RFR(m): 8177290 add copy factory methods for unmodifiable List, Set, Map

Tagir Valeev amaembo at gmail.com
Wed Nov 1 17:45:04 UTC 2017



+        if (coll instanceof ImmutableCollections.AbstractImmutableSet) {
+            return (Set<E>)coll;
+        } else {
+            return (Set<E>)Set.of(coll.stream().distinct().toArray());

I think that good old Set.of(new HashSet<>(coll).toArray()) would
produce less garbage. distinct() also maintains HashSet internally,
but it removes the SIZED characteristic, so instead of preallocated
array you will have a SpinedBuffer which is less efficient than
AbstractCollection.toArray() implementation which just allocates the
array of exact size. What do you think?


+    static final Set<Collector.Characteristics> CH_UNORDERED_NOID
+            = Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.UNORDERED));

Is it really more efficient currently than
Set.of(Collector.Characteristics.UNORDERED)? At least less objects
will be allocated with Set.of

+    Collector<T, ?, List<T>> toUnmodifiableList() {
+        return new CollectorImpl<>((Supplier<List<T>>)
ArrayList::new, List::add,
+                                   (left, right) -> {
left.addAll(right); return left; },
+                                   list -> (List<T>)List.of(list.toArray()),
+                                   CH_NOID);
+    }

Isn't it reasonable to use `e -> List.add(Objects.requireNonNull(e))`
instead of simply `List::add`? In this case if null is added, then
failure will occur much earlier, and the failure stacktrace would be
more relevant. The same for Set/Map.

+                map ->
(Map<K,U>)Map.ofEntries(map.entrySet().toArray(new Map.Entry[0])));

It's the same lambda in two versions of toUnmodifiableMap. Isn't it
better to extract it to the constant to prevent duplication in the
bytecode (or at least to method and refer to it via method reference)?

With best regards,
Tagir Valeev.

With best regards,
Tagir Valeev.

On Wed, Nov 1, 2017 at 12:49 AM, Stuart Marks <stuart.marks at oracle.com> wrote:
> Updated webrev, based on comments from Brian and Roger:
>     http://cr.openjdk.java.net/~smarks/reviews/8177290/webrev.2/
> s'marks
> On 10/30/17 3:50 PM, Stuart Marks wrote:
>> (also includes 8184690: add Collectors for collecting into unmodifiable
>> List, Set, and Map)
>> Hi all,
>> Here's an updated webrev for this changeset; the previous review thread is
>> here:
>> http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-September/049261.html
>> This webrev includes the following:
>> * specification revisions to provide clearer definitions of "view"
>> collections, "unmodifiable" collections, and "unmodifiable views"
>> * new List.copyOf(), Set.copyOf(), and Map.copyOf() "copy factory" methods
>> * new Collectors.toUnmodifiableList, Set, and Map methods
>> * tests for the new API methods
>> I've added some assertions that require some independence between the
>> source collection (or map) and the result of the copyOf() method.
>> I've made a small but significant change to Set.copyOf compared to the
>> previous round. Previously, it specified that the first of any equal
>> elements was preserved. Now, it is explicitly unspecified which of any
>> equals elements is preserved. This is consistent with Set.addAll,
>> Collectors.toSet, and the newly added Collectors.toUnmodifiableSet, none of
>> which specify which of duplicate elements is preserved.
>> (The outlier here is Stream.distinct, which specifies that the first
>> element of any duplicates is preserved, if the stream is ordered.)
>> I've also made some minor wording/editorial changes in response to
>> suggestions from David Holmes and Roger Riggs. I've kept the wording changes
>> that give emphasis to "unmodifiable" over "immutable." The term "immutable"
>> is inextricably intertwined with "persistent" when it comes to data
>> structures, and I believe we'll be explaining this forever if Java's
>> "immutable" means something different from everybody else's.
>> Webrev:
>>      http://cr.openjdk.java.net/~smarks/reviews/8177290/webrev.1/
>> Bugs:
>>      https://bugs.openjdk.java.net/browse/JDK-8177290
>>          add copy factory methods for unmodifiable List, Set, Map
>>      https://bugs.openjdk.java.net/browse/JDK-8184690
>>          add Collectors for collecting into unmodifiable List, Set, and
>> Map
>> Thanks,
>> s'marks

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