default Optional<Boolean> shouldCacheFile(String fileName) {
    return Optional.empty();
  }
Checks whether blocks for the passed file should be cached or not. This method may not be
overridden by all implementing classes. In such cases, the returned Optional will be empty. For
subclasses implementing this logic, the returned Optional would contain the boolean value
reflecting if the passed file should indeed be cached.
@param fileName to check if it should be cached.
@return empty optional if this method is not supported, otherwise the returned optional
        contains the boolean value informing if the file should be cached.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCache.java