List<SomeObject> l;
In this you cannot say List<SomeObject> l = new ArrayList<SubClassOfSomeObjectClass>;
(not allowed)
wheres for
List<? extends SomeObject> l;
you can say
List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>;
(allowed)
But note that in List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>;
you cannot add anything to your list l because ? represents unknown class (Except null of-course).
Update: For your question in the comment What could I possibly do with a list if I cannot add anything to it?
Now consider a case in which you have to write a function to print your list but mind you it must only accept a List having objects which are subclasses of your SomeObject. In this case as I stated above you cannot use
public void printList(List<SubClassOfSomeObjectClass> someList)
So what would you do? You would do something like
public void printList(List<? extends SomeObject> someList) {
for(SomeObject myObj : someList) {
//process read operations on myObj
}