On Mon, 22 Sep 2025 07:22:15 GMT, Per Minborg <pminb...@openjdk.org> wrote:
>> @minborg `EmptyList` already implements it, as any other `List`:
>>
>> public interface List<E> extends SequencedCollection<E> { ... }
>>
>>
>> For `EmptyMap` (and even `SingletonMap` and `SingletonSet`), it's possible,
>> but it will require changing the public interface (`Collections::emptyMap`
>> will have to return `SequencedMap`), which may produce binary compatibility
>> issues. Probably we can invent a binary-compatible signature like this:
>>
>>
>> public static final <K,V,M extends Map<K, V> & SequencedMap<K, V>> M
>> emptyMap() { ... }
>>
>> But it looks ugly.
>
> @amaembo `SequencedMap` already implements `Map`. :-) So, we could say:
>
>
> public static final <K,V> SequencedMap<K, V> emptyMap() { ... }
>
>
> An empty map could also "incidentally" implement `SequencedMap`.
> > @minborg as JLS 13.4.15 says,
> > > Changing the result type of a method, or replacing a result type with
> > > void, or replacing void with a result type, has the combined effect of
> > > deleting the old method and adding a new method with the new result type
> > > or newly void result
> >
> >
> > Changing the result type from `Map` to `SequencedMap` will modify the
> > binary signature, so compiled classes that used this method previously will
> > fail with `NoSuchMethodError`. That's why simply changing `Map` to
> > `SequencedMap` is not quite possible. My trick with `M extends Map<K, V> &
> > SequencedMap<K, V>` employs the fact that the type parameter gets erased to
> > its first bound, which is `Map`, so such kind of return type replacement is
> > safe (but ugly).
>
> Thanks for that JLS reference, Tagir.
>
> @minborg, one would think they can make a method accept wider arguments or
> return narrower results. After all, it matches basic subtype intuitions. Yet,
> one cannot do that; they will get java.lang.NoSuchMethodError from code that
> uses the old method. FWIW, I tripped on it myself a few times.
Yeah, I get that. Another alternative would be not to expose the type in the
API, but instead let the map instance returned from the method implement
`SequencedMap`. A dynamic instanceof would then potentially benefit from this.
Maybe there are situations that are less good as well?
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PR Comment: https://git.openjdk.org/jdk/pull/27406#issuecomment-3318996371
