@Override
    public boolean equals(Object  obj)
    {
       // check for reference equality
       if(this == obj) return true;

       // type check
       if( !(obj instanceof Vertex2D) ) return false;

       // cast to correct type
       Vertex2D point = (Vertex2D)obj;
       if(point == null) return false;

       return ( Math.abs(this.x-point.getX()) < 0.001 && Math.abs(this.y-point.getY()) < 0.001);
    }