private Optional<String> toStringLastKey() {
    return getLastKey().map(CellUtil::getCellKeyAsString);
  }
Opens a HFile.
@param context   Reader context info
@param fileInfo  HFile info
@param cacheConf Cache configuration.
@param conf      Configuration
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java