Why does Set.of disallow duplicate elements?

[Aaron Scott-Boddendijk]

 Dave,

|| Dave said...
|| Okay, I understand this reasoning, but when you want to construct a Set
from an array, you might be tempted to use Set.of(...) because it looks
like it supports an
|| array and indeed, you can do Set.of(new int[] {1, 2 }) I believe?
||
|| Maybe this is just a quirk because of how varargs work.

| Rémi said...
| Set.of(int[]) will call Set.of(E) with E being an int[].
| but
| Set.of(new Integer[] { ... }) calls Set.of(...).
|
| Yes, exactly, it's a known issue with varargs, you have no way to say, i
don't want this varargs to be called with an array.

I think the confusion is the interaction of boxing and varargs.

> List<Integer> list = List.of(1, 2);

is actually, once auto-boxing is applied by the compiler, executed as...

> List<Integer> list = List.of(Integer.valueOf(1), Integer.valueOf(2));

So the equivalent explicit array form should use `Integer[]` not `int[]`...

> Integer[] numbers = new Integer[] {1, 2};
> List<Integer> list = List.of(numbers);

Interestingly, if you actually wanted a `List<Integer[]>` you would then
need to say

> Integer[] numbers = new Integer[] {1, 2};
> List<Integer> list = List.<Integer[]>of(numbers);

Which is explicitly telling the compiler what the type arguments are for
this invocation of the generic method 'of'' (rather than allowing it to use
type-inference)

Regarding the use of `Set.copyOf(Arrays.asList(...))`. I do wonder about
improving the ceremony (because I agree that we want an obvious way of
getting immutable Sets from non-unique inputs) by following the pattern
presented in Optional (`Optional.of` and `Optional.ofNullable`) and
providing `Set.of` and `Set.ofMaybeUnique` (better name needed -
'ofOptionallyUnique'?) - to which the implementation could just be
`Set.copyOf(Arrays.asList(args))` (unless a more efficient path proves
valuable).

`Arrays.asList(...array...)` is not all that expensive. It is _not_ an
ArrayList but a much simpler type with rather trivial implementations for
most methods (and 'always throws' implementations for methods that are
unsupported). So not only does it mean that there's no copying occuring to
make the list but it's even possible that JIT manages enough specialisation
and inlining to elide the allocation entirely (though in practice this
doesn't occur as often as we might like).

--
Aaron Scott-Boddendijk

On Mon, Feb 1, 2021 at 10:35 AM <forax at univ-mlv.fr> wrote:

>
>
>
>
>
> De: "dfranken jdk" <dfranken.jdk at gmail.com>
> À: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> Envoyé: Dimanche 31 Janvier 2021 13:54:44
> Objet: Re: Why does Set.of disallow duplicate elements?
>
>
>
>
> BQ_BEGIN
>
> Okay, I understand this reasoning, but when you want to construct a Set
> from an array, you might be tempted to use Set.of(...) because it looks
> like it supports an array and indeed, you can do Set.of(new int[] {1, 2 })
> I believe?
>
> BQ_END
>
> Set.of(int[]) will call Set.of(E) with E being an int[].
> but
> Set.of(new Integer[] { ... }) calls Set.of(...).
>
>
> BQ_BEGIN
>
>
> Maybe this is just a quirk because of how varargs work.
>
> BQ_END
>
> Yes, exactly, it's a known issue with varargs, you have no way to say, i
> don't want this varargs to be called with an array.
>
>
> BQ_BEGIN
>
>
> I wondered if there was a canonical way to create a Set from an array, but
> couldn't find it, maybe I am missing something?
> I did notice Arrays.asList exists (which makes sense because it creates an
> ArrayList backed by the array), but not Arrays.asSet.
>
> BQ_END
>
> asList() reuse the same backing array, you can not do that with asSet() or
> contains() will be in O(n) in the worst case.
>
>
> BQ_BEGIN
>
>
> So the way I would create a Set from an array would be either
> Arrays.stream(myArr).collect(Collectors.toUnmodifiableSet()) or new
> HashSet<>(Arrays.asList(myArray)) or Set.copyOf(Arrays.asList(myArray)).
>
> BQ_END
>
> yes, the last one is the easy way to create an unmodifiable set from an
> array.
>
>
> BQ_BEGIN
>
>
> I'm not saying the way it is currently implemented is wrong, it's just
> something which can suprise developers as it surprised me. :)
>
> BQ_END
>
> Arrays are currently second class citizen in Java, because they are always
> modifiable and always covariant (String[] can be seen as a Object[]).
> We have talked several times to introduce new variants of arrays,
> non-modifiable one, non-covariant one, etc under the name Array 2.0, but
> Valhalla generics is a blocker for that project.
> Once Valhalla is done, it may be a follow up.
>
>
> BQ_BEGIN
>
>
> Kind regards,
>
> Dave
>
> BQ_END
>
>
> regards,
> Rémi
>
>
> BQ_BEGIN
>
>
>
> Op za 30 jan. 2021 om 21:30 schreef Remi Forax < [ mailto:
> forax at univ-mlv.fr | forax at univ-mlv.fr ] >:
>
> BQ_BEGIN
> Set.of() is the closest way we've got to a literal Set without having
> introduced a special syntax for that in the language.
>
> The idea is that if you conceptually want to write
> Set<String> set = { "hello", "world" };
> instead, you write
> Set<String> set = Set.of("hello", "world");
>
> In that context, it makes sense to reject Set constructed with the same
> element twice because this is usually a programming error.
> So
> Set.of("hello", "hello")
> throws an IAE.
>
> If you want a Set from a collection of elements, you can use
> Set.copyOf(List.of("hello", "hello"))
>
> regards,
> Rémi
>
> ----- Mail original -----
> > De: "dfranken jdk" < [ mailto:dfranken.jdk at gmail.com |
> dfranken.jdk at gmail.com ] >
> > À: "core-libs-dev" < [ mailto:core-libs-dev at openjdk.java.net |
> core-libs-dev at openjdk.java.net ] >
> > Envoyé: Samedi 30 Janvier 2021 19:30:06
> > Objet: Why does Set.of disallow duplicate elements?
>
> > Dear users,
> >
> > While looking at the implementation of Set.of(...) I noticed that
> > duplicate elements are not allowed, e.g. Set.of(1, 1) will throw an
> > IllegalArgumentException. Why has it been decided to do this?
> >
> > My expectation was that duplicates would simply be removed.
> >
> > If I do for instance new HashSet<>(<collection containing duplicates>)
> > it works and duplicates are removed. To me, it looks a bit inconsistent
> > to have duplicates removed for a collection passed in the constructor,
> > but not for a collection (even though it is a vararg array) passed to a
> > static factory method.
> >
> > Kind regards,
> >
> > Dave Franken
>
> BQ_END
>
>
> BQ_END
>
>
>