interface DisplayableItem{}

class Teaser implements DisplayableItem {}

void main() {
    List<Teaser> teasers = new ArrayList<>();
    add1(teasers); //error
    add2(teasers.get(1)); //ok

    for (Teaser teaser : teasers) {
            add2(teaser); //ok
    }

}

private void add1(List<DisplayableItem> items) {
}

private void add2(DisplayableItemitems) {
}