How to turn a directed graph to undirected graph?

My method "addEdge" adds a node to the adjacency list of the selected node, but I want the method to add the selected node to its adjacency list as well and turn it into an undirected graph.

public class Main {

    public static void main(String[] args) {

        Graph g = new Graph();
        Node n1 = new Node("A");
        Node n2 = new Node("B");
        Node n3 = new Node("C");
        Node n4 = new Node("D");
        n1.addEdge(n2);
        n2.addEdge(n3);
        n3.addEdge(n4);
        n4.addEdge(n1);

    }
}
 class Node{
    String node;
    boolean isVisited;
    LinkedList<Node> edge;

     Node(String node){
        this.node = node;
        edge = new LinkedList<>();
    }
    public void addEdge(Node node){
        edge.add(node);
     }

    public LinkedList<Node> getEdge(){
        return edge;
    }
 }