On Mon, 22 Sep 2025 07:22:15 GMT, Per Minborg <[email protected]> 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.
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PR Comment: https://git.openjdk.org/jdk/pull/27406#issuecomment-3318440504
