Point2D Comparator throwing an error

Question

I'm trying to sort a List<Point2D> of points using Collections.sort(). I believe I set up this comparator correct. Anyways, it's throwing an error saying: The method sort(List<T>, Comparator<? superT>) in the type Collections is not applicable for the arguments (List<Point2D>, new Comparator<Point2D.Double>(){}). Does anybody understand why my compiler is throwing this error?

Collections.sort(points, new Comparator<Point2D.Double>() {
    public int compare(Point2D.Double p1, Point2D.Double p2) {
        return Double.compare(p1.getX(), p2.getX());
    }
});

If you have a list of `Point2D` then you need a comparator for `Point2D`. Currently you have a comparator for `Point2D.Double` which is not the same. — Turamarth, Oct 06 '16 at 07:56

score 2 · Accepted Answer · edited Oct 06 '16 at 08:53

2

just remove .Double , your Comparator should be of same type (or parent of type) of your List.

   Collections.sort(points, new Comparator<Point2D>() {
        public int compare(Point2D p1, Point2D p2) {
            return Double.compare(p1.getX(), p2.getX());
        }
    });

edited Oct 06 '16 at 08:53

Amber Beriwal

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answered Oct 06 '16 at 07:57

Amer Qarabsa

5,678
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"Same type" meaning a super type is also valid. – MordechayS Oct 06 '16 at 07:58
yes super type is also valid not sub type, Point2D.Double is subtype of Point2D – Amer Qarabsa Oct 06 '16 at 08:01
its my fault should have cleared that in my answer :) – Amer Qarabsa Oct 06 '16 at 08:17
1

@AmerQarabsa Please edit the answer. It can have parent also. – Amber Beriwal Oct 06 '16 at 08:49
@AmberBeriwal thanks , I mentioned that in my comment yet you are right , better to mention it in the answer – Amer Qarabsa Oct 06 '16 at 08:53
@AmerQarabsa Thank You for considering. Removed my down vote already. – Amber Beriwal Oct 06 '16 at 08:54
@AmberBeriwal Thanks – Amer Qarabsa Oct 06 '16 at 08:56

score 0 · Answer 2 · edited May 23 '17 at 12:06

From the Java documentation of Collections, we can see that in method sort(List<T> list, Comparator<? super T> c), if List is of T then Comparator can be of type T or its parent classes (including T itself).

In your case, you have List<Point2D> &Comparator<Point2D.Double>, and Point2D.Double is not the parent class of Point2D.

Regarding <? super T> refer this link.

Change your code to following:

List<Point2D> points = new ArrayList<Point2D>();
Collections.sort(points, new Comparator<Point2D>() {
    public int compare(Point2D p1, Point2D p2) {
        return Double.compare(p1.getX(), p2.getX());
    }
});

Point2D Comparator throwing an error

2 Answers2