private int crossProduct(Point a, Point b, Point c) {
        int y1 = a.y - b.y;
        int y2 = a.y - c.y;
        int x1 = a.x - b.x;
        int x2 = a.x - c.x;
        return y2 * x1 - y1 * x2;
    }
Cross product to find where c belongs in reference to vector ab.
If result > 0 it means 'c' is on left of ab
   result == 0 it means 'a','b' and 'c' are collinear
   result < 0  it means 'c' is on right of ab
src/com/interview/geometry/JarvisMarchConvexHull.java