2 Dimensional Linked List Array

Question

I am trying to have 2D Linked List Array as:

private LinkedList<Integer>[] adjLst;

graph(int n){
    noOfNodes = n-1;

    for(int i=0;i<=noOfNodes;i++){
        adjLst[i] = new LinkedList<Integer>();
    }
}

But when i am calling it from my main class as:

graph g =new graph(13);

It is throwing an NullPointerException?

Jon Skeet · Accepted Answer · 2013-07-29 22:32:13Z

8

You've never initialized adjLst, so it still has its default value of null. You need something like:

adjLst = new LinkedList<Integer>[noOfNodes + 1];
for (int i = 0; i <= noOfNodes; i++) {
    adjLst[i] = new LinkedList<Integer>();
}

It's not clear why you're subtracting 1 from n and then going up to and including that value, mind you. I'd find this easier to understand:

// Note: fixed case of class. Please follow Java naming conventions
Graph(int n){
    adjLst = new LinkedList<Integer>[n];
    for (int i = 0; i < n; i++) {
        adjLst[i] = new LinkedList<Integer>();
    }
}

answered Jul 29, 2013 at 22:32

Jon Skeet

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