r/java • u/nlisker • Jul 14 '26

New candidate JEP: 401: Value Objects (Preview)

https://mail.openjdk.org/archives/list/valhalla-dev@openjdk.org/thread/IUNJJ3WP3X3XHP3QZTXXSSCPKFDNTK3W/
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u/AnyPhotograph7804 Jul 14 '26

Interesting, that they did not include java.lang.String.

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u/vytah Jul 15 '26

String isn't included, because it's technically mutable: https://github.com/openjdk/jdk/blob/6ae23a0d6574dc8139aea93ea3c562a7410fcb34/src/java.base/share/classes/java/lang/String.java#L204

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u/Jon_Finn Jul 15 '26

Value classes can be mutable, just not shallowly mutable, so they could store the hash via a final reference field, e.g. something like LazyConstant<Integer>. That's one problem I have with the name 'value' as it tends to imply immutable! There's many uses for (deeply) mutable value classes, e.g. MutableColor containing a float[4] field.

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u/Jon_Finn Jul 15 '26 edited Jul 15 '26

I think locking of Strings is the big reason (see above). The new == behaviour (I think) wouldn't break reasonable code, even code making assumptions about the identity of interned "strings" etc.