java - 表达式的类型必须是数组类型，但它被解析为 ArrayList

Question

我试图找出从面积方法调用数组的正确方法，然后应该计算给定点的面积。不确定从每个数组中选择特定 x 和 y 坐标的正确方法是什么。

MyPolygon 类

  import java.util.ArrayList;
import java.awt.geom.Point2D;
import java.awt.geom.Point2D.Double;

/**
 * A class that represents a geometric polygon. Methods are provided for adding
 * a point to the polygon and for calculating the perimeter and area of the
 * polygon.
 */
class MyPolygon {

    // list of the points of the polygon
    private ArrayList<Point2D.Double> points;

    /**
     * Constructs a polygon with no points in it.
     */
    public MyPolygon() {
        points = new ArrayList<Point2D.Double>();
    }

    /**
     * Adds a point to the end of the list of points in the polygon.
     * 
     * @param x
     *            The x coordinate of the point.
     * @param y
     *            The y coordinate of the point.
     */
    public void add(double x, double y) {
        points.add(new Point2D.Double(x, y));
    }

    /**
     * Calculates and returns the perimeter of the polygon.
     * 
     * @return 0.0 if < 2 points in polygon, otherwise returns the sum of the
     *         lengths of the line segments.
     */
    public double perimeter() {

        if (points.size() < 2) {
            return 0.0;
        }

        int i = 0;
        double d = 0;
        double total = points.get(0).distance(points.get(points.size() - 1));

        while (i < points.size() - 1) {
            Point2D.Double point1 = points.get(i);
            // double x = point1.x;
            // double y = point1.y;
            Point2D.Double point2 = points.get(i + 1);
            // double x1 = point2.x;
            // double y1 = point2.y;

            d = point1.distance(point2);
            // d = Math.sqrt(Math.pow(x1 - x,2) + Math.pow(y1 - y, 2));

            total = total + d;
            i++;

        }
        return total;

    }

    /**
     * Calculates and returns the area of the polygon.
     * 
     * @return 0.0 if < 3 points in the polygon, otherwise returns the area of
     *         the polygon.
     */
    public double area() {

        int i = 0;
        double a = 0;
        double total = 0;
        total = total + a;

        if (points.size() < 3) {
            return 0.0;
        }

        for (int m = 0; m < points.size(); m++) {

            total = total + (points[m].x() * points[m + 1].y()) - (points[m].y() * points[m + 1].x());
        }
        return 0.5 * total;
    }

}

测试员类

class PolygonTester {

    public static void main(String args[]) {
        MyPolygon poly = new MyPolygon();
        poly.add(1.0,1.0);
        poly.add(3.0,1.0);
        poly.add(1.0,3.0);
        System.out.println(poly.perimeter());
        System.out.println(poly.area());
    }

}

Answer 1

你的标题实际上已经是解决方案了。您使用 points[m] 这是数组表示法。但点不是数组。这是一个 list 。使用 points.get(int i) 代替，就像在 perimeter() 中所做的那样。