code: 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;
      }
docs: Gets an instance of UnsupportedDateTimeField for a specific named field.
  Names should be of standard format, such as 'monthOfYear' or 'hourOfDay'.
  The returned instance is cached.
  
  @param type  the type to obtain
  @return the instance
  @throws IllegalArgumentException if durationField is null