Java generics wildcards with upper and lower bounds

Asked Dec 27 '17 at 20:49

Active Dec 27 '17 at 20:49

Viewed 383 times

Lets suppose class C extends B, B extends A.

For given function:

public static void add(List<? extends B> list) {
    list.add(new A()); //Doesn't compile
    list.add(new B()); //Doesn't compile
    list.add(new C()); //Doesn't compile
}

Why it's impossible to add any element ? I understand

List<? extends B> list

as a list of elements of type B or C.

For super bound:

public static void add2(List<? super B> list) {
    list.add(new A()); //doesn't compile
    list.add(new B());
    list.add(new C());
}

I understand

List<? super B> list

as a list with elements of type A or B.

I don't understand how does it work :/

asked Dec 27 '17 at 20:49

user109447

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It's not impossible to add any element in the first case: you can add `null`. – Andy Turner Dec 27 '17 at 20:52
Why its impossible to add any element in the first case ? – user109447 Dec 27 '17 at 20:55
https://stackoverflow.com/questions/14306166/java-generic-with-arraylist-extends-a-add-element – PM 77-1 Dec 27 '17 at 20:56
@user109447 because a `List` is a `List extends B>`; none of `A`, `B` or `C` would be safe to add to such a list. – Andy Turner Dec 27 '17 at 21:30

Java generics wildcards with upper and lower bounds

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