RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v6]

[Per Minborg]

On Sun, 16 Mar 2025 19:06:36 GMT, Luca Kellermann <duke at openjdk.org> wrote:

>> Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 246 commits:
>> 
>>  - Merge branch 'master' into implement-jep502
>>  - Clean up exception messages and fix comments
>>  - Rename field
>>  - Rename method and fix comment
>>  - Rework reenterant logic
>>  - Use acquire semantics for reading rather than volatile semantics
>>  - Add missing null check
>>  - Simplify handling of sentinel, wrap, and unwrap
>>  - Fix JavaDoc issues
>>  - Fix members in StableEnumFunction
>>  - ... and 236 more: https://git.openjdk.org/jdk/compare/4e51a8c9...d6e1573f
>
> src/java.base/share/classes/java/util/ImmutableCollections.java line 1488:
> 
>> 1486:             final K k = (K) key;
>> 1487:             return stable.orElseSet(new Supplier<V>() {
>> 1488:                 @Override public V get() { return mapper.apply(k); }});
> 
> This can return `null` (`StableMap` does allow `null` values), so the `getOrDefault` implementation in `AbstractImmutableMap` does not properly work for `StableMap`:
> 
> var map = StableValue.map(Set.of(1), _ -> null);
> // should print "null", but prints "default value"
> System.out.println(map.getOrDefault(1, "default value"));

Thanks for identifying this issue.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23972#discussion_r2020689451