Why different result outputs between the generic type and wildcard type

Question

I know what is the difference between the generic type and the wildcard type, but in this situation I cannot use the same way again to understand.

For short, To compare both Interfaces below

public interface RequestParser {
    <T extends Entity> Message<T> parseRequest(String json);
}

public interface RequestParser {
    Message<? extends Entity> parseRequest(String json);
}

Only the first one can compile the below codes: (User extends Entity)

Message<User> message = requestParser.parseRequest(json);

The one uses wildcard ? cannot succeed.

So what exactly is the difference between them in this situation ...?

Possible duplicate of [What is PECS (Producer Extends Consumer Super)?](https://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super) — Flown, Sep 04 '17 at 07:10
In this case you want to use the produced value and therefore it's also a consumer (linked in the duplicate) — Flown, Sep 04 '17 at 07:12

score 1 · Accepted Answer · answered Sep 04 '17 at 07:50

It's because Message<User> is not a subclass of Message<? extends Entity>.

The issue will be more evident when trying to implement RequestParser.

This will fail without a cast, as T is not necessarily AnotherUser:

public class AnotherUser extends Entity {}

public class RequestParserImpl implements RequestParser {
    <T extends Entity> Message<T> parseRequest(String json) {
         return new Message<AnotherUser>();
    }
}

Whereas this will compile:

public class RequestParserImpl implements RequestParser {
    Message<? extends Entity> parseRequest(String json) {
         return new Message<AnotherUser>();
    }
}

Why different result outputs between the generic type and wildcard type

1 Answers1