Creating an edge from a node in Java

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?

Duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — TylerH, Feb 25 '20 at 15:11

score 3 · Accepted Answer · answered May 31 '11 at 01:40

3

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.

answered May 31 '11 at 01:40

Alvin

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thanks for the suggestion, that actually does make sense. I would have something of the following, ArrayList neighbours – Triple777er May 31 '11 at 01:43

score 1 · Answer 2 · answered May 31 '11 at 01:54

Further to @Alvin's answer (you haven't initialized neighbours nor allowed for expansion), try this:

public List<Edge> neighbours = new ArrayList<Edge>();

Also make use of java's "foreach":

    for (Edge edge : start.neighbours) {
        // .. some code
    }

Consider renaming "neighbours" to "edges", since they are just that - the edges of the node.

Finally, it seems you have a bug in your logic. It seems too complicated. You may be attempting to keep references of neighbours of neighbours. Consider simply looking stuff up when you need it.

I'm not trying to make a simple graph, its just in the form of a graph structure. I'm implementing link state routing. So I need to consider each neighbour and its port number so I can communicate with each node. Thanks for your input. — Triple777er, May 31 '11 at 02:16

Creating an edge from a node in Java

2 Answers2