code: public static void merge(File mergeDir, List<File> sortedFiles, File mergedFile, Comparator<String> comparator) {
          merge(mergeDir, sortedFiles, mergedFile, comparator, MERGE_FACTOR);
      }
docs: Merge multiple files into a single file. Multi-pass approach where the merge factor controls how many passes that
  are required. This method uses the default merge factor for merging.
  @param mergeDir merge directory. If null, then it defaults to system temp.
  @param sortedFiles sorted files that are to be merged
  @param mergedFile final output file
  @param comparator comparator used to merge files (sorting)