做一个家庭作业问题，第一个项目插入到链表插入罚款，当我插入更多的值，他们似乎没有秩序，因为current.next仍然==null根据调试器，我不明白为什么我的生活。

public void insert(String key)    
{    
 Link newLink = new Link(key);    
Link current = first;     

 Main.nodeCount++;
 while(current != null && key.compareTo(current.dData) > 0)  
 {              
 if(current.next != null)
 current = current.next;
 else
 break;
 } // end while                                                                                                                                    

 if(isEmpty())
 {
 first = newLink;
 last = newLink;
 return;
 }

 if (current == first )        
 {
 if(key.compareTo(current.dData) < 0)
 {
 newLink.next = current;
 current.previous = newLink;
 first = newLink;
 return;
 }//end if

 if(key.compareTo(current.dData) > 0)
 {
 current.next = newLink;
 first.next = newLink;
 newLink.previous = current;
 return;
 }//end if
 }
 if (current == last)
 {
 if(key.compareTo(current.dData) < 0)
 {
    current.previous.next = newLink;
    newLink.previous = current.previous;
    newLink.next = current;
    current.previous = newLink;
    last = current;
 }

 if(key.compareTo(current.dData) > 0)
 {
    newLink.previous = current;
    current.next = newLink;
    last = newLink;
    return;
 }//end if
 return; 
 }//end if

 if (current != first && current != last)
 {
 current.previous.next = newLink;
 newLink.previous = current.previous;
 newLink.next = current;
 current.previous = newLink;    
 }