多角形の面積を計算する的な

PolyArea.java

/**
* Copyright (c) 2014 pistatium
* 
* This software is released under the MIT License.
* 
* http://opensource.org/licenses/mit-license.php
*/

import java.util.*;
import java.lang.Math;

/**
 * 多角形の面積を計算する用のクラス
 * 
 */
public class PolyArea {
    
    private static class Point {
        public double x;
        public double y;
        public Point(double x, double y) {
            this.x = x;
            this.y = y;
        }
    }
    private ArrayList<Point> points;
    
    public PolyArea() {
        points = new ArrayList<Point>();
    }
    public void add(double x, double y) {
        points.add(new Point(x, y));
    }
    
    /**
     * 外積を使って面積を計算する
     */
    public double calcArea() {
        int count = points.size();
        
        // 3点以下は０を返す
        if (count < 3) {
            return 0.0;
        }
        
        double sum = 0.0;

        for (int i = 0; i < count; i++) {
            int i_next = (i + 1) % count;
            int i_next2 = (i + 2) % count;
            Point p1 = points.get(i);
            Point p2 = points.get(i_next);
            Point p3 = points.get(i_next2);
            sum += (p1.x - p3.x) * p2.y; 
        }
        return Math.abs(0.5 * sum);
    }
}

Good code! I found this!

    PolyArea area = new PolyArea();
    area.add(0,0);
    area.add(3,0);
    area.add(3,3);
    area.add(0,3);
    System.out.println(area.calc());

9.0