Adding an item to the list:
List< ? extends X > doesn't allow to add anything, except for null into the list.
List< ? super X > allows to add anything that is-a X (X or its subtype), or null.
Getting an item from the list:
- When you get an item from List< ? extends X >, you can assign it to a variable of type X or any supertype of X, including Object.
- When you get an item from List< ? super X >, you can only assign it to a variable of type
Object.
Some examples:
List list1 = new ArrayList();
list1.add(null); //OK
Number n = list1.get(0); //OK
Serializable s = list1.get(0); //OK
Object o = list1.get(0); //OK
list1.add(2.3); //ERROR
list1.add(5); //ERROR
list1.add(new Object()); //ERROR
Integer i = list1.get(0); //ERROR
List list2 = new ArrayList();
list2.add(null); //OK
list2.add(2.3); //OK
list2.add(5); //OK
Object o = list2.get(0); //OK
list2.add(new Object()); //ERROR
Number n = list2.get(0); //ERROR
Serializable s = list2.get(0); //ERROR
Integer i = list2.get(0); //ERROR
