why wrote:

  public static String getDigit(String s) {
    if (s == null) { return null; }

    int len = s.length();
    if (len == 0) { return ""; }
      //could use s.toCharArray()
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < len; i++) {
      char c = s.charAt(i);
      //could use Character.isDigit(char)
      if (c >= '0' && c <= '9') {
        sb.append(c);
      }
    }
    return sb.toString();
  }