Comparator with double type

问题:

I have written the following code:

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

If I let's say have two double numbers, 3.2 - 3.1, the difference should be 0.1. When I cast the number to an int, however, the difference ends up as 0, which is not correct.

I therefore need compare() to return a double, not an int. The problem is, my getX field is a double. How can I solve this problem?

回答1:

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;
    }    
}

回答2:

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.

回答3:

Since Java 1.8 you can also use

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

回答4:

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;
}

回答5:

I just want to expand on Peter Lawrey answer on JDK 8, if you do it like this:

public class NewClass2 implements Comparator<Point> {
    public int compare(Point p1, Point p2) {
        return Double.compare(p1.getY(), p2.gety());
    }    
}

You could define this comparator using a lambda expression pretty easily

(Point p1,Point p2) -> Double.compare(p1.getY(), p2.gety())  

Better yet, you could use a member reference like this:

Double::compare

回答6:

Use Double.compare(/**double value 1*/, /**double value 2*/); with a new Comparator for your model class double value.

public static List<MyModel> sortByDouble(List<MyModel> modelList) {
        Collections.sort(modelList, new Comparator<MyModel>() {
            @Override
            public int compare(MyModels1, MyModels2) {
                double s1Distance = Double.parseDouble(!TextUtils.isEmpty(s1.distance) ? s1.distance : "0");
                double s2Distance = Double.parseDouble(!TextUtils.isEmpty(s2.distance) ? s2.distance : "0");
                return Double.compare(s1Distance, s2Distance);
            }
        });
        return modelList;
    }

回答7:

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);

回答8:

Well, you could multiply those double values by an appropriate factor before converting into integer, for eg. in your case since its only one decimal place so 10 would be a good factor;

return (int)(p1.getY()*10 - p2.getY()*10);

回答9:

Double min  = Arrays.stream(myArray).min(Double::compare).get();