docs: public static synchronized UnsupportedDurationField getInstance(DurationFieldType type) {
          UnsupportedDurationField field;
          if (cCache == null) {
              cCache = new HashMap<DurationFieldType, UnsupportedDurationField>(7);
              field = null;
          } else {
              field = cCache.get(type);
          }
          if (field == null) {
              field = new UnsupportedDurationField(type);
              cCache.put(type, field);
          }
          return field;
      }