code: public FileIndexBuilder setStartingRecordPosition(long recordPosition) {
          checkIfDestroyed();
          if (index.size() != 1)
              throw new IllegalStateException("Initial record is already initialised to " + index.get(0));
          index.set(0, startingByte = recordPosition);
          return this;
      }
docs: Sets the starting record to a specified pointer (in bytes).
  
  @param recordPosition starting record position in file (in bytes)
  @return this