Java wildcard generic propblem: Compiler assumes different types for same object?

. Yes, you know that any element that's already in the collection should be able to be added to it, but the type doesn't reflect that.? and? Extends Foo should only be used when you only need to retrieve objects from the collection and not add to it.

Can you show why you need to use a wildcard as opposed to, say Collection.

You can't add a string object in to a collection of type Collection, because the type Collection itself means that you don't know the type of the collection, hence the compiler won't allow you to add arbitrary types in it.

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