Question

I am testing the different sort methods (Selection, bubble insertion) and I am trying to use Comparator at the same time.

So far I have two classes; Main and Selectionsort.

My main looks like this:

public class Main {

    /**
     * @param args
     */
    public static void main(String[] args) {
        String[] anArray = {"Ludo", "matador", "ChessTitan", "Rottefælden"};

        for (int i=0; i<anArray.length-1; i++) {
            for (int j=i+1; j<anArray.length; j++) {
                if (anArray[j].compareTo(anArray[i]) < 1) {

                    String temp = anArray[i];
                    anArray[i] = anArray[j];
                    anArray[j] = temp;

                }
            }

        }

        for (String string : anArray) {
            System.out.println(string);
        }
    }
}

And my selectionSort looks like this:

public class SelectionSort implements Comparator<String> {

    @Override
    public int compare(String o1, String o2) {      
        return o1.compareTo(o2);
    }
}

What I want to do is use my Comparator when I use selection sort.

How is this possible?

Answer 1

replace this line:

if (anArray[j].compareTo(anArray[i]) < 1) {

with this:

if (comparator.compare(anArray[j],anArray[i]) < 1) {

where comparator is an instance of the comparator you want to use.

---

Since your Comparator has no state, you’ll probably want to assign it to a static final field.

private static final Comparator<String> COMP = new SelectionSort();

So now the above code would read

if (COMP.compare(anArray[j],anArray[i]) < 1) {

---

Here’s a solution for your last question. Create two overloaded static methods, one with a Comparator, one without, then also create a Comparator that uses the natural order (amazingly no such thing is available to my knowledge in the JDK). Something like this:

private static final Comparator<? extends Comparable> NATURAL_ORDER = new Comparator<Comparable>() {
    @Override
    public int compare(final Comparable o1, final Comparable o2) {
        return o1.compareTo(o2);
    }
};

private static <T> Comparator<T> naturalOrder() {
    return (Comparator<T>) NATURAL_ORDER;
}

public static <T> void sort(final T[] array) {
    if (!Comparable.class.isAssignableFrom(array.getClass().getComponentType())) {
        throw new IllegalArgumentException(
              "Array Component Type must implement Comparable");
    }

    sort(array, naturalOrder());
}

public static <T> void sort(final T[] array, final Comparator<? super T> comparator) {
    // implement sort here
}