private byte[] blockForWrite(int index) throws IOException {
    if (index >= blockCount) {
      int additionalBlocksNeeded = index - blockCount + 1;
      disk.allocate(this, additionalBlocksNeeded);
    }

    return blocks[index];
  }
Gets the block at the given index, expanding to create the block if necessary.
jimfs/src/main/java/com/google/common/jimfs/RegularFile.java