Question

I’m currently reviving an old homework assignment, where I’m writing a program that among other functions, involves finding the shortest path in a graph using Dijkstra’s algorithm.

I think I’ve got it right for the most part, but I keep getting NullPointerException at line 58 when executing if(currentNode.getAktuell()).

I’ve been trying several solutions back and forth but can’t seem to figure out what is wrong but prioQueue.poll(); returns null when the queue is empty. I’ve tried to handle that last currentNode that eventually turns into null but have not been able to find a working solution, so I’m starting to think that I’ve missed out on something here.

I would really appreciate it if someone familiar with dijkstras algorithm could help me out here. There’s probably a better solution to the algorithm but I only want help with finding out what is wrong with the one I’ve written, and not “the answer” using someone else’s algorithm.

public static List<String> shortestPath(Graph<String> graph, String från, String till){

    //if(!pathExists(graph, från, till))
    //return null;

    PriorityQueue<DjikstraObjekt<String>> prioQueue = new PriorityQueue<DjikstraObjekt<String>>();
    LinkedHashMap<String, DjikstraObjekt<String>> samling = new LinkedHashMap<String, DjikstraObjekt<String>>();

    for(String bla : graph.getNodes())
        samling.put(bla, new DjikstraObjekt<String>(bla, Integer.MAX_VALUE, null, false));
    samling.get(från).updateVikt(0);
    prioQueue.add(samling.get(från));

    while(!samling.get(till).getAktuell())
    {

        DjikstraObjekt<String> currentNode = prioQueue.poll();
        if(currentNode==null)
            break;
        if(currentNode.getAktuell())
            continue;


        currentNode.aktuellNod();

        for(ListEdge<String> edge : graph.getEdgesFrom(currentNode.getNode()))
        {
            System.out.println("get edges from");
            int nyVikt = edge.getVikt() + currentNode.getVikt();
            DjikstraObjekt<String> toNode = samling.get(edge.getDest());
            if(!toNode.getAktuell() && nyVikt < toNode.getVikt()) {
                toNode.updateVikt(nyVikt);
                toNode.setFrån(currentNode.getNode());
                prioQueue.add(toNode);
            }
        }

    }       

    List<String> djikstaList = new ArrayList<String>();
    for(int i=0;i<samling.size();i++){
        if(samling.get(i).getNode()!=från){
            System.out.println(samling.get(i).getNode());
            djikstaList.add(samling.get(i).getNode());
        }       
    }

    return djikstaList;
}


public class DjikstraObjekt<E> implements Comparable<DjikstraObjekt<E>> {
    private E nod;
    private int vikt;
    private E frånNod;
    private boolean aktuellNod=false;

    public DjikstraObjekt(E nod, int vikt, E frånNod, boolean aktuellNod){

        this.nod=nod;
        this.vikt=vikt;
        this.frånNod=frånNod;
        this.aktuellNod=aktuellNod;

    }
    public E getNode() {
        return nod;
    }
    public void updateVikt(int nyvikt){
        vikt=nyvikt;
    }
    public int getVikt() {
        return vikt;
    }
    public boolean getAktuell() {
        return aktuellNod;
    }
    public void aktuellNod(){
        aktuellNod=true;
    }
    public void setFrån(E från)
    {
        frånNod = från;
    }
    public int compareTo(DjikstraObjekt<E> other) {
        return getVikt() - other.getVikt();
    }
}

Heres my listEdge class:

public class ListEdge<E> {

    private E dest;
    private String namn;
    private Integer vikt;


    public ListEdge(E dest, String namn, Integer vikt){
        this.dest=dest;
        this.namn=namn;
        this.vikt=vikt;

    }

    public E getDest(){
        return dest;
    }
    public void ändraVikt(Integer nyVikt){
        if(vikt<0)
            throw new IllegalArgumentException();
        vikt=nyVikt;

        }
    public String getNamn(){
        return namn;
    }
     public int compareTo(ListEdge other) {
         return this.vikt.compareTo(other.getVikt());
 }

    public int getVikt(){
        return vikt;
    }
    public String toString(){
        return "till " + dest + " med " + namn +" "+ vikt;
    }
}

These should be the relevent methods from my ListGraph class:

public List<E> getNodes(){
    List<E> temp = new ArrayList<E>();
    for(E test : noder.keySet()){
        temp.add(test);

    }
return temp;
}

public List<ListEdge<E>> getEdgesFrom(E nod) {
        List<ListEdge<E>> temp = new ArrayList<ListEdge<E>>();
        if(noder.containsKey(nod)){
            try{
                for(Map.Entry<E, List<ListEdge<E>>> test : noder.entrySet()){
                    if(test.getKey().equals(nod)){
                        System.out.println(nod+" "+test.getKey());
                        for(ListEdge<E> e: test.getValue()){
                            temp.add(e);
                    }
                }
            }
        }
            catch(NoSuchElementException E){

            }

        }
        return temp;
    }

Answer 1

I couldn’t reconstruct the NullPointerException you told us about. As Leandro pointed out, the problem might lay with your implementation of ListEdge and Graph.

I did an implementation of both classes myself to test your code.

The only problem I could find was in the end where you create the result list:

for(int i=0;i<samling.size();i++){
        if(samling.get(i).getNode()!=från){

This will always Result in a NullPointerException because get() expects a key and in your case that’s a String, not an int. To iterate over the Map use something like

List<String> djikstaList = new ArrayList<String>();
for(String key : samling.keySet()){
    if(samling.get(key).getNode()!=från){
        System.out.println(samling.get(key).getNode());
        djikstaList.add(samling.get(key).getNode());
    }       
}

Furthermore, i assume you wan’t to return the actual path from from to to so you would need to add a getter getFrån() to DijkstraObjekt and then build up the list like this:

   String fromNode = samling.get(to).getNode();
   djikstaList.add(to);
   while(fromNode != from){   
       fromNode = samling.get(fromNode).getFrån();
       djikstaList.add(fromNode);
   }

After this the List will contain the complete path (including Start and End node) in reverse order.

If wanted, I can post all of my classes I used for testing/debugging.

Cheers
tannerli