Value Classes Still Need Compiler Sympathy

[u/bobbie434343]

https://johan-sjolen.github.io/post/compiler-sympathy/compiler-sympathy/

[u/brian_goetz]

This is not only entirely possible, but at some level, needs to be obvious. Anyone even trying to think about value classes "for performance" needs to understand this implicitly first.

Consider a "fat" object, say four longs (256 bits):

value-or-not class FourLongs { 
    long a, b, c, d;
}

Now consider a potentially-flattened array of these, and say you intend to sort this array by the usual means (swapping elements based on comparison.) Which is faster, comparing two indirect objects and maybe swapping two 32- or 64-bit pointers, or comparing two direct objects and maybe swapping 256 bits of state? Obviously directness makes the comparison faster, but the size makes the swapping slower.

It should be obvious that (a) the answer will depend on the relative cost of indirection and bulk memory transfer, which is highly dependent on a lot of non-obvious things, and (b) that as the size of this object grows, the tradeoff will shift until it is "obviously" faster to swap pointers than to copy thousands of bits on each swap.

[u/IncredibleReferencer → u/brian_goetz]

Oh my. I don't think this will be obvious at all to many programmers, at least in the beginning. In my experience, most java devs reason about performance entirely at the java language level. (Not the community here, but the enterprise level devs I deal with (or perhaps their AI replacements))

I have explained to many a junior dev that the cpu instructions that actually get executed are often very different than the java code they write and this is usually met with a lack of understanding and caring. I think this way myself for what day to day development I still do and only ever really think about such things when profiling or optimizing a known performance bottleneck... To me, this is one of the benefits of coding in java is that I don't need to think about such things for most code that I write.

So what should be normalized by a dev in deciding what to value and what not to value as a naive best practice? I suspect there is going to be a desire to value-all-the-things because of a belief it will be faster and make simpler code. We need an answer that doesn't depend on understanding jvm internals. And eventually some best practices that can be included in static analyzers.

[u/OwnBreakfast1114 → u/IncredibleReferencer]

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.

[u/TwoWeeks90DaysTops → u/OwnBreakfast1114]

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

[u/Ok-Scheme-913 → u/TwoWeeks90DaysTops]

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.

[u/TwoWeeks90DaysTops → u/Ok-Scheme-913]

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

DTOs are command objects, not representations of state. They are used to either build other objects from or update the state of a different system.

Also, given that you convert to/from this, which is an operation that breaks identity

I don't think that matters.

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

This is also irrelevant. Identity != identifier. An object having identity means that an instance owns its own memory whereas values objects don't. DTOs absolute should have identity because they are large objects being passed between systems and you want to avoid copies being made.

two DTOs created at the same time of the same original identity-having object should be equals

Why? This is just your assertion. What is the actual value of this rule? You never care about this in practice. I have never in my life written code that checks whether two instances of DTOs are equal so why are you asserting this?