private boolean linkLast(Node<E> node){
  if (count >= capacity)   return false;
  Node<E> l=last;
  node.prev=l;
  last=node;
  if (first == null)   first=node;
 else   l.next=node;
  ++count;
  notEmpty.signal();
  return true;
}

private boolean linkLast ( Node < E > node ) { if ( count >= capacity ) return false ; Node < E > l = last ; node . prev = l ; last = node ; if ( first == null ) first = node ; else l . next = node ; ++ count ; notEmpty . signal ( ) ; return true ; }
links node as last element , or returns false if full .
