code: public PatternInfo getIntervalPattern(String skeleton, int field)
      {
          if ( field > MINIMUM_SUPPORTED_CALENDAR_FIELD ) {
              throw new IllegalArgumentException("no support for field less than SECOND");
          }
          Map<String, PatternInfo> patternsOfOneSkeleton = fIntervalPatterns.get(skeleton);
          if ( patternsOfOneSkeleton != null ) {
              PatternInfo intervalPattern = patternsOfOneSkeleton.
                  get(CALENDAR_FIELD_TO_PATTERN_LETTER[field]);
              if ( intervalPattern != null ) {
                  return intervalPattern;
              }
          }
          return null;
      }
docs: Get the interval pattern given the largest different calendar field.
  @param skeleton   the skeleton
  @param field      the largest different calendar field
  @return interval pattern  return null if interval pattern is not found.
  @throws IllegalArgumentException  if getting interval pattern on
  a calendar field that is smaller
  than the MINIMUM_SUPPORTED_CALENDAR_FIELD