双向链表java实现

package com.liu.Link;

class DoublyLinkedApp
{
public static void main(String[] args)
   {                             
   DoubleLink theList = new DoubleLink();

   theList.insertFirst(22);     
   theList.insertFirst(44);
   theList.insertFirst(66);

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

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

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

   theList.displayForward();    

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

   theList.displayForward();     
   }  
} 


public class DoubleLink {
    private Link6 first;
    private Link6 last;
    public DoubleLink()
    {
        first = null;
        last = null;
    }
    public boolean isEmpty()
    {
        return first == null;
    }
    public void insertFirst(long d)
    {
        Link6 newLink = new Link6(d);
        if(isEmpty())
        {
            last = newLink;
        }else
            first.previous = newLink;
        newLink.next = first;
        first = newLink;
    }
    public void inesrtLast(long d)
    {
        Link6 newLink = new Link6(d);
        if(isEmpty())
            first = newLink;
        else
        {
            last.next = newLink;;
            newLink.previous = last;
        }
        last = newLink;
    }
    
    //插入到某个值的后面
    public boolean insertAfter(long key,long d)
    {
        Link6 current = first;
        while(current.dData != key)
        {
            current = current.next;
            if(current == null)
            {
                return false;
            }
            
        }
        Link6 newLink = new Link6(d);
        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 Link6 deleteFirst()
    {
        Link6 temp = first;
        if(first.next == null)//这种情况是只存在一个数据的时候
            last = null;
        else
            first.next.previous = null;
        first = first.next;
        return temp;
    }
    
    public Link6 deleteLast()
    {
        Link6 temp = last;
        if(first.next == null)
            first = null;
        else
            last.previous.next = null;
        last = last.previous;
        return temp;
    }
    
    public Link6 deleteKey(long key)
    {
        Link6 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.print("List (first-->last): ");
        Link6 current = first;
        while(current != null)
        {
            current.displayLink();
            current = current.next;
        }
        System.out.println("");
    }
    
    public void displayBackward()
    {
        System.out.print("List (last-->first): ");
          Link6 current = last;           
          while(current != null)         
          {
             current.displayLink();      
             current = current.previous; 
          }
          System.out.println("");
          
    }
    
}

class Link6
{
    public long dData;
    public Link6 next;
    public Link6 previous;
    
    public Link6(long d)
    {
        dData = d;
    }
    
    public void displayLink()
    {
        System.out.print(dData+" ");
    }
}