static boolean isNumeric(String string) {
    return string
        .chars()
        .allMatch(x -> '0' <= x && x <= '9');
  }

  static int[] veryGeneralFunction(List<String> strings) {
    return strings
        .stream()                           // transform into stream
        .filter(x -> isNumeric(x))          // get only the numeric elements
        .sorted()                           // sort them
        .mapToInt(x -> Integer.parseInt(x)) // transform each element to int
        .toArray();                         // make an array
  }

  public static void main(String[] args) {
    List<String> strings = Arrays.asList("123", "a", "0", " ", "999", "9a9");
    for (int i : veryGeneralFunction(strings)) {
      System.out.println(i);
    }
  }