Java Null Pointer Exception with array of ArrayList

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);

Answer 2

That's because you don't allocate adj[v]. You can't call the add method on null.

You could do

   for (int i = 0; i < edge; i++) {
        u = input.nextInt();
        v = input.nextInt();
        if (adj[v]==null) adj[v] = new ArrayList();
        adj[v].add(u); 
        if (adj[u]==null) adj[u] = new ArrayList();
        adj[u].add(v); 
    }

Answer 3

You've created an array for sure but you've not allocated each element of the array. As such when you're trying to add to the non-existant elements, you experience a NullPointerException.

As a side-note, what you're trying to achieve would look a lot better if you create a List of List. In other words:

List<List<Integer>> yourList = new ArrayList<List<Integer>>();

You can then initialize each of your individual list inside yourList and then put elements into it.

Answer 4

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

says adj is a reference to 1000 ArrayList references. You need to initialize each of those ArrayLists as well.

private final int MAX = 1000;
static List<Integer>[] adj = (List<Integer>[]) new ArrayList[MAX];
public static void main(String[] args) {
    for (int i = 0; i < MAX; i++){
       adj[i] = new ArrayList();
    }

    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
    }