[lworld] RFR: 8346307: [lworld] Clarify identity vs value in Class, Objects, and document limitations of value objects [v9]

[Roger Riggs]

On Thu, 26 Jun 2025 17:43:52 GMT, ExE Boss <duke at openjdk.org> wrote:

>> Roger Riggs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:
>> 
>>  - A few editorial changes
>>    Highlight that in preview-mode, Objects can have either identity or value subclasses.
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - "value interface" is a left over from an earlier design and is not allowed by the JEP 401 JLS.
>>    An interface will never have the ACC_IDENTITY modifier, hence 'isValue()' will be true but is not significant.
>>    Class.toGenericString() should only include the "value" prefix for record and regular classes.
>>    Correct the test to match the toGenericString() output.
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - Merge
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - Merge branch 'lworld' into 8346307-throws-identityexception
>>  - ... and 6 more: https://git.openjdk.org/valhalla/compare/81fbc414...6cbade17
>
> src/java.base/share/classes/java/lang/System.java line 477:
> 
>> 475:      *      <div class="preview-comment">
>> 476:      *          The "identity hash code" of a {@linkplain Class#isValue() value object}
>> 477:      *          is computed by combining the hash codes of the value object's fields recursively.
> 
> Doesn’t this work by recursively calling the `identityHashCode` algorithm?
> Suggestion:
> 
>      *          The "identity hash code" of a {@linkplain Class#isValue() value object}
>      *          is computed by combining the identity hash codes of the value object's
>      *          fields recursively.

The identity hashcode of an Identity object is not recursive, its just the identity hash code of the object.
For value objects, the "recursion" is implied, though repeating it reinforces the concept.
The identity hashcode of a field, whose type is a value class,  is the combination of the
identity hashcodes of its fields.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1327#discussion_r2169820802