code: public static Function<Path, Path> indexFile() {
          return new Function<Path, Path>() {
              @Nullable
              @Override
              public Path apply(@Nullable Path dataFile) {
                  final String dataFileName = dataFile.getName();
                  return new Path(dataFile.getParent(), dataFileName.replace("-Data.db", "-Index.db"));
              }
          };
      }
docs: Return a function which determines the SSTable index file when supplied with the SSTable data file.
  @return Function.