无法实现嵌套 class 比较器

Question

我正在实现一个点 class 并尝试使用嵌套 class 来实现一个比较器，它将根据两点的斜率进行比较。我在实现这样的比较器时遇到了问题，并且不明白如何在我的 main() 函数中使用它。

这是我在尝试编译时收到的错误消息：

Point.java:20: error: non-static variable this cannot be referenced from a static context
       if (Point.this.slopeTo(pt1) > Point.this.slopeTo(pt2)) {
                ^
Point.java:20: error: non-static variable this cannot be referenced from a static context
       if (Point.this.slopeTo(pt1) > Point.this.slopeTo(pt2)) {
                                          ^
Point.java:22: error: non-static variable this cannot be referenced from a static context
       } else if (Point.this.slopeTo(pt1) < Point.this.slopeTo(pt2)) {
                       ^
Point.java:22: error: non-static variable this cannot be referenced from a static context
       } else if (Point.this.slopeTo(pt1) < Point.this.slopeTo(pt2)) {
                                                 ^
4 errors

以下是我的代码：

import java.util.Comparator;
import java.lang.Comparable;
import java.util.Arrays;
import edu.princeton.cs.algs4.StdDraw;
import edu.princeton.cs.algs4.StdOut;


public class Point implements Comparable<Point> {
    private final int x;
    private final int y;

    // constructs the point (x, y)
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    private static class bySlope implements Comparator<Point> {
        public int compare(Point pt1, Point pt2) {
            if (Point.this.slopeTo(pt1) > Point.this.slopeTo(pt2)) {
                return 1;
            } else if (Point.this.slopeTo(pt1) < Point.this.slopeTo(pt2)) {
                return -1;
            }
            return 0;
        }
    }

    // draws this point
    public void draw() {
        StdDraw.point(x, y);
    }

    // draws the line segment from this point to that point
    public void drawTo(Point that) {
        StdDraw.line(this.x, this.y, that.x, that.y);
    }

    // string representatio
    public String toString() {
        return "(" + x + ", " + y + ")";
    }

    // compare two points by y-coordinates, breaking ties by x-coordinates
    public int compareTo(Point that) {
        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;
            }
        }
    }

    // the slope between this point and that point
    public double slopeTo(Point that) {
        double lineSlope;
        // horizontal line
        if (that.y == this.y && that.x != this.x) {
            lineSlope = (1.0 - 1.0) / 1.0;
        } else if (that.y != this.y && that.x == this.x) {
            lineSlope = Double.POSITIVE_INFINITY;
        } else if (that.y == this.y && that.x == this.x) {
            lineSlope = Double.NEGATIVE_INFINITY;
        } else {
            lineSlope = (that.y - this.y) / (that.x - this.x);
        }
        return lineSlope;
    }

    // compare two points by slopes they make with this point
    public Comparator<Point> slopeOrder() {
        return new bySlope();
    }

    public static void main(String args[]) {
        Point[] myPoints = new Point[3];

        myPoints[0] = new Point(1,2);
        myPoints[1] = new Point(3,4);
        myPoints[2] = new Point(7,8);

        Arrays.sort(myPoints, new Point.bySlope());

        for (int i = 0; i < myPoints.length; i++) {
            StdOut.println(myPoints[i].toString());
        }
    }
}

Answer 1

您应该将内部 class 实例提供给 Arrays.sort，以便从父 class 实例的角度比较点。为此，您不应在 main() 函数中创建新实例，而应从 Point 实例中获取它。

所以，在 main 函数中你应该使用这样的东西：

Point pivot;
... // set up pivot point here
Arrays.sort(myPoints, pivot.slopeOrder());

您还应该从嵌套的 class 定义中删除 "static"，使其成为内部 class，从而可以实际访问其父成员：

private class bySlope implements Comparator<Point> {

无法实现嵌套 class 比较器

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