code: private Class<?> ptype(@NonNull final Field field) {
          Class<?> ptype = ptypes.get(field);
          if (ptype == null) {
              final ParameterizedType parameterizedType = (ParameterizedType) field.getGenericType();
              ptype = (Class<?>) parameterizedType.getActualTypeArguments()[0];
              ptypes.put(field, ptype);
          }
          return ptype;
      }
docs: ---------------------------------------------------------------------------------------------------------- ptypes