class World {

  private List<String> countries;
  private final DataFetcher df;

  public World() {
    this.countries = new ArrayList<>();
    this.df = new DataFetcher();
  }

  /**
   * Calls {@link DataFetcher} to fetch data from back-end.
   *
   * @return List of strings
   */
  public List<String> fetch() {<FILL_FUNCTION_BODY>}
}

    var data = df.fetch();
    countries = data.isEmpty() ? countries : data;
    return countries;
  
<no_super_class>