Implement Java Comparator in Array.sort()

Question

Assume I have an array of two dimensional arrays which represent points I want to compare. For example I can compare the distance from the origin. What I can do is create a class Point:

class Point implements Comparable<Point>{
     // class variables, constructor
     public int compareTo(Point p) {
         return (x*x + y*y).compareTo(p.x*p.x + p.y*p.y);
     }
}

and fill up an array of type Point with all the points and then use Array.sort(pointsArray). This can be also done on the following ways:

1) Arrays.sort(points, Comparator.comparing(p -> p[0]*p[0] + p[1]*p[1]));

or

2) Arrays.sort(points, (p1, p2) -> p1[0]*p1[0] + p1[1]*p1[1] - p2[0]*p2[0] - p2[1]*p2[1]);

without creating any new data type. The explanation can be found in this question.

Now, what if I would need to compare the x coordinate first and, if the comparison shows no difference, the y coordinate, i.e. :

class Point implements Comparable<Point>{
     // class variables, constructor
     public int compareTo(Point p) {
         int cmp = x.compareTo(p.x);
         if(cmp == 0) return y.compareTo(p.y);
         return cmp;
     }
}

How can this be translated in 1) and 2) ?

Comparator.<Point>comparingInt(p -> p.x).thenComparingInt(p -> p.y). (Or comparingDouble, or whatever). — Andy Turner
– Andy Turner, Commented Sep 26, 2019 at 13:25

Kayaman · Accepted Answer · 2019-09-26 13:27:19Z

1

Use the shortest way possible, e.g. Comparator.comparing(Point::getX).thenComparing(Point::getY).

There's no reason to use the longer versions. They're less readable, and it's too easy to make mistakes. As an example, here's one possible implementation

(p1, p2) -> {
    if(p1.x == p2.x) {
        return p1.y.compareTo(p2.y);
    }
    return p1.x.compareTo(p2.x);
}

Takes a lot longer to understand what's happening there, doesn't it?

answered Sep 26, 2019 at 13:27

Kayaman

73.8k6 gold badges91 silver badges128 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

CaspianTerrazzo · Accepted Answer · 2019-09-26 13:32:11Z

0

Just use brackets to make a greater name space for lambda function to compare

Arrays.sort(points, (p1, p2) -> {
    int x1 = p1[0];
    int x2 = p2[0];
    int y1 = p1[1];
    int y2 = p2[1];
     // just for readability no need to make local variables
    if(x1 == x2){
      // compare y1 and y2 values here return 1 for greater 0 for equal -1 for less then
    }else{
    // x compare

    }



});

answered Sep 26, 2019 at 13:32

CaspianTerrazzo

2,15815 silver badges29 bronze badges

Collectives™ on Stack Overflow

Implement Java Comparator in Array.sort()

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related