Mastering Java: The Ultimate Quiz for 'Thinking in Java'

Why does List<? extends Fruit> not accept new Apple() when calling add()?

1. Because the list is bound to Fruit objects only

2. Because the compiler cannot determine what specific subtype of Fruit should be accepted

3. Because Apple is not a subtype of Fruit

4. Because you can only add null to such a List

The correct answer is: Because the compiler cannot determine what specific subtype of Fruit should be accepted

List<? extends Fruit> means that the elements in the list can be any subtype of Fruit. When calling add(), the compiler cannot determine which specific subtype is expected in the list, so it will not allow adding a new Apple object. Option A is incorrect because the list is not limited to only Fruit objects, but rather any subtype of Fruit. Option C is incorrect because Apple is a subtype of Fruit. Option D is incorrect because any non-null object can be added to the list, not just null.