@Override
  public Optional<Boolean> isAlreadyCached(BlockCacheKey key) {
    boolean result =
      l1Cache.isAlreadyCached(key).orElseGet(() -> l2Cache.isAlreadyCached(key).orElse(false));
    return Optional.of(result);
  }
Returns the list of fully cached files
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CombinedBlockCache.java