Records -- current status

Kevin Bourrillion kevinb at
Fri Mar 16 21:14:02 UTC 2018

On Fri, Mar 16, 2018 at 11:55 AM, Brian Goetz <brian.goetz at>

There are a number of potentially open details on the design for records.
> My inclination is to start with the simplest thing that preserves the
> flexibility and expectations we want, and consider opening up later as
> necessary.
> One of the biggest issues, which Kevin raised as a must-address issue, is
> having sufficient support for precondition validation. Without foreclosing
> on the ability to do more later with declarative guards, I think the recent
> construction proposal meets the requirement for lightweight enforcement
> with minimal or no duplication.  I'm hopeful that this bit is "there".

Agreed. Even if we had solved declarative guards, we'd still benefit from
what you're doing here when we need a defensive copy etc.

> Our goal all along has been to define records as being “just macros” for a
> finer-grained set of features.  Some of these are motivated by boilerplate;
> some are motivated by semantics (coupling semantics of API elements to
> state.)  In general, records will get there first, and then ordinary
> classes will get the more general feature, but the default answer for "can
> you relax records, so I can use it in this case that almost but doesn't
> quite fit" should be "no, but there will probably be a feature coming that
> makes that class simpler, wait for that."
> Some other open issues (please see my writeup at
> for reference),
> and my current thoughts on these, are outlined below. Comments welcome!
>  - Extension.  The proposal outlines a notion of abstract record, which
> provides a "width subtyped" hierarchy.  Some have questioned whether this
> carries its weight, especially given how Scala doesn't support case-to-case
> extension (some see this as a bug, others as an existence proof.)  Records
> can implement interfaces.

I also suggest we avoid abstract records. A reference to one may seem like
a proper record but it will behave badly with regard to equals(). I don't
see the upside compared to a common interface, and then you don't have to
have the novel parameterized extends clause.

 - Concrete records are final.  Relaxing this adds complexity to the
> equality story; I'm not seeing good reasons to do so.


>  - Additional instance fields.  These are a much bigger concern. While the
> primary arguments against them are of the "slippery slope" variety, I still
> have deep misgivings about supporting unrestricted non-principal instance
> fields, and I also haven't found a reasonable set of restrictions that
> makes this less risky.  I'd like to keep looking for a better story here,
> before just caving on this, as I worry doing so will end up biting us in
> the back.

Lazy-initialized derived values are common enough. I'm not grasping what
there is to be afraid of here - I thought that preventing custom eq/hc
addressed the concerns. I understand the spirit of "start restrictive and
open up later", but I think we should still have some halfway convincing
explanation of why this was worth worrying about.

>  - Mutability and accessibility.  I'd like to propose an odd choice here,
> which is: fields are final and package (protected for abstract records) by
> default, but finality can be explicitly opted out of (non-final) and
> accessibility can be explicitly widened (public).

I agree that field accessibility should play by the normal rules.

On the other hand. As much as I want everyone to stick to immutable records
as much as possible, it seems very costly to me to have to introduce a new
keyword for "not final", and have users keep track of which things have
which defaults. Let this just be "best practice", like it already is for
regular fields (make them final unless you have good reason not to).

 - Accessors.  Perhaps the most controversial aspect is that records are
> inherently transparent to read; if something wants to truly encapsulate
> state, it's not a record.  Records will eventually have pattern
> deconstructors, which will expose their state, so we should go out of the
> gate with the equivalent.  The obvious choice is to expose read accessors
> automatically.  (These will not be named getXxx; we are not burning the
> ill-advised Javabean naming conventions into the language, no matter how
> much people think it already is.)  The obvious naming choice for these
> accessors is fieldName().  No provision for write accessors; that's
> bring-your-own.

Method and field named identically is a slight concern. If we gain field
references using the same syntax as method references there would probably
be no way to refer to such a field. I'm pretty sure this is not worth
worrying about though.

 - Core methods.  Records will get equals, hashCode, and toString.  There's
> a good argument for making equals/hashCode final (so they can't be
> explicitly redeclared); this gives us stronger preservation of the data
> invariants that allow us to safely and mechanically snapshot / serialize /
> marshal (we'd definitely want this if we ever allowed additional instance
> fields.)  No reason to suppress override of toString, though.

Agree with all this.

> Records could be safely made cloneable() with automatic support too (like
> arrays), but not clear if this is worth it (its darn useful for arrays,
> though.)

People just really need to not use arrays anymore, and especially not with
records. imho we should have added immutable List and ImmutableIntArray
etc. classes a very long time ago. I know we won't now due to our value
type aspirations. In the meantime we're in a weird place. Arrays are
completely terrible except as micro-optimizations to be used with great

Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at

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