Doubly Linked Lists operation example in core java

Doubly Linked List basic operation example:

/**
 *
 */

/**
 * @author Abhinaw.Tripathi
 *
 */

class Link
{
 public long dData;
 public Link next;
 public Link previous;

 public Link(long d)
 {
this.dData=d;
 }

 public void displayLink()
 {
 System.out.println(dData+ " ");
 }

}

class DoublyLinkedList
{
  private Link first;
  private Link last;

  public DoublyLinkedList()
  {
first=null;
last=null;
  }
  public boolean isEmpty()
  {
return (first==null);
  }

  public void insertFirst(long dd)
  {
 Link newLink=new Link(dd);
 if(isEmpty())
 {
 last=newLink;

 }
 else
  first.previous=newLink;
 newLink.next=first;
 first=newLink;
  }

  public void insertLast(long dd)
  {
 Link newLink=new Link(dd);
 if(isEmpty())
 first=newLink;
 else
 {
 last.next=newLink;
    newLink.previous=last;
 }
 last=newLink;
  }

  public Link deleteFirst()
  {
 Link temp=first;
 if(first.next == null)
 last=null;
 else
 first.next.previous=null;
 first=first.next;
 return temp;
  }

  public Link deleteLast()
  {
 Link temp=last;
 if(first.next == null)
 first=null;
 else
 last.previous.next=null;
 last=last.previous;
 return temp;
  }


  public boolean insertAfter(long key,long dd)
  {
 Link current =first;
 while(current.dData!=key)
 {
 current=current.next;
 if(current == null)
 return false;
 }

 Link newLink=new Link(dd);

 if(current==last)
 {
 newLink.next=null;
     last=newLink;
 }
 else
 {
 newLink.next=current.next;
 current.next.previous=newLink;
 }

 newLink.previous=current;
 current.next=newLink;
 return true;
  }

  public Link deletekey(long key)
  {
 Link current=first;
 while(current.dData!=key)
 {
 current=current.next;
 if(current==null)
 return null;
 }
 if(current == first)
 first=current.next;
 else
 current.previous.next=current.next;

 if(current==last)
 last=current.previous;
 else
 current.next.previous=current.previous;
 return current;
  }

  public void displayForward()
  {

 System.out.println("List (first-----.last:)");
 Link current =first;
 while(current!=null)
 {
 current.displayLink();
 current=current.next;
 }
 System.out.println(" ");
  }

  public void displayBackward()
  {
 System.out.println("List (last---->first)");
 Link current=last;
 while(current!=null)
 {
 current.displayLink();
 current=current.previous;
 }
 System.out.println("  " );
  }

}


public class DoublyLinkedApp
{
public static void main(String[] args)
{
DoublyLinkedList theList=new DoublyLinkedList();
theList.insertFirst(22);
theList.insertFirst(44);
theList.insertFirst(66);

theList.insertLast(11);
theList.insertLast(33);
theList.insertLast(55);

theList.displayForward();
theList.displayBackward();

theList.deleteFirst();
theList.deleteLast();
theList.deletekey(11);

theList.displayForward();
theList.displayBackward();

theList.insertAfter(22, 77);
theList.insertAfter(33, 88);

theList.displayForward();

}

}


The desire output will be

List (first-----.last:)
66 
44 
22 
11 
33 
55 

List (last---->first)
55 
33 
11 
22 
44 
66 
  
List (first-----.last:)
44 
22 
33 

List (last---->first)
33 
22 
44 
  
List (first-----.last:)
44 
22 
77 
33 
88