Java Geometry Algorithm findArea(Point2D p1, Point2D p2, Point2D p3)

Description

Given three points each represents the vertex of a triangle, return the area of this triangle.

License

Open Source License

Declaration

public static double findArea(Point2D p1, Point2D p2, Point2D p3) 

Method Source Code

//package com.java2s;
import java.awt.geom.Point2D;

public class Main {
    /**//  w ww.  java 2  s.c om
     *  Given three points each represents the vertex of a triangle,
     *  return the area of this triangle.
     */
    public static double findArea(Point2D p1, Point2D p2, Point2D p3) {
        if (!preprocess(p1, p2, p3)) {
            return 0.0;
        } else {
            double p4x;
            double p4y;
            double m13 = findSlope(p1.getX(), p1.getY(), p3.getX(), p3.getY());
            double m24 = 0;
            if (Double.isNaN(m13)) {
                p4x = p2.getX();
                p4y = p1.getY();
            } else if (m13 == 0) {
                p4x = p1.getX();
                p4y = p2.getY();
            } else {
                m24 = -1 / m13;
                p4y = (p2.getX() - m24 * p2.getY() - p1.getX() + m13 * p1.getY()) / (m13 - m24);
                p4x = p2.getX() - p2.getY() * m24 + p4y * m24;
                if (findSlope(p2.getX(), p2.getY(), p4x, p4y) == m13) {
                    // 3 points on the same line
                    return 0;
                }
            }
            double dist24 = Math.sqrt(Math.pow((p2.getX() - p4x), 2) + Math.pow((p2.getY() - p4y), 2));
            double dist13 = Math.sqrt(Math.pow((p1.getX() - p3.getX()), 2) + Math.pow((p1.getY() - p3.getY()), 2));
            double area = 0.5 * dist13 * dist24;
            return area;
        }
    }

    /**
     * Return false when two of the points are the same, because
     * the area would be 0.
     */
    private static boolean preprocess(Point2D pa, Point2D pb, Point2D pc) {
        if ((pa.getX() == pb.getX()) && (pa.getY() == pb.getY())) {
            return false;
        } else if ((pa.getX() == pc.getX()) && (pa.getY() == pc.getY())) {
            return false;
        } else if ((pb.getX() == pc.getX()) && (pb.getY() == pc.getY())) {
            return false;
        } else {
            return true;
        }
    }

    /**
     * Find the slope of the line formed by two points
     * (p1x, p1y) and (p2x, p2y).
     */
    public static double findSlope(double p1x, double p1y, double p2x, double p2y) {
        double nominator = p2x - p1x;
        double denominator = p2y - p1y;
        if (denominator == 0) {
            return Double.NaN;
        } else {
            return nominator / denominator;
        }
    }
}

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