code: public void addLast( AbstractJournalOperation op )
      {
          if (tail == null)
          {
              head = tail = op;
          }
          else
          {
              tail.setNext(op);
              tail = op;
          }
          
          op.setNext(null);
          size++;
      }
docs: Append an operation at the end of the queue
  @param op a journal operation