是否需要在直通方法中使用有界通配符泛型？

Question

I know that in the following method in Collection<E> :

public void addAll(Collection<? extends E> subcollection);

We use Collection<? super E> Collection<? super E> there to allow a collection that solely exists of sub-elements, example:

List<Drivable> drivables = new ArrayList<>();
List<Car> cars = new ArrayList<>();
//we need the wildcard here, because of the following line:
drivables.addAll(cars);

However, are such bounded wildcards needed in my following method?

public static <E> Collection<E> requireNonEmpty(final Collection<E> collection) throws NoSuchElementException {
    if (collection.isEmpty()) {
        throw new NoSuchElementException("collection must be non-empty");
    }
    return collection;
}

This uses a similar idiom as Objects.requireNonNull(T object) , which actually returns the T object .

So, is there any benefit (or is it even wrong?) to write my method as the following?

public static <E> Collection<? super E> 
    requireNonEmpty(final Collection<? extends E> collection);

Answer 1

The use of Collection<? extends E> subcollection Collection<? extends E> subcollection makes the passed parameter a universal donor - ie it can be read from with freedom.

You do not need to read objects from the passed collection so you do not need any wildcards.

Incidentally using Collection<? super E> subcollection Collection<? super E> subcollection makes it a universal recipient - ie you can add to it.

See When to use extends and super for a clear introduction.

Also a good answer covering this Generic lower unbound vs upper bounded wildcards .