Java-双链表实现

Question

I'm learning data structures. So I'm trying to implement doubly linked list in Java. I'm trying to implement insertAtAnyPosition method

Using this method, I can able to insert the value at any specified index.

public class DoublyLinkedList {

    Node head = null;

    public void insert(int data) {
        Node node = new Node();
        node.data = data;
        node.next = null;

        if(head == null)
            head = node;
        else {
            Node n = head;
            while(n.next != null  ) {
                n = n.next;
            }
            n.next = node;
            node.previous = n.next;
        }
    }

    public void insertAtStart(int data) {

        Node node = new Node();
        node.previous = null;
        node.data = data;

        head.previous = node;
        node.next = head;
        head = node;
    }

    public void insertAtAnyPosition(int index, int data) {
        Node node = new Node();
        node.data = data;

        Node n = head;

        for (int i = 0; i < index-1; i++) {
            n = n.next;
        }
        Node n1 = n.next.previous;

        node.previous = n1;
        node.next = n.next;
        n.next = node;
        n1 = node;

    }

    public void display() {
        Node n = head;

        while(n.next != null) {
            System.out.println(n.data);
            n = n.next;
        }
        System.out.println(n.data);
    }

}

Node Class

public class Node {

    int data;
    Node next;
    Node previous;
}

Main method

public static void main(String[] args) {
        // TODO Auto-generated method stub

        DoublyLinkedList list = new DoublyLinkedList();

        list.insert(12);
        list.insert(4);
        list.insert(1);
        list.insert(30);
        list.insertAtAnyPosition(4,100); // Null Pointer Exception thrown when trying to insert at 4th Index.
        list.display();

    }

"Null Pointer Exception" is thrown when I try to insert element at 4th Index . And the exception is thrown in this line Node n1 = n.next.previous; But the above code works, when i try to insert element at index 1,2,3 but not in 4. Can anyone please help me how to resolve this issue?

Answer 1

Culprit is n.next.privious which must be causing null pointer exception for you

public void insertAtAnyPosition(int index, int data) {
        Node node = new Node();
        node.data = data;

        Node n = head;

        for (int i = 0; i < index-1; i++) {
            n = n.next;
        }
        Node n1 ; 
        if(n.next== null){
.       n1 = n;
}else{
       n1 =n.next.previous;
}
        node.previous = n1;
        node.next = n.next;
        n.next = node;
        n1 = node;

Answer 2

n.next.previous is no different from just n , of course. Unless n is the very last node (which it is, if you insert in position 4), in which case n.next doesn't exist, which gives you the NullPointerException .

Answer 3

Since your 4th position is the end you need to consider a case for end as well. Where you'll have to set the new inserted node next to null and the previous node (that was last) and set it's next to the new node