public void addArrow(Node startNode, Node endNode) {
        Arrow arrow = new Arrow(startNode, endNode);
        if (!arrows.contains(arrow)) {
            arrows.add(arrow);
            for (Cell cell : cells) {
                if (cell.getId().equals(startNode.getId())) {
                    cell.addArrow(arrow);
                }
            }
        }
        repaint();
    }