java - Class for Doubly-Linked List -

i have code class handling doubly-linked list doesn't have empty head or tail node. have modified code obtained supposed doubly-linked list throwing following exception. how prevent exception occurring , changes required remove empty head or tail node?

exception in thread "main" java.lang.nullpointerexception     @ p4dll.removelast(p4dll.java:105)     @ p4dll.main(p4dll.java:197)

code:

class listnode {     object element;     listnode next;     listnode prev;      public listnode( object anelement )     {         element = anelement;         next = null;         prev = null;     }      public listnode( object anelement,  listnode nextptr, listnode prevptr)     {         element = anelement;         next = nextptr;         prev = prevptr;     } } // end class listnode  public class p4dll {      private listnode head;     private listnode tail;     private int size;      public p4dll()     {         head = null;         tail = null;         size = 0;     }      public void addatfirst( object anelement )     {         //create new node stored @ beginning of list         listnode newnode = new listnode( anelement);         newnode.next = head;         head = newnode;         size++;          if(tail == null)             tail = head;     }      public void addatlast( object anelement )     {         //create new node stored @ begginning of list         listnode newnode = new listnode( anelement);          if(tail == null)         {             head = tail = newnode;         }         else         {             tail.next = newnode;             tail = tail.next;         }          size++;     }      public object removefirst( )     {         //the list has no elements remove         if ( isempty( ) )         {             return null;         }          //get ptr first data node         listnode ptr = head.next;          //save data in node can returned         object data = head.next.element;          //link around node         ptr.next.prev = ptr.prev;         ptr.prev.next = ptr.next;          size--;          //return data in node         return data;     }      public object removelast( )     {         //if list has no elements there nothing return         if ( isempty( ) )         {             return null;         }          //get ptr last data node         listnode ptr = tail.prev;          //save data in node can returned         object data = tail.element;          //link around node         ptr.next.prev = ptr.prev; // p4dll.java:105         ptr.prev.next = ptr.next;          size--;          //return data in node         return data;     }      public object getfirstelement( )     {         if ( size == 0 )         {             return null;         }         else         {             return head.next.element;         }     }      public object getlastelement( )     {         if ( size == 0 )             return null;         else             return tail.prev.element;     }      public int getnumelements( )     {         return size;     }      public boolean isempty( )     {         return size == 0;     }      public void displaylist( )     {         if ( isempty( ) )         {             system.out.println ( "the list empty.\n" );         }         else         {             system.out.println ( this.tostring( ) );         }     }         public string tostring( )     {         string returnstr = "";          if ( ! isempty( ) )         {             listnode ptr = head.next;              while ( ptr != tail )             {                 returnstr += ptr.element.tostring ();                 returnstr += "\n";                 ptr = ptr.next;             }         }          return returnstr;     }   // makes list empty  public void clear( )  {      if ( ! isempty( ) )      {          head.next = tail;          tail.prev = head;          size = 0;      }  }    public static void main ( string[] args )   {         p4dll list = new p4dll( );         list.addatfirst ( "abe" );         list.addatfirst ( "beth");         list.addatlast ( "ed" );         list.displaylist ();         system.out.println( "the number of elements in list "                             + list.getnumelements() );          system.out.println( "\nnow remove last element , display.\n" );         list.removelast(); // p4dll.java:197         list.displaylist ();   }  } 

you code producing nullpointerexception inside removelast() method. fix it! :-d

it seems copied code somewhere, including spelling mistakes:
http://www.cramster.com/answers-jul-09/computer-science/doubly-linked-list-xi5te-program-implements-class-doubly-linked_619688.aspx

the code written assuming head , tail nodes. you'll need re-write method (and others) account new reality.

edit:

i finished reviewing diff between code , code found in above link. suggestion scrap , write linked list scratch. not written assuming head , tail nodes, it's broken in @ least 1 way. you've noticed since size-- missing supplied removelast() , had add yourself. there's issue of comments talking "pointers", , spelling mistakes, , etc.

here's starting point. working implementation of removelast():

public object removelast( ) {     if (size == 0) return null;     final object result = last.element;     if (size == 1)         first = last = null;     else {         last = last.prev;         last.next = null;     }     size--;     return result; } 

since you're not using head or tail nodes, i'm using fields called first , last better clarity.