class Queue
{
 private Cell front,back;

 public Object first()
 {
  return front.first;
 }

 public void leave()
 {
  front=front.next;
  if(front==null)
     back=null;
 }

 public void join(Object obj)
 {
  if(front==null)
     {
      front = new Cell(obj,null);
      back=front;
     }
  else
     {
      back.next = new Cell(obj,null);
      back=back.next;
     }
 }

 public boolean isEmpty()
 {
  return front==null;
 }

 private static class Cell
 {
  Object first;
  Cell next;

  Cell(Object f,Cell n)
  {
   first=f;
   next=n;
  }
 }
}