Java Null Pointer Exception with array of ArrayList
<p>I have found many related post here but couldn't get my answer.Why this run-time error ?</p>

<pre><code>static List&lt;Integer&gt;[] adj = (List&lt;Integer&gt;[]) 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 &lt; edge; i++) {
            u = input.nextInt();
            v = input.nextInt();
            adj[v].add(u); // Null pointer Exception
            adj[u].add(v); // Null pointer Exception
        }
</code></pre>
