RFR: 8299444 java.util.Set.copyOf allocates needlessly for empty input collections [v2]

[Stuart Marks]

On Mon, 9 Jan 2023 08:33:09 GMT, Viktor Klang <duke at openjdk.org> wrote:

>> Currently Set.copyOf allocates both a HashSet and a new empty array when the input collection is empty.
>> 
>> This patch avoids allocating anything for the case where the parameter collection's isEmpty returns true.
>
> Viktor Klang has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - 8299444: java.util.Set.copyOf allocates needlessly for empty input collections
>    
>        Modifies ImmutableCollections.listCopy:
>        Introduces a check for isEmpty to avoid allocation in the case of an empty input collection.
>  - 8299444: java.util.Set.copyOf allocates needlessly for empty input collections
>    
>    Modifies Map.copyOf:
>    Introduces a check for isEmpty to avoid allocation in the case of an empty input Map.

src/java.base/share/classes/java/util/ImmutableCollections.java line 169:

> 167:     @SuppressWarnings("unchecked")
> 168:     static <E> List<E> listCopy(Collection<? extends E> coll) {
> 169:         if (coll instanceof List12 || (coll instanceof ListN && ! ((ListN<?>)coll).allowNulls)) {

Maybe replace the cast with an instanceof pattern here?

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PR: https://git.openjdk.org/jdk/pull/11847