Null pointer when using doubly linked list

Question

I am trying to create 2d doubly linked circular array, reading data from a txt file and creating nodes automatically. My program is reading the first line properly, but when it reaches next line and time to create next node, null pointer occurs. I dont understand why it is happening please help me.

public class project1 {
    public static void main(String[] args) {
        File file = new File("Input0.txt");
        List mList = new List();
        try {

            Scanner sc = new Scanner(file);

            while (sc.hasNextLine()) {
                String line = sc.nextLine();
                Node kNode = new Node(line.charAt(0));
                mList.insertLast(kNode);
                for (int j = 1; j < line.length(); j++) {
                    System.out.println(line.charAt(j));
                }
            }
            sc.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

class Node {
    int data = 0;
    char key;
    Node nPrev, nNext, tNode, prev, next;

    Node() {
    }

    Node(char c) {
        key = c;
    }

    Node(Node x, Node p, Node q) {
        tNode = x;
        nPrev = p;
        nNext = q;
    }

    Node(int x, Node p, Node q) {
        data += x;
        prev = p;
        next = q;
    }
}

class List {
    Node head;

    List() {
        head = new Node();
        head.prev = head;
        head.next = head;
    }

    void insertFirst(char x) {
        insertBefore(head.next, x);
    }

    void insertFirst(Node x) {
        insertBefore(head.next, x);
    }

    void insertLast(char x) {
        insertAfter(head.prev, x);
    }

    void insertLast(Node x) {
        insertAfter(head.prev, x);
    }

    void insertAfter(Node pos, int i) {
        Node n = new Node(i, pos, pos.next);
        pos.next.prev = n;
        pos.next = n;
    }

    void insertAfter(Node pos, Node x) {
        Node n = new Node(x, pos, pos.next);
        pos.next.prev = n;
        pos.next = n;
    }

    void insertBefore(Node pos, int i) {
        Node n = new Node(i, pos.prev, pos);
        pos.prev.next = n;
        pos.prev = n;
    }

    void insertBefore(Node pos, Node x) {
        Node n = new Node(x, pos.prev, pos);
        pos.prev.next = n;
        pos.prev = n;
    }
}

these are the error. Null pointer happens when its trying to create second node. it creates first node properly than says Null pointer right after.

line 77 = pos.next = n;

line 69 = insertAfter(head.prev, x);

line 18 = mList.insertLast(kNode);

Could you please post the complete code. You missed out the insertBefore(node,char) function definition. — Aditya
– Aditya, Commented Jan 20, 2014 at 0:30

Peter · Accepted Answer · 2014-01-20 00:38:42Z

0

You are missing to set the pointers for the newly inserted node:

 void insertBefore(Node pos, int i) {
    Node n = new Node(i, pos.prev, pos);
    pos.prev.next = n;
    pos.prev = n;
    /** should contain also something similar to: **/
   n.prev = pos;
   n.next = pos;
}

Actually what you are doing when inserting the first node you are setting your head.prev to n and your head.next to n. So at the next insert you are passing n (which is head.next now) and try to invoke n.prev.next with n.prev == null and the line following n.prev which again is null.

answered Jan 20, 2014 at 0:38

Peter

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