code: public <T extends Widget & HasValidators<?>> void add(final T field) {
          fields.put(field, field.validate(false));
          if (field.isAttached()) {
              updateStateAndNotify();
          }
          registrations.put(field, field.addValidationChangedHandler(new ValidationChangedHandler() {
              @Override
              public void onValidationChanged(ValidationChangedEvent event) {
                  fields.put(field, event.isValid());
                  if (fireEvents) {
                      updateStateAndNotify();
                  }
              }
          }));
      }
docs: Adds a field to the group.
  
  @param <T> the generic type
  @param field the field