How to override compareTo method for doubles?

Question

I'm currently having some trouble understanding how the compareTo method works for the Comparable class, and how to override it. I have array of pairs, each of which hold 2 double values, that I'm trying to sort. This is what I tried:

static class Pair implements Comparable<Pair>
{
    double x;
    double y;
    Pair(double x, double y)
    {
        this.x = x;
        this.y = y;
    }
    public double compareTo(Pair other)
    {
        return y - other.y;
    }
}

However, it doesn't compile and instead gives me this error:

Main.java:5: error: Pair is not abstract and does not override abstract method compareTo(Pair) in Comparable
    static class Pair implements Comparable<Pair>
           ^
Main.java:14: error: compareTo(Pair) in Pair cannot implement compareTo(T) in Comparable
        public double compareTo(Pair other)
                      ^
  return type double is not compatible with int
  where T is a type-variable:
    T extends Object declared in interface Comparable
2 errors

It worked when it was with integers, but not with doubles, why is that? And how could I make it work with doubles? Thanks.

Answer 1

For example:

   static class Pair implements Comparable<Pair> {

        double x;
        double y;

        Pair(double x, double y) {
            this.x = x;
            this.y = y;
        }

        public int compareTo(Pair other) {
            if (y > other.y) {
                return 1;
            } else if(y<other.y)  {
                return -1;
            } else {
                return 0;
            }
        }
    }

or (better variant):

    static class Pair implements Comparable<Pair> {

        double x;
        double y;

        Pair(double x, double y) {
            this.x = x;
            this.y = y;
        }

        public int compareTo(Pair other) {
            return Double.compare(this.y, other.y);
        }
    }