Java generics: List = new LinkedList>() is prohibited?

Question

How come, in Java, I can write

List list = new LinkedList();

but not

List>          


        

Answer 1

Consider what would happen if you could.

List> list1 = new LinkedList>();
List> list2 = list1; // this shouldn't be allowed, because of the below
Container foo = new Container();
foo.add("hi");
list2.add(foo); // legal, because Container is a subtype of Container
Container bar = list1.get(0);
Double x = bar.get(0); // type error; it is actually a String object
                       // this indicates that type safety was violated

However, using List> doesn't have this problem because you cannot put anything (except null) into a list of this type (because there is no type that is guaranteed to be a subtype of "? extends Container").