Wild card, adding to a list throws UnsupportedOperationException

Question

public static void wildCard2(List<? super Base> lst){
    for (int i=0; i<2; i++){
        System.out.println(lst.get(i));
    }
    lst.add(new Base());

}

As shown in the code above, I have a class called Base. Can anyone explain why this fails at RunTime and throws UnsupportedOperationException ?

Check http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p — Adisesha, Feb 01 '16 at 08:40
@Nikunj - he posted error in subject - `UnsupportedOperationException` while adding element to list. — Sabir Khan, Feb 01 '16 at 08:47
you need to show your calling part too specifically which `List` is getting passed for `lst` — Sabir Khan, Feb 01 '16 at 08:48

Robby Cornelissen · Answer 1 · 2016-02-01T09:08:11.807

The add(E e) method defined on the List<E> interface is specified as an optional operation.

If you're getting an UnsupportedOperationException at runtime, this means that the List implementation that you're using does not support this operation.

It might be an unmodifiable type created by calling Collections.unmodifiableList(), an instance of a Guava ImmutableList, or something else entirely. Check the runtime type to find out:

System.out.println(lst.getClass().getName());

score 0 · Answer 2 · answered Feb 01 '16 at 08:55

0

You cannot .add() from such a List, you can create a copy of the list to a known-modifiable implementation like ArrayList.

answered Feb 01 '16 at 08:55

Abdelhak

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Wild card, adding to a list throws UnsupportedOperationException

2 Answers2