SortedSet<String> getCachedFileNamesForTest() {
    SortedSet<String> fileNames = new TreeSet<>();
    for (BlockCacheKey cacheKey : map.keySet()) {
      fileNames.add(cacheKey.getHfileName());
    }
    return fileNames;
  }
Used in testing. May be very inefficient.
@return the set of cached file names
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/LruBlockCache.java