Java Generics for Upper bound & lower bound wild cards
collections, generics, java, wildcard
Solution
This is because when you are using upper bound, you cannot add elements to collection, only read them.
this means that these are some of legal assignments:
List<? extends Number> l = new ArrayList<Integer>();
List<? extends Number> l = new ArrayList<Double>();
so you cannot guarantee that when adding object, it will hold correct types of objects. for better explatation please follow: How can I add to List<? extends Number> data structures?
Problem
I was reading java generics, I came across an interesting query. My question is as follows. For an upper bounded wildcard ``` public static void printList(List<? extends Number> list) { for (int i = 0; i < 10; i++) { list.add(i);// gives compilation error } } ``` For a lower bounded wildcard ``` public static void printList(List<? super Integer> list) { for (int i = 0; i < 10; i++) { list.add(i);// successfully compiles } } ``` I am confused with this because looking at the Sun Oracle documentation I understand that the code should compile for point 1 as well Upper Bound Wildcard Lower Bound Wildcard Can anyone please help me to understand this?