Java quicksort - is it possible to compare objects without using a comparator?

Question

Is there a way to quickSort the elements of an array in an ArrayList and at the same time change the order of the ArrayList based on the array sorted without using the Comparator<> function?

    public ArrayList<PatientArray> ageSorter(ArrayList<PatientArray> pa) {
        if (pa.size() <= 1) {
            return pa;
        }

        ArrayList<PatientArray> sorted;
        ArrayList<PatientArray> smaller = new ArrayList<PatientArray>();
        ArrayList<PatientArray> greater = new ArrayList<PatientArray>();

        PatientArray middle = pa.get(0);
        int i;
        PatientArray j;
        for (i = 1; i < pa.size(); i++) {
            j = pa.get(i);

            if ((new SortAge().compare(j, middle)) < 0) { // this object comparator
                smaller.add(j);
            } else {
                greater.add(j);
            }
        }
        smaller = ageSorter(smaller);
        greater = ageSorter(greater);
        smaller.add(middle);
        smaller.addAll(greater);
        sorted = smaller;

        return sorted;
    }

    class SortAge implements Comparator <PatientArray>{
    public int compare(PatientArray a1, PatientArray a2){
        return a1.age-a2.age;
    }

Answer 1

The simplest way to avoid use of a Comparator would be to perform the comparison yourself directly in your quicksort code:

if (pa.get(i).age < middle.age)

While you haven't asked for general review comments, I'll note that there are lots of unnecessary commands in your code.

public ArrayList<PatientArray> ageSorter(ArrayList<PatientArray> pa) {
    if (pa.size() <= 1) {
        return pa;
    }

    ArrayList<PatientArray> smaller = new ArrayList<PatientArray>();
    ArrayList<PatientArray> greater = new ArrayList<PatientArray>();

    PatientArray pivot = pa.get(0);
    for (int i = 1; i < pa.size(); i++) {
        if (pa.get(i).age < pivot.age) {
            smaller.add(j);
        } else {
            greater.add(j);
        }
    }
    smaller = ageSorter(smaller);
    greater = ageSorter(greater);
    smaller.add(middle);
    smaller.addAll(greater);
    return smaller;
}

Also note that typically quicksort is implemented so that the sorting is done in-place - ie without creating new arrays.

As @Holger points out in comments below, choice of pivot (as first element) is also poor. The reasons, and alternatives, are explained here

While technically your algorithm is quicksort it's probably not quick.

Answer 2

You can use the sort method on List class introduced on Java 8 . So your method will be as follow:

public List<PatientArray> ageSorter(ArrayList<PatientArray> pa) {
   pa.sort(Comparator.comparingInt(a -> a.age));
   return pa;
}