Making a subclass to return itself without overriding

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Say I have an interface Interfacethat has a method aMethod() that should return itself, but also has a default implementation. Then I also have a class Foo that implements Interface and adds the additional method aFooMethod().

interface Interface {

    public default Interface aMethod() {
        // Some code
        return this;
    }
}

class Foo implements Interface {

    public void aFooMethod() {
        // Some code
    }
}

Now I cannot call new Foo().aMethod().aFooMethod(); because aMethod() returns an Interface.

Now I could obviously go ahead and override the method in Foo:

class Foo implements Interface {

    @Override
    public Foo aMethod() {
        Interface.super.aMethod();
        return this;
    }

    public void aFooMethod() {
        // Some code
    }
}

The question I have is if there is a good way of forcing this, that a subclass has to return itself without it having to override that method?

My best aproach is to say something like this:

interface Interface<I extends Interface<I>> {

    @SupressWarnings("unchecked")
    public default I aMethod() {
        // Some code
        return (I)this;
    }
}

But this is not what I would consider good as it has to use a SupressWarnings("unchecked"). Is there any better way?

inheritance generics java
1 Answers

I recently asked a question about this very same thing, but in C#. Both the problem and the answer are still the same though: Something like this isn't possible.

The pattern you mentioned

interface Interface<I extends Interface<I>> {}

seems like a good approach at first, but falls apart as soon as there is more than one class that implements Interface. And the problem is not the necessity to suppress a warning, it's actually something way bigger. If I create two classes like this:

class A implements Interface<A> {}
class B implements Interface<A> {}

it will still work. The constraint I extends Interface<I> only forces the type parameter I to be an Interface<I>, and A Does implement Interface<A>, so it satisfies the constraint even when Interface<A> is implemented by B.

I've been trying to find ways around this, but sadly, neither Java nor C# allow for anything along these lines.

user_65148340
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