@Override public SelectedGroup<T> useGroup(final String group){
  Objects.requireNonNull(group,"group must not be null");
  return new SelectedGroup<>(find(group));
}

@ Override public SelectedGroup < T > useGroup ( final String group ) { Objects . requireNonNull ( group , "group must not be null" ) ; return new SelectedGroup < > ( find ( group ) ) ; }
use the specific group .
