在 Array.sort() 中实现 Java 比较器

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, ie :

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) ?

Answer 1

Use the shortest way possible, eg 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?

Answer 2

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

    }



});