LinkedList();
    int middle = l.size() / 2;
    for (int i = 0; i < middle; i++) {
        left.add(l.get(i));
    }
    for (int i = middle; i < l.size(); i++) {
        right.add(l.get(i));
    }

    left = mergeSort(left);
    right = mergeSort(right);

    l = merge(left, right);
    return l;
}

private static List merge(List left, List right) {
    List result = new LinkedList();
    while (left.size() > 0 || right.size() > 0) {
        if (left.size() > 0 && right.size() > 0) {
            if ((int) left.get(0) <= (int) right.get(0)) {
                result.add(left.get(0));
                left.remove(0);
            } else {
                result.add(right.get(0));
                right.remove(0);
            }
        } else if (left.size() > 0) {
            result.add(left.get(0));
            left.remove(0);
        } else if (right.size() > 0) {
            result.add(right.get(0));
            right.remove(0);
        }
    }
    return result;
}

Hope someone can clear this up for me.

Answer 1

The idea would be to change this:

l = merge(left, right);
return l;

into this (not sure if the order is preserved – I suppose it is):

l.clear();
l.addAll(merge(left, right)); //no need to return the list, it is the same