// adapted from Waite, Lafore  "Data Structures and 
// Algorithms"
import java.io.*;          
class Link
   {
   public double dData;              // data item
   public Link next;                 // next link in list
// -------------------------------------------------------------
   public Link(double d)             // constructor
      { dData = d; }
// -------------------------------------------------------------
   public void displayLink()         // display this link
      { System.out.print(dData + " "); }
// -------------------------------------------------------------
   }  // end class Link

class FirstLastList{
   private Link first;               // ref to first item
   private Link last;                // ref to last item


   public FirstLastList(){            // constructor
      first = null;                  // no items on list yet
      last = null;
   }
 

   public boolean isEmpty(){          // true if no links
       return first==null; 
   }

   public void insertLast(double dd){ // insert at end of list
      Link newLink = new Link(dd);   // make new link
      if( isEmpty() )                // if empty list,
         first = newLink;            // first --> newLink
      else
         last.next = newLink;        // old last --> newLink
      last = newLink;                // newLink <-- last
      }

  // PRECONDITION: nonempty list
   public double deleteFirst(){       // delete first link                               
       double temp = first.dData;
      if(first.next == null)         // if only one item
         last = null;                // null <-- last
      first = first.next;            // first --> old next
      return temp;
      }


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




class LinkQueue{
   private FirstLastList theList;

   public LinkQueue()                // constructor
      { theList = new FirstLastList(); }  // make a 2-ended list


   public boolean isEmpty() {         
       return theList.isEmpty(); 
    }

   public void insert(double j){      
       theList.insertLast(j); 
    }

   public double remove(){ 
      return theList.deleteFirst();  
   }

   public void displayQueue(){
      System.out.print("Queue (front-->rear): ");
      theList.displayList();
   }
}  // end class LinkQueue



class LinkQueueApp{
   public static void main(String[] args) throws IOException {
      LinkQueue theQueue = new LinkQueue();
      theQueue.insert(20);                 // insert items
      theQueue.insert(40);
      theQueue.insert(60);                 // insert items
      theQueue.insert(80);

      theQueue.displayQueue();             // display queue

      theQueue.remove();                   // remove items
      theQueue.remove();

      theQueue.displayQueue();             // display queue
      }  // end main()
   }  // end class LinkQueueApp