@Override
  public String toString() {
    return this.hfileName + '_' + this.offset;
  }
Construct a new BlockCacheKey
@param hfileName The name of the HFile this block belongs to.
@param offset    Offset of the block into the file
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/BlockCacheKey.java