Double Linked Lists

Question

Hi everyone,
I have some problems about Double Linked Lists which I can not overcome. When I execute the following codes, there is an exception occured that is called NullPointer Exception in inser method.
So how can I fix it ??
class DoublyLinkedNode {
     // properties
     DoublyLinkedNode nextNode;
     DoublyLinkedNode previousNode;
     int size;
     int data;
     // constructors
     public DoublyLinkedNode(int data,DoublyLinkedNode nextNode, DoublyLinkedNode previousNode){
          this.data = data;
          this.nextNode = nextNode;
          this.previousNode = previousNode;
          size = 0;
     // methods
     public void setNext(DoublyLinkedNode next){
          nextNode = next;
     public void setPrevious(DoublyLinkedNode pre){
          previousNode = pre;
     public DoublyLinkedNode getNext(){
          return nextNode;
     public DoublyLinkedNode getPrevious(){
          return previousNode;
     public int getData(){
          return data;
     public void setData(int value){
          data = value;
class DoubleLinkedList {
     // properties
     int size;
     DoublyLinkedNode tail;
     DoublyLinkedNode head;
     String s = "";     
     // constructors
     public DoubleLinkedList(){
          head = null;
          tail = null;
          size = 0;
     // methods
     public boolean isEmpty(){
          if(head == null)
               return true;
          else
               return false;
     public DoublyLinkedNode next(){          
          return head.getNext();
     public DoublyLinkedNode previous(){
          return head.getPrevious();     
     public void insert(int data){
          head = new DoublyLinkedNode(data, head.getNext(),head.getPrevious());
          size++;
     public String toString(){
          s += head.getData() + "";
          head = head.getNext();
          if(isEmpty())
               return s;
          else
               return toString();               
     public static void main(String args[]){
          DoubleLinkedList list = new DoubleLinkedList();
          list.insert(111);
          list.insert(21);          
          list.insert(311);
          list.insert(41);
          list.insert(51);                    
          list.insert(16);
          list.insert(117);
          System.out.println(list.toString());
//          for(int a = 0; a < 5; a++){
//               list.previous();
}

Answer 1

Again, I changed insert method which is now correct I think.
But now, toString cause only appear the last object on the console.
So how can I fix toString method ???
     public void insert(int data){
          DoublyLinkedNode pointer = head.getPrevious();
          DoublyLinkedNode pointer2 = pointer.getNext();
          head = new DoublyLinkedNode(data,pointer2, pointer );     
     }