sorting a list using Collections causes errors

Question

i have an object of type MatOfDMAtch and i converted to a list and i want to sort it as shown below using Collections, but when i run the code i receive the below errors.

please let me know why i am receiving these errors and how to solve it.

CODE:

List dMatchList = matDMatch.toList();
    System.out.println("dMatchList.size(): " + dMatchList.size());

    sortMAtches(0, 100, dMatchList);
}

private static void sortMAtches(double minDist, double maxDist, List list) {
    // TODO Auto-generated method stub
    java.util.Collections.sort(list);
    /*for (int i = 0; i < list.size(); i++) {
        System.out.println("lsit[" + i + "] = " + list.get(i));
    }*/
}

ERRORS:

Exception in thread "main" java.lang.ClassCastException: org.opencv.features2d.DMatch cannot be cast to java.lang.Comparable
at java.util.ComparableTimSort.countRunAndMakeAscending(Unknown Source)
at java.util.ComparableTimSort.sort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at java.util.Arrays.sort(Unknown Source)
at java.util.Arrays$ArrayList.sort(Unknown Source)
at java.util.Collections.sort(Unknown Source)
at test.FeaturesMatch.sortMAtches(FeaturesMatch.java:96)
at test.FeaturesMatch.main(FeaturesMatch.java:91)

UPDATE:

now i used the comparator interface, but as you see in the acode below at the commented out line, i cant use .compareTo() method! how to use it?

List<DMatch> dMatchList = matDMatch.toList();
    DMatch[] dMatArray = matDMatch.toArray();
    System.out.println("dMatchArray.length(): " + dMatArray.length);
    System.out.println("dMatchList.size(): " + dMatchList.size());

    java.util.Collections.sort(dMatchList, compa);
}

static Comparator<DMatch> compa = new Comparator<DMatch>() {

    public int compare(DMatch arg0, DMatch arg1) {
        // TODO Auto-generated method stub
        return arg0.distance.???; //compareTo() does not exist??
    }
};

score 1 · Answer 1 · answered Mar 30 '15 at 11:00

1

Either your DMatch class must implement the Comparable interface or you must call Collections.sort( ... ) with an apropriate Comparator that can compare your DMatch objects.

answered Mar 30 '15 at 11:00

Florian Schaetz

10,454
5
32
58

Jordi Castilla · Accepted Answer · 2015-03-30T11:59:51.897

1

You must implement a custom Comparator like this (change getYourValueToCompare() by your getter):

Collections.sort(dMatchList, new Comparator<DMatch>() {
    public int compare(DMatch a1, DMatch a2) {
        return a1.getYourValueToCompare().compareTo(a2.getYourValueToCompare());
    }
});

NOTE: Remember to implement equals method for DMatch class or you won't see any sorting!!

edited Mar 30 '15 at 11:59

answered Mar 30 '15 at 11:02

Jordi Castilla

26,609
8
70
109

Follow my instructions and implement inner comparator, you will get compareTo method without problem, also **REMEMBER CREATE EQUALS METHOD IN DMatch CLASS** – Jordi Castilla Mar 30 '15 at 11:59

score 0 · Answer 3 · answered Mar 30 '15 at 11:01

0

Class org.opencv.features2d.DMatch do not implement the interface java.lang.Comparable. So it is not comparable by default. You have to write your own Comparator.

And call java.util.Collections.sort(list, new MyDMatchComparator());

public class MyDMatchComparator implements Comparator<DMatch>{

    @Override
    public int compare(DMatcho1, DMatch o2) {
       //compare logic
    }
}

answered Mar 30 '15 at 11:01

Jens

67,715
15
98
113

@user2121 Thats why your class do not implment Comparable. You have to implement it your own way for instance: `if (arg0.distance ==arg0.distance) return 0; if (arg0.distance ==arg0.distance) return -1` or what your logic is. – Jens Mar 30 '15 at 11:59

sorting a list using Collections causes errors

3 Answers3