Doubly-linked list with data structure

[Java Tip]

Code:

public 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; // cannot find it
    }
    Link newLink = new Link(dd); // make new link

    if (current == last) // if last link,
    {
      newLink.next = null; 
      last = newLink; 
    } else // not last link,
    {
      newLink.next = current.next; 
      current.next.previous = newLink;
    }
    newLink.previous = current; 
    current.next = newLink; 
    return true; // found it, insert
  }

  public Link deleteKey(long key){
    Link current = first; 
    while (current.dData != key)
    {
      current = current.next;
      if (current == null)
        return null; // cannot find it
    }
    if (current == first) // found it; first item?
      first = current.next; 
    else
      current.previous.next = current.next;

    if (current == last) // last item?
      last = current.previous; 
    else
      // not last
      current.next.previous = current.previous;
    return current; // return value
  }

  public void displayForward() {
    System.out.print("List (first to last): ");
    Link current = first; // start at beginning
    while (current != null) // until end of list,
    {
      current.displayLink();
      current = current.next; // move to next link
    }
    System.out.println("");
  }

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

  public static void main(String[] args) {
    DoublyLinkedList theList = new DoublyLinkedList();

    theList.insertFirst(22);
    theList.insertFirst(44);
    theList.insertLast(33);
    theList.insertLast(55);

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

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

    theList.displayForward();

    theList.insertAfter(22, 77); // insert 77 after 22
    theList.insertAfter(33, 88); // insert 88 after 33

    theList.displayForward();
  }

}

class Link {
  public long dData; // data item

  public Link next; // next link in list

  public Link previous; // previous link in list

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

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

}