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javanullpointerexceptiondequedoubly-linked-list

Nullpointer exception when trying to create a deque in Java

I have been trying to create a deque in Java, but I get a nullpointer exception when I add a new node. Here is the helper class I use for the doubly-linked list: 

private class Node {
    private Item item;
    private Node next;
    private Node prev;
}

Then I add nodes to the beginning of the list as follows:

    public void addFirst(Item item) {
    Node oldfirst = first;
    first = new Node();      
    first.prev = null;
    first.item = item;
    first.next = oldfirst;
    oldfirst.prev = first;        
    N++;
}

I get a NullPointer exception for the oldfirst.prev = first; line. Also, when I compile the code I get the following warning:

Warning: The field Deque2<Item>.Node.prev is never read locally

What might I be doing wrong?

Solution

  • The exception means that oldfirst is null. And you initialise it as Node oldfirst = first, so it means that first is null.