Question

I have read that the following code snippet will result in a compiler error

void tryAddingToList(List<? extends SuperType> list) {
  list.add(new SubType());
}

Now my question is:
Who exactly sets the restriction and how is the restriction set? Does the compiler specifically check for the add method call in such a method implementation?

If yes, what if I implement my own List and instead of add, introduce a new addToList method that does the same thing as add, and call that in the tryAddingToList method?

Even better, what if I implement my get method such that it also “secretly” adds something to the list (don’t ask me why anyone would do that; I’m just curious).

Answer 1

You should use super instead of extends here.

void tryAddingToList(List<? super SubType> list) {
    list.add(new SubType());
}

Assume there are 3 types SuperType, SubType1, SubType2. In your original code, List<? extends SuperType> could be List<SubType2>, and you can’t add a SubType1 to List<SubType2>.