Question about Interface Collection<E>

[yma16]

In Interface Collection<E>, it has

addAll(Collection<? extends E> c)

removeAll(Collection<?> c)

Why does it use <? extends E> and <?> in these two method? Why not use

removeAll(Collection<? extends E> c) ?

Thank you.

[sunde887]

Have you tried reading what the two things do and how they differ? Or if they differ? If not, here is a link to where you should be able to find your answer:
Lesson: Generics (The Java™ Tutorials > Bonus)

The ? is a wildcard, and <? extends X> is a bounded wildcard to help you narrow your search. It has something to do with reading and writing.

For example, with the removeAll method, it really doesn't care what the type of the passed in argument is. However, when adding a collection you only want types of E and it's subclasses to be added to the collection. You don't want to allow a collection of type Ham to be added to a collection of Type Hamster, that wouldn't make much sense.

My understanding of generics is admittedly rusty so my explanation may have some flaws. That is where the tutorials will come in handy. Check them out.

[pbrockway2]

Why not use removeAll(Collection<? extends E> c) ?

I guess the reason is the same the reason why remove() is declared as "boolean remove(Object)" while add() is declared as "boolean add(E)".

There's discussion at stackoverflow.