docs: public static synchronized UnsupportedDateTimeField getInstance(
              DateTimeFieldType type, DurationField durationField) {
  
          UnsupportedDateTimeField field;
          if (cCache == null) {
              cCache = new HashMap<DateTimeFieldType, UnsupportedDateTimeField>(7);
              field = null;
          } else {
              field = cCache.get(type);
              if (field != null && field.getDurationField() != durationField) {
                  field = null;
              }
          }
          if (field == null) {
              field = new UnsupportedDateTimeField(type, durationField);
              cCache.put(type, field);
          }
          return field;
      }