Polygon.java

import java.awt.geom.Line2D;
import java.util.ArrayList;
import java.util.Random;
 
public class Polygon {
    private ArrayList<Point> points = new ArrayList<Point>();
 
    public static void main(String[] args) {
        Polygon poly = new Polygon();
        poly.randomPolygon();
        for (int i = 0; i < poly.points.size(); i++) {
            System.out.print(poly.points.get(i).getX() + ",");
            System.out.print(poly.points.get(i).getY() + ";");
        }
    }
 
    private void addPoint(double x, double y) {
        points.add(new Point(x, y));
    }
 
    public void randomPolygon() {
        Random rand = new Random();
 
        addPoint(rand.nextDouble() * 10, rand.nextDouble() * 10);
        addPoint(rand.nextDouble() * 10, rand.nextDouble() * 10);
        
        for (int i = 0; i < 1000000; i++) {
        	int segment = rand.nextInt(points.size());
            points.add(segment, new Point());
            
            if(!isConvex()) {
            	points.remove(segment);
            } 
        }
    }
    
    private boolean isConvex() {
        for (int j = 0; j < points.size(); j++) {
            Point point1 = points.get(j);
            Point point2 = points.get((j + 1) % points.size());
            Point point3 = points.get((j + 2) % points.size());
            if(!counterClockwise(point1, point2, point3)) {
            	return false;
            }
        }
         return true;
    }
    
    
    private boolean counterClockwise(Point point1, Point point2, Point point3) {
        return (point2.getX() - point1.getX())*(point3.getY() - point1.getY()) - (point2.getY() - point1.getY())*(point3.getX() - point1.getX()) > 0;
    }

    private double getArea() {
    	double area = 0.0;
    	for(int i = 0; i < points.size() ; i++) {
    		  int j = (i+1) % points.size();
    		  area += points.get(i).getX() * points.get(j).getY();
    		  area -= points.get(i).getY() * points.get(j).getX();
    	}
    	return Math.abs(area/2);
    }
 
    // checks that point3 and point4 are on opposite sides of the line passing
    // through point1 and point2
    private boolean intersects(Point point1, Point point2, Point point3,
            Point point4) {
 
        // subtract a very small amount, to fix the edge case where a ray tested
        // passes through an intersection
        return Line2D.linesIntersect(point1.getX(), point1.getY(), point2
                .getX() - 0.001, point2.getY() - 0.001, point3.getX(), point3
                .getY(), point4.getX(), point4.getY());
    }
 
    private double getSlope(Point point1, Point point2) {
        double slope = (point1.getY() - point2.getY()) / (point1.getX() - point2.getX());
        return slope;
    }
 
    /**
     * If a ray extending from the new point, intersects the edges of the
     * polygon in and odd number of places it must be contained otherwise it is
     * not.
     */
    public boolean contains(double x, double y) {
 
        int intersections = 0;
        for (int i = 0; i < points.size(); i++) {
            // using mod so it wraps back around connecting the first point to
            // the last point
            Point point1 = points.get((i + 1) % points.size());
            Point point2 = points.get(i);
 
            if (intersects(point1, point2, new Point(x, y), new Point(x, 10))) {
                intersections++;
            }
        }
        return intersections == 1;
    }
}