java黑皮书课后习题：11.15凸多边形面积

public static void main(String[] args) {
	System.out.print("Enter the number of the points: ");
	Scanner input = new Scanner(System.in);
	int n = input.nextInt();
	System.out.println("Enter the coordinates of the points: ");
	double[][] points = new double[n][2];
	for(int i = 0;i < n;i++) {
		points[i][0] = input.nextDouble();
		points[i][1] = input.nextDouble();
	}
	System.out.print("The total area is " + getArea(points));
 

}  

//三角形面积公式 S = 0.5 * |->AB| * | ->AC |
public static double getS(Point2D p1,Point2D p2,Point2D p3) {
	double s1 = p1.distance(p2);
	double s2 = p2.distance(p3);
	double s3 = p1.distance(p3);
	double s = (s1 + s2 + s3) / 2;
	return Math.sqrt(s * (s - s1) * (s - s2) * (s - s3));
}

public static double getArea(double[][] p) {
	Point2D[] points2D = new Point2D[p.length];
	for(int i = 0;i < points2D.length;i++) {
		points2D[i] = new Point2D(p[i][0],p[i][1]);
	}
	double area = 0;
	for(int i = 0;i < points2D.length - 2 ;i++)
		area += getS(points2D[0],points2D[i+1],points2D[i+2]);
	return area;
}