Question

I’m new to list and I have a problem with the method below:
the problem is: java.lang.NullPointerException

Code:

public static List<Integer> input(List<Integer> l)
{
        Node<Integer> pos=l.getFirst();
        System.out.println("Enter num (!=999)");
        int x = reader.nextInt();
        l.insert(null, x);
        while(x!=999)
        {
                System.out.println("Enter num (!=999)");
                l.insert(pos, x);
                pos = pos.getNext();
        }

        return l;
}

Silly me I forgot the input message inside the while…

Answer 1

If you initialized l as an empty list, then the call to

Node<Integer> pos=l.getFirst();

would return null. Then later, you call pos.getNext(); on a null instance, hence the NullPointerException. One way to fix this would be to handle the possibility of the empty list inside your while loop, like this:

while (x != 999)
{
    System.out.println("Enter num (!=999)");
    l.insert(pos, x);
    if (pos == null)
        pos = l.getFirst();
    else
        pos = pos.getNext();
}

When you run this, you’ll see the second problem in your code, which you should be able to resolve.

Good luck!