code: private static DateTimeFormatter createFormatterForStyle(String style) {
          if (style == null || style.length() != 2) {
              throw new IllegalArgumentException("Invalid style specification: " + style);
          }
          int dateStyle = selectStyle(style.charAt(0));
          int timeStyle = selectStyle(style.charAt(1));
          if (dateStyle == NONE && timeStyle == NONE) {
              throw new IllegalArgumentException("Style '--' is invalid");
          }
          return createFormatterForStyleIndex(dateStyle, timeStyle);
      }
docs: Select a format from a two character style pattern. The first character
  is the date style, and the second character is the time style. Specify a
  character of 'S' for short style, 'M' for medium, 'L' for long, and 'F'
  for full. A date or time may be omitted by specifying a style character '-'.
  
  @param style  two characters from the set {"S", "M", "L", "F", "-"}
  @throws IllegalArgumentException if the style is invalid