class Vector2d extends AbstractVector implements Serializable {

    public double smallestAngleBetween(Vector2d otherVector)
    {
        double dotProduct = dot(otherVector);
        return Math.acos(dotProduct);
    }

    public  Vector2d(double x, double y);
    public  Vector2d(Vector2d vec);
    public  Vector2d(Vector2i vec);

}