java深度First Search递归

Question

目前我尝试了解有关java的更多信息并尝试编写图表代码。 我有一个HashMap。目前我想实施DFS，但它不会起作用。

我想将从startNode可以实现的所有节点添加到List

原则上代码是正确的还是正确的？如果我测试我的程序，我会得到一个NullPointerException。但我不知道为什么。我知道一个NPE，但在我看来，我的代码不应该抛出NPE。因此，请不要将其标记为NullPointer问题的副本。

非常感谢您的帮助！

这是我的代码：

/**
 * For a given node the method returns all nodes the outgoing edges of the node directly lead to.
 * 
 * @return list containing all directly connected nodes.
 */
@Override
public List<Node> getAdjacentNodes(Node startnode) {
    List<Node> nodes = null;
    if (startnode != null && this.nodes.values().contains(startnode)) {
        return startnode.getAdjacentNodes();
    }
    nodes = new LinkedList<Node>();
    return nodes;
}

@Override
public List<Node> getAdjacentNodes(int startnode) {
    return getAdjacentNodes(nodes.get(startnode));
}public List<Node> depthFirstSearch(Node startNode){
    LinkedList<Node> nodeList = null;
    resetState();

    //Get a List with all Adjacent Nodes from the startNode
    List<Node> node = getAdjacentNodes(startNode);
    //Test for each of the ADjacent Nodes from startNode if the current status is white
    for(Node child : node) {

        //Is the status white, run the recursive method
        if(child.status == Node.WHITE) {
            runDFS(child);
        }

    }

    return nodeList;
}

public LinkedList<Node> runDFS(Node child) {
    //Create a LinkedList as in the depthFirstSearch-Method
    LinkedList<Node> nodeList = null;
    //Set the status from the current child to GRAY
    child.status = Node.GRAY;
    //For each Child from the AdjacentsNodes from the child check if color is white
    for(Node nextNode : getAdjacentNodes(child)) {
        if (hasWhiteNeighbor(nextNode) == true) {
            //is color white execute the runDFS method again to get to the last node
            runDFS(nextNode);
        }
        //If the color is not white set it to black and add the node to List, because last node was found
        else {
            child.status = Node.BLACK;
            nodeList.add(child);
        }
    }
    return nodeList;
}