List and ArrayList peculiar behavior in generics

Asked Aug 07 '18 at 07:11

Active Aug 07 '18 at 07:13

Viewed 82 times

Following code works fine while it should not compile.

I am not sure why assignment of a List to String is permitted.

It only happens to List interfaces. Changing to a Array List working fine as expected and it no longer compiles.

public class Test {
        public static <T extends List<Number>> T newList() {
            return null;
        }

        public static void main(String[] args) {
            String s = newList();
        }
    }

Please help me understand this behavior.

edited Aug 07 '18 at 07:13

asked Aug 07 '18 at 07:11

T-Bag

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7

There could possibly a subclass of String that implements the List interface (But this cannot happen as String is final) – Thiyagu Aug 07 '18 at 07:13
3

Type inference doesn't take into account the final-ness of classes (like you can write `` more directly). So yes, what @user7 said. – Andy Turner Aug 07 '18 at 07:15
@AndyTurner Nice. I did not know that. Unless this question is not a duplicate, could you make your comment an answer? Additionally, is there any documentation about that (maybe in the JLS)? – Seelenvirtuose Aug 07 '18 at 07:20
1

@Seelenvirtuose in fact, the same wouldn't happen with ArrayList, because a class couldn't extend both ArrayList and String. – Andy Turner Aug 07 '18 at 07:20
3

@Seelenvirtuose it is a duplicate, it's just one of those nigh-on impossible to find ones, because it's hard to think of the search term. – Andy Turner Aug 07 '18 at 07:21
@AndyTurner Exactly. I'm sure I read that here some time back – Thiyagu Aug 07 '18 at 07:23
2

@ernest_k Good find! – Seelenvirtuose Aug 07 '18 at 07:27
@user7 I think tge explanation of yours is short and clear – St.Antario Aug 07 '18 at 23:18

List and ArrayList peculiar behavior in generics

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