How to add elements to a list of Long Serializable

Question

I am currently writing a set of generic DAOs with generic signatures such as DAO<PK extends Serializable> (you've all seen the pattern).
I am using Hibernate to persist to the database whose method is;
public Serializable save(Object object)
However, I am working in batches to save groups of Objects at a time & would like to save the returned Serializable types from this method to a list.
List< ? extends Serializable> someList = new ArrayList< Long >
for ( loop 0..n ) {
someList.add( save( objectToPersist ) );
This will not even compile as the IDE gives the error;
The method add(capture-of ? extends Serializable) in the type List<capture-of ? extends Serializable> is not applicable for the arguments (Serializable)
Ive obviously got the wrong end of the stick somewhere but I thought List<? extends Serializable> was a subclass of Collection<?> etc.
Or am I trying to subclass a super type...
Apologies for a stupid question as Ive just started using Generics & I've got the Tiger 5 book to read this weekend so Im sure itll make sense by then, but this is confusing me.
Thanks

Answer 1

As a side note
        List<? extends Serializable> someList = new ArrayList< Long >();
        someList.add(null);  //works
        someList.add(new Long(21));  //wont compilehttp://www.javabeat.net/articles/33-generics-in-java-50-3.html
http://www.javabeat.net/articles/33-generics-in-java-50-4.html
Came across these links discussing why the above wont compile