code: static long parseGenerationStampInMetaFile(File blockFile, File metaFile
        ) throws IOException {
      String metaname = metaFile.getName();
      String gs = metaname.substring(blockFile.getName().length() + 1,
          metaname.length() - FSDataset.METADATA_EXTENSION.length());
      try {
        return Long.parseLong(gs);
      } catch(NumberFormatException nfe) {
        throw (IOException)new IOException("blockFile=" + blockFile
            + ", metaFile=" + metaFile).initCause(nfe);
      }
    }
docs: Find generation stamp from block file and meta file.
  @param blockFile
  @param metaFile
  @return
  @throws IOException