code: public FileIndexBuilder appendNextRecord(long recordSize) {
          checkIfDestroyed();
          if (recordSize < 0)
              throw new IllegalArgumentException("Size cannot be negative.");
          if (currentRecord == step) {
              index.add(startingByte + currentByte);
              currentRecord = 0;
          }
          currentByte += recordSize;
          ++currentRecord;
          return this;
      }
docs: Appends next record to this index builder and returns this.
  
  @param recordSize size of record measured in bytes
  @return this