What is the point of Collection<?>

Question

I have been reading Effective Java and I have come across the unbounded Collection type <?>. However, the reading has led me to believe that you can't put any elements, other than null, into an instance of Collection<?>.

So, my question is this: what is the purpose of instantiating a Collection<?> when we can only insert null elements as it seems pointless. I've been trying to figure out this concept, but it just doesn't seems to make much sense. Any help would be much appreciated.

Answer 1

Collection<?> allows you to create, among other things, methods that accept any type of Collection as an argument. For instance, if you want a method that returns true if any element in a Collection equals some value, you could do something like this:

public boolean collectionContains(Collection<?> collection, Object toCompareTo) {
    for (Object o : collection) {
        if (toCompareTo.equals(o)) return true;
    }
    return false;
}

This method can be called by any type of Collection:

Set<String> strings = loadStrings();
collectionContains(strings, "pizza");

Collection<Integer> ints = Arrays.toList(1, 2, 3, 4, 5);
collectionContains(ints, 1337);

List<Collection<?>> collections = new ArrayList<>();
collectionContains(collections, ILLEGAL_COLLECTION);