Question

I get an error whenever I try to create a simple edge from a node.
Basically, I’ve created two of my own classes called Node and Edge.

The Node class is as follows:

public class Node {

    public String ident;
    public int numLinks;

    public Edge[] neighbours;


    public Node (String ident) {
        this.ident = ident;

    }

    public void setNeighbour (Node start, Node end, int cost, int portNum) {

    }
}

And my Edge class is as follows:

public class Edge {

   Node start;
   Node end;
   int cost;
   int portNum;

   public Edge (Node a, Node b, int cost, int portNum) {
       this.start = a;
       this.end = b;
       this.cost = cost;
       this.portNum = portNum;
   }
}

In my main class, I create two nodes, namely, the start and the end node. The cost and the port number (port number that both these nodes listen to on), I read from a text file and am saving them into an array list named “linkCostList” and “portNumList”.

Now, since each start node can have more than one edge (I’m basically creating a graph), I’m calling the setNeighbour() method in the following way:

for (int i = 0; i < startNode.numLinks; i++) {
    nextNode = new Node (String name of node I read from text file)
    startNode.setNeighbour (startNode, nextNode, linkCostList.get(i), portNumList.get(i));
}

My setNeighbour method is as follows:

public void setNeighbour (Node start, Node end, int cost, int portNum) {
    for (int i = 0; i < start.numLinks; i++) {
        neighbours[i] = new Edge (start, end, cost, portNum);
    }
}

Whenever I compile it, I get an error of the following sort:

Exception in thread "main" java.lang.NullPointerException
    at Node.setNeighbour(Node.java: *line number*)
    at Start.startlsr(Start.java: *line number*)
    at graph.main(lsr.java: *line number*)
}

I understand that this is a null pointer exception, so, somewhere in that loop, I must be doing something wrong. Could anyone please help me figure it out?

Answer 1

Have you initialzed neighbours in your Node class? It looks like the exception is from accessing a null array (neighbours[i]).

Also It looks like the neighbours array will grow/shrink dynamically? In this case, instead of using array, consider using ArrayList so you don’t have to grow neighbours yourself.