code: static boolean format(Configuration conf,
                                  boolean force,
                                  boolean isConfirmationNeeded
                                  ) throws IOException {
      boolean allowFormat = conf.getBoolean("dfs.namenode.support.allowformat",
                                            true);
      if (!allowFormat) {
        throw new IOException("The option dfs.namenode.support.allowformat is "
                              + "set to false for this filesystem, so it "
                              + "cannot be formatted. You will need to set "
                              + "dfs.namenode.support.allowformat parameter "
                              + "to true in order to format this filesystem");
      }
      Collection<URI> dirsToFormat = NNStorageConfiguration.getNamespaceDirs(conf);
      Collection<URI> editDirsToFormat =
          NNStorageConfiguration.getNamespaceEditsDirs(conf);
  
      FSNamesystem nsys = new FSNamesystem(new FSImage(conf, dirsToFormat,
                                           editDirsToFormat, null), conf);
      try {
        if (!nsys.dir.fsImage.confirmFormat(force, isConfirmationNeeded)) {
          return true; // aborted
        }
        nsys.dir.fsImage.format();
        return false;
      } finally {
        nsys.close();
      }
    }
docs: Verify that configured directories exist, then
  Interactively confirm that formatting is desired
  for each existing directory and format them.
  
  @param conf
  @param isConfirmationNeeded
  @return true if formatting was aborted, false otherwise
  @throws IOException