NullPointer列表中的列表

Question

这是一项家庭作业，所以解释和指针优先于解决方案。我们之前的任务是使用邻接矩阵创建Graph模板。这项作业分配是使用邻接列表修改前一作业。我将之前基于矩阵的代码注释掉以供参考。

    import java.util.*;

public class Graph {
    public Node rootNode;
    public List<Node> nodes = new ArrayList<Node>();
    Map<Integer, List<Integer>> adjList;
    //public int[][] adjMatrix;
    int size;

    public void setRootNode(Node n) {
        rootNode = n;
    }

    public Node getRootNode() {
        return rootNode;
    }

    public void addNode(Node n) {
        nodes.add(n);
    }

    // This method will be called to make connect two nodes
    public void connectNode(Node src, Node dst) {
//      if (adjMatrix == null)
//      {
//          size = nodes.size();
//          adjMatrix = new int[size][size];
//      }

        int srcIndex = nodes.indexOf(src);
        int dstIndex = nodes.indexOf(dst);
//      adjMatrix[srcIndex][dstIndex] = 1;
//      adjMatrix[dstIndex][srcIndex] = 1;
        if (!adjList.containsKey(srcIndex))
            adjList.put(srcIndex, new ArrayList<Integer>());
        adjList.get(srcIndex).add(dstIndex);

        if (!adjList.containsKey(dstIndex))
            adjList.put(dstIndex, new ArrayList<Integer>());
        adjList.get(dstIndex).add(srcIndex);

    }

    private Node getUnvisitedChildNode(Node n) {
        int index = nodes.indexOf(n);
        for (int j = 0; j < size; j++)
            //if (adjMatrix[index][j] == 1 && ((Node) nodes.get(j)).visited == false)
            if (adjList.containsKey(index) && ((Node) nodes.get(j)).visited == false)
                return nodes.get(j);
        return null;
    }

    // BFS traversal: iterative version
    public void bfs() {
        // BFS uses a Queue data structure
        Queue<Node> q = new LinkedList<Node>();
        q.add(rootNode);
        printNode(rootNode);
        rootNode.visited = true;
        while (!q.isEmpty()) {
            Node n = q.remove();
            Node child = null;
            while ((child = getUnvisitedChildNode(n)) != null) {
                child.visited = true;
                printNode(child);
                q.add(child);
            }
        }
    }

    // DFS traversal: iterative version
    public void dfs() {
        // DFS uses a Stack data structure
        Stack<Node> s = new Stack<Node>();
        s.push(rootNode);
        rootNode.visited = true;
        printNode(rootNode);
        while (!s.isEmpty()) {
            Node n = s.peek();
            Node child = getUnvisitedChildNode(n);
            if (child != null) {
                child.visited = true;
                printNode(child);
                s.push(child);
            } else
                s.pop();
        }
    }

    // Utility methods for clearing visited property of node
    private void reset() {
        for (Node n : nodes)
            n.visited = false;
    }

    // Utility methods for printing the node's label
    private void printNode(Node n) {
        System.out.print(n.label + " ");
    }

    // get all neighboring nodes of node n.
    public List<Node> getNeighbors(Node n) {
        int srcIndex = nodes.indexOf(n);
        List<Node> neighbors = new ArrayList<Node>();
//      for (int j = 0; j < adjMatrix[srcIndex].length; j++)
//          if (adjMatrix[srcIndex][j] == 1)
        for (int j = 0; j < adjList.get(srcIndex).size(); j++)
            if (adjList.get(srcIndex).contains(j))
                neighbors.add(nodes.get(j));
        return neighbors;
    }

    // implement recursive version of dfs
    public void dfs(Node n) {
        printNode(n); // visit the node
        n.visited = true;
        for (Node node : getNeighbors(n))
            if (!node.visited)
                dfs(node);
    }

    static class Node {
        public char label;
        public boolean visited = false;

        public Node(char label) {
            this.label = label;
        }
    }

    public static void main(String[] args) {
        // Lets create nodes as given as an example in the article
        Node n0 = new Node('0');
        Node n1 = new Node('1');
        Node n2 = new Node('2');
        Node n3 = new Node('3');
        Node n4 = new Node('4');
        Node n5 = new Node('5');
        Node n6 = new Node('6');
        Node n7 = new Node('7');
        Node n8 = new Node('8');

        // Create the graph, add nodes, create edges between nodes
        Graph g = new Graph();
        g.addNode(n0);
        g.addNode(n1);
        g.addNode(n2);
        g.addNode(n3);
        g.addNode(n4);
        g.addNode(n5);
        g.addNode(n6);
        g.addNode(n7);
        g.addNode(n8);
        g.setRootNode(n8);

        g.connectNode(n0, n1);
        g.connectNode(n0, n3);
        g.connectNode(n0, n8);
        g.connectNode(n1, n7);
        g.connectNode(n3, n2);
        g.connectNode(n3, n4);
        g.connectNode(n3, n4);
        g.connectNode(n4, n8);
        g.connectNode(n2, n7);
        g.connectNode(n2, n5);
        g.connectNode(n5, n6);


        // Perform the DFS and BFS traversal of the graph
//      System.out.print("DFS Traversal (Iterative):    ");
//      g.dfs();
//      g.reset();

        System.out.print("\nDFS Traversal (Recursive):   ");
        g.dfs(g.getRootNode());
        g.reset();

        System.out.print("\nBFS Traversal (Iterative):   ");
        g.bfs();
        g.reset();
    }
}

我在connectNode方法中获取了一个Null指针，我在其中引入了ArrayList。再次请记住这是功课。提前感谢您的帮助。

编辑：如何在stackoverflow中显示行号？

Answer 1

if (!adjList.containsKey(srcIndex))你的adjList为空。您永远不会在对象中设置此列表。