Null Pointer Exception

[Rose Jac]

Dijkstra algorithm:
What changes should i make?

public final class Dijkstra
{
    HashMap<Integer, String> verticesMap = new HashMap();
    HashMap<String, HashMap<String, Integer>> adjacencyMap = new HashMap();
    Dijkstra(ArrayList<Node> nodeList)
    {
        for(Node n : nodeList)
        {
            verticesMap.put(n.id, n.name);
        }
        for(Node n : nodeList)
        {
            HashMap<String, Integer> hm = new HashMap();
            for(Node n1 : n.directAdjacencyList)
            {
                hm.put(verticesMap.get(n1.id), 1);
                adjacencyMap.put(n.name, hm);
            }
        }
        System.out.println(" vertices map " + verticesMap);
        System.out.println(" adjacencyMap " + adjacencyMap);
        System.out.println(verticesMap.get(5));
        System.out.println(verticesMap.get(1));
        doDijkstra(verticesMap.get(1), verticesMap.get(5));
    }
    public Map<String, Integer> getAdjacentVertices(String vertexName){
        return adjacencyMap.get(vertexName);
    }
    public int getAdjacentCost(String startVertexName, String endVertexName){
        return adjacencyMap.get(startVertexName).get(endVertexName);
    }

    public int doDijkstra(String vertexStartName, String vertexEndName)
    {
        ArrayList<String> pathList = new ArrayList();
        HashMap<String, Integer> costPerNode = new HashMap<String, Integer>();
        ArrayList<String> processed = new ArrayList<String>();
        ArrayList<String> inQueue = new ArrayList<String>();
        costPerNode.put(vertexStartName, 0);

        Iterator<String> iterator = getAdjacentVertices(vertexStartName).keySet().iterator();
        while(iterator.hasNext())
        {
            String next = iterator.next();
            costPerNode.put(next, getAdjacentCost(vertexStartName, next));
            inQueue.add(next);
        }
        System.out.println( " Q " + inQueue);
        processed.add(vertexStartName);
        System.out.println(" processed " + processed);
        
        while(!inQueue.isEmpty())
        {
            iterator = getAdjacentVertices(inQueue.get(0)).keySet().iterator();    ///null pointer 
            while(iterator.hasNext())
            {
                String next = iterator.next();
                if(!processed.contains(next))
                   inQueue.add(next);
                if(costPerNode.get(next) == null)
                {
                    int cost = costPerNode.get(inQueue.get(0)) + getAdjacentCost(inQueue.get(0), next);
                    costPerNode.put(next, cost);
                }
                else if(costPerNode.get(inQueue.get(0)) + getAdjacentCost(inQueue.get(0), next) < costPerNode.get(next))
                {
                    costPerNode.put(next, costPerNode.get(inQueue.get(0)) + getAdjacentCost(inQueue.get(0), next));
                    inQueue.add(next);
                }
            }
            processed.add(inQueue.get(0));
            inQueue.remove(0);
        }
        processed.clear();
        inQueue.clear();

        if(costPerNode.get(vertexEndName) != null)
            return costPerNode.get(vertexEndName);
        else
            return 0;

    }
        
    public static void main(String args[])
    {
        Node a = new Node(1, "A");
        Node b = new Node(2, "B");
        Node c = new Node(3, "C");
        Node d = new Node(4, "D");
        Node e = new Node(5, "E");
        ArrayList<Node> nodeList = new ArrayList();
        nodeList.add(a);
        nodeList.add(b);
        nodeList.add(c);
        nodeList.add(d);
        nodeList.add(e);
        for(Node n : nodeList)  n.directAdjacencyList = new ArrayList();
        a.directAdjacencyList.add(b);
        a.directAdjacencyList.add(c);
        b.directAdjacencyList.add(a);
        b.directAdjacencyList.add(d);
        c.directAdjacencyList.add(a);
        c.directAdjacencyList.add(d);
        e.directAdjacencyList.add(d);

        Dijkstra da = new Dijkstra(nodeList);          //null pointer 
    }
}

output:
run:
vertices map {1=A, 2=B, 3=C, 4=D, 5=E}
Exception in thread "main" java.lang.NullPointerException
adjacencyMap {E={D=1}, A={B=1, C=1}, B={D=1, A=1}, C={D=1, A=1}}
E
A
Q [B, C]
processed [A]
at simulation.Dijkstra.doDijkstra(Dijkstra.java:73)
at simulation.Dijkstra.<init>(Dijkstra.java:43)
at simulation.Dijkstra.main(Dijkstra.java:155)
Java Result: 1
BUILD SUCCESSFUL (total time: 2 seconds)


help please!

[Rose Jac]

Ok. I figured it out. I forgot to add D's directadjacentnodes.
Thanks!

[Ralph Cook]

I suggest you put this code into a debugger. Most of them allow you to set a break for unhandled exception, set one for null pointer and see where the deref of the null is.

I don't know if you're truncating the information or what, but mostly java exceptions can/do produce stack traces, which include the line number of the line where the exception occurred.

You do not say whether you know what a null pointer exception is, or what attempts you have made to figure out your problem. I am not inclined to debug your program for you from scratch; this forum really does better with more specific questions than "what's wrong here?", though the worst of those don't even give the output.

Do you have/use eclipse, netbeans, or something similar? Do you have any way of stepping through your code?

rc