code: public FileIndexBuilder setStartingRecordNumber(long recordNumber) {
          checkIfDestroyed();
          if (index.size() != 1)
              throw new IllegalStateException("Initial record is already initialised to " + index.get(0));
          startingRecordNumber = recordNumber;
          return this;
      }
docs: Sets the starting record number to a specified one.
  
  @param recordNumber starting record number
  @return this