How to use generics with inheritance?

Question

I wrote some generics with inheritance, but cannot get it to work typesafe. My goal is to provide a general BaseService on that I can exection the action(base) method, no matter if the base object is a Foo extends Base or Bar extends Base.

The following does not work, why?

class Base;
class Foo extends Base;

interface BaseService<T extends Base> {
    void action(T base);
}

class FooService implements BaseService<Foo> {
    void action(Foo foo) {
    }
}


//usage:
//complains that the method is not applicable for the argument,
//and that I should change action(T) to action(Base). Why?
Base base;
getService().action(base);


//just to demonstrate the problem
BaseService<? extends Base> getService() {
    return new FooService();
}

Possible duplicate of "[Is `List` a subclass of `List`? Why aren't Java's generics implicitly polymorphic?](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicit)." — wchargin, Apr 01 '13 at 01:52
It already is typesafe. Too typesafe. I don't think you need Generics at all for this. — user207421, Apr 01 '13 at 02:05
@membersound That proves that it's typesafe, 'too typesafe' as I said. — user207421, Apr 01 '13 at 04:18
This isn't a good candidate for generics because you're always returning a `FooService` - being able to treat it as a `BaseService` for example would break type safety. — Paul Bellora, Apr 01 '13 at 04:33
Could you make a suggestion in how to refactor the code so that it works the way I intended, but without generics? — membersound, Apr 01 '13 at 10:38

score 1 · Accepted Answer · answered Apr 02 '13 at 12:45

Generics exist for the purpose of static type checking. As you write

My goal is to provide a general BaseService on that I can exection the action(base) method, no matter if the base object is a Foo extends Base or Bar extends Base.

you indicate that you don't want any static type checking. So to accomplish that, the argument type should be Base instead of T.

As the return type of getService() is BaseService<? extends Base>, the compiler can't even know what T is (other than that it's some unknown subtype of Base) so you can't call the action method at all. But even if the return type of getService() would be FooService, the argument type T would be Foo, so you could not call it with type Base.

You should probably think a bit more about what you actually want the compiler to check by the use of generics.

How to use generics with inheritance?

1 Answers1