java - Generic Doubly Linked List Implementation -

so have basic generic implantation of generic doubly linked list. have created insert method going add node according order.

public class doublyll <t extends comparable<t>> { dnode<t> head; dnode<t> tail;  public void insertinorder(t item) { //to create "ordered" linked list     dnode <t> n = new dnode<>();      if (head == null) {         head = n;         n.data = item;     }     else {         dnode<t> temp = head;         while (temp != null && n.data.compareto(temp.data) > 0) { // line 18             temp = temp.next;         }         if (temp == head) { //inserting node in first             head.prev = n;             n.next = head;             head = n;             n.data = item;         }         else if (temp == null) { // inserting @ last             tail.next = n;             n.prev = tail;             tail = n;             n.data = item;         }          else { //inserting in middle             n.prev = temp.prev;             n.next = temp;             temp.prev.next = n;             temp.prev = n;             n.data = item;         }      }       }  @override public string tostring() {     dnode temp = head;     string str = "";     while (temp != null) {         str += temp.data + " ";         temp = temp.next;     }     return str; }    public static void main(string[] args) {     doublyll<integer> list = new doublyll<>();     list.insertinorder(2);     list.insertinorder(222); //line 62     list.insertinorder(22222);     system.out.println(list);  } }  class dnode<t> { t data; dnode prev; dnode next; } 

however, when i'm running i'm getting nullpointerexception @ line 18 , 62. can rid of make ordered list like, "2, 22, 2222?

it's hard what's problem without stack trace looks instead of

if (head == null) {     head = n;     n.data = item; } 

you should have

if (head == null) {     head = n;     tail = n;     n.data = item; } 

otherwise tail remains null.