java - Arrays.sort(pointsBySlope, p.SLOPE_ORDER) 是否取决于pointsBySlope[] 是什么？

Question

Point 是一个类，它有两个实例变量，它们是点的坐标。 (x,y)Point 实现了 Comparable 接口(interface)。但除了通过坐标进行比较之外，任何点 (x1,y1) 还可以与 (x2,y2) 进行比较，即它们相对于另一个点 (x0,y0) 的相对斜率。以下是 Point 类:

import java.util.Comparator;

public class Point implements Comparable<Point> {

// compare points by slope
public final Comparator<Point> SLOPE_ORDER = new Ordering(); 
private final int x;                              // x coordinate
private final int y;                              // y coordinate

// create the point (x, y)
public Point(int x, int y) {
    /* DO NOT MODIFY */
    this.x = x;
    this.y = y;
}

private class Ordering implements Comparator<Point>{

    @Override
    public int compare(Point p1, Point p2) {
        if(slopeTo(p1) - slopeTo(p2) < 0) return -1;
        else if(slopeTo(p1) - slopeTo(p2) > 0) return 1;
        else return 0;
    }

}

// slope between this point and that point
public double slopeTo(Point that) {
    /* YOUR CODE HERE */
    if(this.y == that.y && this.x == that.x) return Double.NEGATIVE_INFINITY;
    else if(this.y == that.y) return 0;
    else if(this.x == that.x) return Double.POSITIVE_INFINITY;
    else return ((double)that.y - (double)this.y)/((double)that.x - (double)this.x);
}

// is this point lexicographically smaller than that one?
// comparing y-coordinates and breaking ties by x-coordinates
public int compareTo(Point that) {
    /* YOUR CODE HERE */
    if(this.y < that.y){
        return -1;
    }else if(this.y > that.y){
        return 1;
    }else{
        if(this.x < that.x) return -1;
        else if(this.x > that.x) return 1;
        else return 0;
        }
    }
}

现在我编写了一个简单的测试类来测试按斜率排序是否有效。

public class Test {

public static void main(String[] args) {
    In in = new In(args[0]);
    int N = in.readInt();
    Point[] points = new Point[N];
    Point[] pointsBySlope = new Point[N];
    for (int i = 0; i < N; i++) {
        int x = in.readInt();
        int y = in.readInt();
        points[i] = new Point(x, y);
        pointsBySlope[i] = points[i];
    }
    Arrays.sort(points);
    Arrays.sort(pointsBySlope);

    Point p = points[2];
    Arrays.sort(pointsBySlope, p.SLOPE_ORDER);

    p = points[3];
    Arrays.sort(pointsBySlope, p.SLOPE_ORDER);
    for(int i=0;i<pointsBySlope.length;i++){
        System.out.println(pointsBySlope[i].toString()+" , ");
    }
    System.out.println("\n\n"+"=================================");
    Arrays.sort(pointsBySlope);
    Arrays.sort(pointsBySlope, p.SLOPE_ORDER);
    for(int i=0;i<pointsBySlope.length;i++){
        System.out.println(pointsBySlope[i].toString()+" , ");
    }
}

}

输出为

(19000, 10000) , 
(18000, 10000) , 
(14000, 10000) , 
(21000, 10000) , 
(32000, 10000) , 
(1234, 5678) , 

(19000, 10000) , 
(14000, 10000) , 
(18000, 10000) , 
(21000, 10000) , 
(32000, 10000) , 
(1234, 5678) ,

为什么两种情况下的输出不同？最初按自然顺序对 pointsBySlope 数组进行排序，然后按 SLOPE_ORDER 排序，当直接按 SLOPE_ORDER 排序时，如何产生不同的输出？

Answer 1

在这两种排序中，p 都是 (19000, 10000)。然后，每个 (14000, 10000)、(18000, 10000)、(21000, 10000)、(32000, 10000) 的“到 p 的斜率”为 0。这意味着您的比较器不会对这些点重新排序；它只会让它们保持排序之前的顺序。对于你的第二个输出，这恰好是点的自然顺序；对于第一个输出，它不是。