code: void readHeader() throws IOException {
      int version = in.readInt();
      if (version != BlockCrcInfoWritable.LATEST_BLOCK_CRC_FILE_VERSION) {
        throw new IOException("Version " + version + " is not supported.");
      }
      numBuckets = in.readInt();
      currentBucket = -1;
      numRecordsReadInBucket = 0;
      numRecordsInBucket = 0;    
    }
docs: Read header of the file
  @throws IOException