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....@gmail.com>
To: "core-libs-dev" <core-libs-...@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
