ClassCastException while sorting an array using Comparator

Question

I have like this class to compare Y Cordinates (descending order) and I have predefined Point class .

In main class ,I am sending Compare Y array which is keeping points(type =Point) Arrays.sort(array); and it gives me ClassCastExeption ,

how can ı fix this problem.

public class CompareY  implements Comparator<Point> {

public CompareY(){
}

@Override
public int compare(Point a1, Point a2) {

    if (a1.y > a2.y)
        return -1;
    else if (a1.y < a2.y)
        return 1;
    else {
        if (a1.x < a2.x)
            return 1;
        else if (a1.x > a2.x)
            return -1;
        else
            return 0;
    }

}

and show us how you use your construct. btw you can achieve line breacks in questions by adding to whitespaces at the end of each line — Dude
– Dude, Commented May 9, 2015 at 12:15
Can you show the code where you get the error. Looks like you are trying to sort an Object array that contains objects of different types. — Chetan Kinger
– Chetan Kinger, Commented May 9, 2015 at 12:35
If x and y are int, you can use return a1.y - a2.y to avoid verbosity, if not, then Math.signum() will help. — Alex Salauyou
– Alex Salauyou, Commented May 9, 2015 at 12:45
@serdar See my answer for an explanation on why you get a ClassCastException and how to fix this. I think the question that you have posted is complete and no additional code needs to be posted from your end to narrow down the issue. +1 — Chetan Kinger
– Chetan Kinger, Commented May 9, 2015 at 12:53

Chetan Kinger · Accepted Answer · 2015-05-09 12:59:10Z

3

The Arrays.sort(Object[] a) method expects a type that implements Comparable. If you are creating an array of CompareY objects, you should note that CompareY does not implement Comparabe and therefore, you get a ClassCastException at runtime since CompareY cannot be cast to a Comparable. Similarly, if you are creating an array of Point objects, you will still get a ClassCastException since Points does not implement Comparable

To fix the issue :

Create an array of Point objects.
Use the Arrays.sort method that takes a Comparator.

Example :

Point[] points = new Point[]{new Point(..),new Point(...)};
Arrays.sort(points,new CompareY());

Alternately, you can have the Point class implement Comparable and define the natural ordering for Points. You can then use the Array.sort(Object[] a) method directly.

edited May 9, 2015 at 12:59

answered May 9, 2015 at 12:48

Chetan Kinger

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5 Comments

serdar özer Over a year ago

ı already implement comparable interface in point class and it is work properly.ı used Array.sort(points,new CompareY()); it works .thank you for answer.

Chetan Kinger Over a year ago

@serdarözer I am posting this as part of the question. Are you still getting a ClassCastException after implementing Comparable? Can you edit your question with the code that creates the array and passes it to the Arrays.sort method please?

Chetan Kinger Over a year ago

@serdarözer So you are saying that you are facing the issue even after implementing Comparable? I don't think that's possible. That being said, I believe my answer pointed (pun intended) you in the right direction so don't forget to upvote and accept the answer :)

serdar özer Over a year ago

no just write after this piece of code Array.sort(points,new CompareY()); and ı accepted answer but ı dont have enough reputation for upvote :-)

Chetan Kinger Over a year ago

@serdarözer My bad. I keep forgetting the restriction on upvotes until a certain rep is reached ;)

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