oop - Class implementing Interface with subtype arguments

Question

Why I cannot do that? I get compilation error.

public interface A {

  void update(Object a);
}

public class B implements A{
     void update(Long a) {
     }

}

That is Java 8

I do not see here violating any OO principle.

That's really make my time difficult to implement a generic API...

(I try to get Generics out of the play because the generic API gets counter-intuitive)

Answer 1

You get a compilation error because void update(Long a) does not implement void update(Object a).

You can implement it as follows:

public class B implements A {
    void update(Object o) {
        if (!(o instanceof Long)) {
            // possibly throw an exception

        }
        Long a = (Long) o;
        ...
    }
}