You do not need to cast. LinkedList implements List so you have no casting to do here.
Even when you want to down-cast to a List of Objects you can do it with generics like in the following code:
LinkedList ll = someList;
List extends Object> l = ll; // perfectly fine, no casting needed
Now, after your edit I understand what you are trying to do, and it is something that is not possible, without creating a new List like so:
LinkedList ll = someList;
List
and I'll explain why this is not possible otherwise. Consider this:
LinkedList ll = new LinkedList();
List l = ll; // ERROR, but suppose this was possible
l.add((Object) new Integer(5)); // now what? How is an int a String???
For more info, see the Sun Java generics tutorial. Hope this clarifies.