Question

I have found many related post here but couldn’t get my answer.Why this run-time error ?

static List<Integer>[] adj = (List<Integer>[]) new ArrayList[1000];

    public static void main(String[] args) {
        int edge, u, v, source;
        Scanner input = new Scanner(System.in);
        edge = input.nextInt();
        for (int i = 0; i < edge; i++) {
            u = input.nextInt();
            v = input.nextInt();
            adj[v].add(u); // Null pointer Exception
            adj[u].add(v); // Null pointer Exception
        }

Answer 1

First you need to initialize each elements of your array. Because until you do this, your reference in the array are not pointing to any object.

So, before that for-loop, you can add this one, to initialize your List inside the Array: –

for (List<Integer> elem: adj) {
    elem = new ArrayList<Integer>();
}

Also, it would be better if you have List of List rather than an array of List. So, you can declare your list as: –

static List<List<Integer>> adj = new ArrayList<List<Integer>>();

An advantage of using an ArrayList is that, you don’t have to limit your size at the beginning. So, you can add any number of elements. But, if it is a requirement to create a fixed size list, you can pass the size parameter in your ArrayList constructor.

And then you need to change your element adding code from: –

adj[v].add(u);

to: –

adj.get(v).add(u);