[Records] Transparency and effects on collections

Remi Forax forax at univ-mlv.fr
Tue Mar 17 09:57:36 UTC 2020

Hi Daniel, 
Pooling of Java objects in memory usually do more harm than good, because you artificially change the liveness of the pooled objects which doesn't work well with modern GC algorithms. 
Obviously, if objects are stored in a database, it can still be a win but usually the pool is more or less coupled with the ORM and use off-heap memory. 

EnumSet/EnumMap are specialized because Enum.ordinal() is a perfect hashcode function, so you can implement fast and compact set and map. It's not clear to me why there is a need for a specialized version of set/map for record given has you said a record is a class. Do we miss something ? 

About filtering fields value on stream, adding a method isEqual that takes a mapping function and a value should be enough, 
Predicate<Point> filter = Predicate.isEqual(Company::name, "Apple"); 
I remember the lambda EG discuss that method (and its primitive variations), i don't remember why it's was not added. 

About introducing a typesafe representation of fields, we currently provide a non-typesafe representation, j.l.r.RecordComponent. 
There is no typesafe representation because what :: means on a field is still an open question. 
And if we go that way, a static final field is not the best representation in term of classfile because it is initialized too early. A ldc constantdynamic + a static method or something along that line is a better idea. 


> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Lundi 16 Mars 2020 21:24:04
> Objet: Fwd: [Records] Transparency and effects on collections

> Received on the -comments list.

> -------- Forwarded Message --------
> Subject: 	[Records] Transparency and effects on collections
> Date: 	Wed, 11 Mar 2020 07:45:32 -0700 (PDT)
> From: 	Daniel Latrémolière [ mailto:daniel.latremoliere at gmail.com |
> <daniel.latremoliere at gmail.com> ]

> To: 	[ mailto:amber-spec-comments at openjdk.java.net |
> amber-spec-comments at openjdk.java.net ]

> I understand that records are transparent and have correct equals/hashcode, then
> are useful as keys in collections. When trying to find classes to evolve to
> records, I found classes having more or less the same use-cases in memory than
> a multi-column primary key would have in SQL.
> ------------------------------------------------------------------------
> Records are a sugar above classes, like enum but for another use case, is it
> planned to have more evolved collections (like enum has with EnumSet/EnumMap)?
> Given records are explicitly transparent, API for pooling records would need to
> use this explicit transparency to allow partial queries and not only the
> Set/Map exact operations.

> If this is the case, it would probably need some specialised subtype of Set,
> like a new RecordSet (similar to a simple table without join, contrary to SQL).
> Current Java's Stream API would probably be perfect with some small
> enhancements, JPA-like (on a sub-type RecordStream<R>) allowing to refer
> directly to the field to be filtered.

> In this case, the compiler would need to generate for each record one static
> field per instance field of the record to allow typed queries, like in the
> following example.

> |record R(|||String foo, ...)| {||
> || ...||
> ||}||

> desugarized more completely in:

> |class R {||
> || public static final RecordField<R, String> FOO;||
> || private String foo;||
> || ...||
> ||}||

> It would allow some typed code for partial querying, like:

> |RecordSet<R> keyPool;||
> ||Predicate<String> fooFilter;||
> ||keyPool.stream().filter(R.FOO, fooFilter).forEach(...);||

> Thanks for your attention,
> Daniel.
> ------------------------------------------------------------------------
> NB: in desugarization, I used standard static fields, like JPA, and not an enum
> containing all meta-fields (which would probably be more correct and
> efficient). This is due to the lack of JEP 301 (needed for typed constants in
> enum). If allowed, the desugarized record will become something like:

> |class R {||
> || public static enum META<X> implements RecordField<R, X> {||
> || FOO<String>(String.class);||
> || }||
> || private String foo;||
> || ...||
> ||}||

> In query, it would be used as R.META.FOO for filtering on field "foo" of the
> record:

> |keyPool.stream().filter(R.META.FOO, fooFilter).forEach(...)|
> ------------------------------------------------------------------------
> PS: I am not interested in interning records but using them in pools defined by
> programmer. Pooling would improve memory and performance to deduplicate
> records, because equals would more frequently succeed at identity test without
> continuing to real equality test (field by field). Having specialized
> implementations of collections, using fields of records following the order
> given by user, would probably be useful for performance against simple Set/Map:
> structures like a hierarchical Map of Map of ..., field by field, can be more
> efficient if partial queries are frequently used or if the pool is big.
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