Factory methods for SequencedSet and SequencedMap
Joseph D. Darcy
joe.darcy at oracle.com
Fri Jan 17 17:30:40 UTC 2025
On 1/16/2025 11:26 PM, Rafael Winterhalter wrote:
> Would it even be possible to change the return types of Set.of(...)
> and Map.of(...) without breaking binary compatibility?
In short, no.
The methods in question are *static* methods. Switching to covariant
overrides with more precise return types works for subclasses because of
bridge methods.
In a bit more detail, in a covariant override a single method in the
source code gets translated into multiply methods in the class file.
References to methods in the class file use the argument types and
return type so if an old class file refers to the previously declared
source-level return type, there is the bridge method present to be
linked to (for binary compatibility) and then executed.
-Joe
>
> I also think that the randomization of Set.of(...) and Map.of(...) is
> a good property as it uncovers bugs early if one relies on iteration
> order. This especially since those methods are often used in tests
> where production code would use a proper HashSet which cannot
> guarantee iteration order for good reasons. Exactly here I think the
> new interfaces are a good addition as it uncovers such misconceptions.
> If code relies on insertion order, providing a Set.of(...) does no
> longer compile, what is a good thing.
>
> To me, adding SequencedSet.of(...) and SequencedMap.of(...) sounds
> like the right approach, with implementations similar to that of
> Set.of(...) and Map.of(...). As for megamorphism, I think the chance
> of encounter at a call site is similar, as Set12 and SetN from the Set
> interface are typically combined with HashMap. As for a possible
> SequencedSet12 and SequencedSetN, I think they would normally be seen
> with LinkedHashSet.
>
> Best regards, Rafael
>
> Am Fr., 17. Jan. 2025 um 00:36 Uhr schrieb David Alayachew
> <davidalayachew at gmail.com>:
>
> I should also add, the documentation went out of their way to
> specify that iteration order is unspecified.
>
> Also, I see Rémi's comment, but that's even more unconvincing to me.
>
> Map.of has an upper limit of 10 entries, and Map.ofEntries has an
> upper limit of that Java max file size limit thing. You all know
> what I am talking about.
>
> Point is, both of these static factories were meant to be used on
> a small number of entries. If it truly has just been not done
> until now, then the bug database will confirm that easily.
>
> When I get back, I can check myself.
>
>
> On Thu, Jan 16, 2025, 6:25 PM David Alayachew
> <davidalayachew at gmail.com> wrote:
>
> I guess let me ask the obvious question.
>
> Chesterton's fence -- why wasn't this done before? I refuse to
> believe that this idea wasn't thought up years ago, which
> leads me to believe there was a reason that it hasn't been done.
>
> Is there any way we can look this up in the bug database or
> something?
>
>
> On Thu, Jan 16, 2025, 2:28 PM Jens Lideström
> <jens at lidestrom.se> wrote:
>
> Having the result Map.of and Set.of preserve the insertion
> order would
> often be convenient.
>
> More often than not programs iterate over the contents of
> a maps and
> sets at some point. For example to present the values in a
> GUI, for
> serialisation, or even for error printouts. In all those
> cases having a
> fixed iteration order is much better than having a random
> iteration
> order.
>
> Often it is even a subtle bug to have a random iteration
> order. For
> example, I ran in to a situation where jdeps printed a
> error message
> containing a list of modules. But the list was in a
> different order on
> each run of the program! It took me a while to figure out
> that it was
> actually the same list. A possible explanation is that
> jdeps is
> implemented using Map.of or Set.of.
>
> Because of this I think I would be better if the most
> commonly used
> standard collection factories produced collections with a
> fixed
> iteration order.
>
> Guavas ImmutableMap and ImmutableSet also preserve
> insertion order.
>
> Regards,
> Jens Lideström
>
>
> On 2025-01-16 08:44, Remi Forax wrote:
>
> > -------------------------
> >
> >> From: "Rafael Winterhalter" <rafael.wth at gmail.com>
> >> To: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> >> Sent: Thursday, January 16, 2025 8:13:17 AM
> >> Subject: Factory methods for SequencedSet and SequencedMap
> >
> >> Hello,
> >
> > Hello,
> >
> >> I am happily taking SequencedSet and SequencedMap into
> use, but one
> >> inconvenience I encounter is the lack of factory
> methods for the two.
> >> In code where many (initial) collections have zero or
> one element (for
> >> later aggregation), I now write Set.of()/Set.of(one) and
> >> Map.of()/Map.of(key, value), as it makes the code
> shorter and more
> >> readable. Those collections are of course implicitly
> sequenced, but
> >> now I must make the variable type of the surrounding
> monad Set and
> >> Map, and simply assume that a LinkedHashSet or
> LinkedHashMap is used
> >> when a collection of more than one element is set,
> without requiring
> >> the interface type. This does not require any type
> casting, as I rely
> >> on the iteration order only, but the code loses some of
> its
> >> expressiveness.
> >> I did not find any discussion around introducing
> factories for
> >> SequencedSet.of(...) and SequencedMap.of(...), similar
> to those that
> >> exist in the Set and Map interfaces. Was this ever
> considered, and if
> >> not, could it be?
> >
> > Thanks for re-starting that discussion, it was talked a
> bit, but not as
> > it should be.
> >
> > So the issue is that currently we do not have any
> compact, unmodifiable
> > and ordered Set/Map implementation,
> > one use case is when you have data that comes from a
> JSON object as a
> > Map and you want to keep the inserted order, if by
> example the JSON is
> > a config file editable by a human, an other example is
> in unit tests
> > where you want to help the dev to read the output of the
> test so the
> > code that creates a Set/Map and what is outputed by the
> test should be
> > in the same order.
> > Currently there is no good solution for those use cases
> because
> > Set|Map.copyOf() does not keep the ordering.
> >
> > I see two solutions, either we add a new
> > SequenceSet|SequenceMap.of/copyOf, or we change the
> impleemntation of
> > Set|Map.of()/copyOf().
> > Python had gone for the latter solution, which has the
> advantage a
> > being simple from the user POV, but from an algorithm
> expert POV, a Set
> > and a SequencedSet are different concepts we may want to
> emphasis ?
> >
> >> Best regards, Rafael
> >
> > regards,
> > Rémi
>
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