code: public boolean isInitialized() {
      for (int i = 0; i < fields.getNumArrayEntries(); i++) {
        if (!isInitialized(fields.getArrayEntryAt(i))) {
          return false;
        }
      }
      for (final Map.Entry<FieldDescriptorType, Object> entry :
               fields.getOverflowEntries()) {
        if (!isInitialized(entry)) {
          return false;
        }
      }
      return true;
    }
docs: See {@link Message#isInitialized()}.  Note:  Since {@code FieldSet}
  itself does not have any way of knowing about required fields that
  aren't actually present in the set, it is up to the caller to check
  that all required fields are present.