docs: public synchronized void printList(){
          Entry<E, T> entry = this.head.next;
          
          System.out.println("\n|----- key list----|");
          while( entry != this.tail)
          {
              System.out.println(" -> " + entry.key );
              entry = entry.next;
          }
          System.out.println("|------- end --------|\n");
      }