Comparator with double type

Since Java 1.8 you can also use

Comparator.comparingDouble(p -> p.getY())

I suggest you use the builtin method Double.compare(). If you need a range for double values to be equal you can use chcek for that first.

return Double.compare(p1.getY(), p2.gety());

or

if(Math.abs(p1.getY()-p2.getY()) < ERR) return 0;    
return Double.compare(p1.getY(), p2.gety());

The problem with using < and > is that NaN will return false in both cases resulting in a possibly inconsistent handling. e.g. NaN is defined as not being equal to anything, even itself however in @suihock's and @Martinho's solutions, if either value is NaN the method will return 0 everytime, implying that NaN is equal to everything.

You don't need to return double.

The Comparator interface is used to establish an ordering for the elements being compared. Having fields that use double is irrelevant to this ordering.

Your code is fine.

Sorry, I was wrong, reading the question again, this is what you need:

public class NewClass2 implements Comparator<Point> {
    public int compare(Point p1, Point p2) {
        if (p1.getY() < p2.getY()) return -1;
        if (p1.getY() > p2.getY()) return 1;
        return 0;
    }    
}