Comparator with double type

Question

I have written the following code:

public class NewClass2 implements Comparator
{
    public int compare(Point p1, Point p2)
    {
        retur         


        

Answer 1

It is so convinent in Java 8, choose anyone just as you wish:

Comparator<someClass> cp = (a, b) ->  Double.compare(a.getScore(), b.getScore());

Comparator<someClass> cp = Comparator.comparing(someClass::getScore);

Comparator<someClass> cp = Comparator.comparingDouble(someClass::getScore);

Answer 2

int compare(Double first, Double second) {
    if (Math.abs(first - second) < 1E-6) {
        return 0;
    } else {
        return Double.compare(first, second);
    }
}

Answer 3

The method compare should return an int. It is a number that is either:

• Less than zero, if the first value is less than the second;
• Equal to zero, if the two values are equal;
• Greater than zero, if the first value is greater than the second;

You don't need to return a double. You must return an int to implement the Comparator interface. You just have to return the correct int, according to the rules I outlined above.

You can't simply cast from int, as, like you said, a difference of 0.1 will result in 0. You can simply do this:

public int compare(Point p1, Point p2)
{
    double delta= p1.getY() - p2.getY();
    if(delta > 0) return 1;
    if(delta < 0) return -1;
    return 0;
}

But since comparison of floating-point values is always troublesome, you should compare within a certain range (see this question), something like this:

public int compare(Point p1, Point p2)
{
    double delta = p1.getY() - p2.getY();
    if(delta > 0.00001) return 1;
    if(delta < -0.00001) return -1;
    return 0;
}

Answer 4

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;
    }    
}

Answer 5

Since Java 1.8 you can also use

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

Answer 6

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.