public static double getDistance(Node node1, Node node2) {
		if (node1 == null || node2 == null) {
			System.out.println("WARNING: Link.getDistance(): node == null");
			return -1;  // TODO: il faudrait mieux renvoyer une exception
		}
		double delta_x = node1.pos_x - node2.pos_x;
		double delta_y = node1.pos_y - node2.pos_y;
		double distance = Math.sqrt(delta_x*delta_x + delta_y*delta_y);
		return distance;
	}