Class that implements an interface with a method returning List of interfaces generates compilation error

Question

Let's say I have these four classes:

public interface A {}

public interface B {
    public A getA();
}

public class C implements A {}

public class D implements B {
    public C getA() {
        return new C();
    }
}

This compiles well. However if I have this code I get a compilation error:

public interface A {}

public interface B {
    public List<A> getA();
}

public class C implements A {}

public class D implements B {
    public List<C> getA() {
        return new ArrayList<C>();
    }
}

public class E implements B {
    public List<A> getA() {
        return new ArrayList<C>();
    }
}

What is the reason that allows to return a C in the method getA, in the first example, but generates an error in the second example when I return a List?

It looks to me that both examples should compile or throw an error.

Thanks.

EDIT I read the post using Dog/Animal, however my doubt is different. I have added the class E. If you call getA in class E you get a List as defined in the interface. Then why it fails compiling for class E?

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-implicitly-p) — Tunaki, Sep 30 '15 at 09:37

Thusitha Thilina Dayaratne · Answer 1 · 2015-10-01T09:23:50.107

3

This is relate to java generics. You have defined the return type of method in interface as List<A> but you are trying to return a List<C> from the actual implementation. If you actually wanna return a List<C> from the method then you have to change your interface method as follows

public List<? extends A> getA();

Explanation why can't return new ArrayList<C>();

Let say it is valid to return as u mentioned. Then

List<A> listA = getA(); // This will compile if allowed
listA.add(new A()); // this will also valid since reference type is A.

So you can see that it is no point of allowing the way you mentioned

edited Oct 01 '15 at 09:23

answered Sep 30 '15 at 09:42

Thusitha Thilina Dayaratne

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Hi Thusitha. Thanks for your reply. Yes, it compiles with your change. What I want to know is why the second example fails. This is just something I want to learn not something I have to make work. – daniel sp Sep 30 '15 at 09:48
have you read the explanation given in the http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p. – Thusitha Thilina Dayaratne Sep 30 '15 at 10:01
Yes I read, and I still don't why you cannot have an object of type E with a method getA that returns List and internally it returns ArrayList. Is a different thing. In this post it says that if you have a list DEFINED as List you cannot use it as a List, and I agree. But here is the opposite case. I am returning a list of Animal that contains only Dog. Ok you are free to add cats. – daniel sp Sep 30 '15 at 11:21

Class that implements an interface with a method returning List of interfaces generates compilation error

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