code: static <T> FieldSetter<T> getFieldSetter(final Class<T> clazz, String fieldName) {
      try {
        Field field = clazz.getDeclaredField(fieldName);
        return new FieldSetter<T>(field);
      } catch (NoSuchFieldException e) {
        throw new AssertionError(e); // programmer error
      }
    }
docs: Secret sauce for setting final fields; don't make it public.