default Optional<Boolean> isAlreadyCached(BlockCacheKey key) {
    return Optional.empty();
  }
Checks whether the block for the passed key is already cached. 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 block for the passed key is already cached or not.
@param key for the block we want to check if it's already in the cache.
@return empty optional if this method is not supported, otherwise the returned optional
        contains the boolean value informing if the block is already cached.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCache.java