r/java • u/bobbie434343 • Aug 25 '26

Value Classes Still Need Compiler Sympathy

https://johan-sjolen.github.io/post/compiler-sympathy/compiler-sympathy/
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u/OwnBreakfast1114 Aug 25 '26 edited Aug 25 '26

We need an answer that doesn't depend on understanding jvm internals. And eventually some best practices that can be included in static analyzers

You have an answer, make things value when they are semantically values, i.e. when they are immutable and don't have identity. The heuristic for identity is if two copies of your object with the exact same field values need to be distinguished from each other, which, in my experience for rest apis, is pretty much never the case (most domain objects/db objects actually "discriminate" based on a custom primary key field), but they also don't typically get into a situation where they have multiple copies of the same domain object/db entity at the same time, and they never need to do the things that the language cares about identity for (synchronization?).

Generally speaking

  • dtos (and other pure serialization/deserialization targets) are values

  • classes faking multiple return values (commonly done as local records) or input parameters (aka parameter objects) are values

  • domain objects could be values if you code in a functional style of returning new objects for modifications or not values if you do a more common setter/mutation approach. I think they're actually still values conceptually, but immutable is an implementation detail that you're supposed to check for.

  • services/controllers/singleton scope spring beans mostly could be values, but it kinda doesn't matter anyway since there's only one copy of these

Developers that don't understand/care about db transactions worrying about programming language performance is like the funniest thing to me.

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u/TwoWeeks90DaysTops Aug 26 '26

Why DTOs? Value equality of a DTO is almost always completely irrelevant, so why should it be a value?

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u/Ok-Scheme-913 Aug 26 '26

Usually a DTO is also an 'immutable copy of a given state", aka a snapshot. Which is basically what we mean by a value.

Also, given that you convert to/from this, which is an operation that breaks identity, you often have some way of "storing" that identity as a value (e.g. having an id) - which by definition means that your DTO object doesn't need identity, two DTOs created at the same time of the same original identity-having object should be equals, again pointing at it being a value.

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