About the possibility of using JEP468 to inspire optional fields in records for domain contracts modeling.

[david Grajales]

Hi Amber team. I would like to share some thoughts based on real use cases
I am dealing with at work.


I know is very likely you have already thought about something like
this and I don't know if this would be the best solution for the use
case presented.

My intention is not to propose this as "the solution" but instead just
present a personal use-case based on my actual job and a naive
solution that crossed my mind in the first 15 minutes.

So please pay more attention to the use case and "the problem" rather
than the proposed "solution" which is actually just a mean to explain
the problem


Nowadays to create a new record object we must set the value to all it
fields, even the ones that might be null.

record User (String name, String email){}
var userWithoutEmail = new User ("name", null)

This is good since it forces us to set all fields to a valid state and null
can be a valid state. This is useful for things like JSON serialization;
null fields in JSON are usually not serialized, which saves some bandwidth,
in very large JSON structures that only have an small set of mandatory
fields, this is very common in banks that happens to be international,
since they usually use the same Core for all countries but each country may
have different requirements (for example in some countries it's mandatory
for people to provide 2 lastnames but in other countries usually you only
have one lastname).

To achieve this I usually do the opposite of withers, let me introduce you
to the "withouts"

record User (String name, String email){

   public static withoutEmail(String name){

      return new User(name, null);
   }
}
var userWithoutEmail = User.withoutEmail(name);

The example is very basic for explanatory purposes and for a record with
only 2 fields this can be an overkill, but for bigger records it makes a
lot of sense, specially if only few fields between dozens are actually
mandatory.

Why not use a class instead? Serialization. records' serialization uses the
record constructor, which means it's safer since you can make format and
safety validations in the data before the record is built.

nowadays to make sure all the mandatory fields are set, I do something like
this.

record User (String name, String email){
User{
if(name == null)
throw new IllegalArgumentException("name field it's mandatory");
}
}


With derived record creation (or something equivalent but for direct
creation) and nullability it would be possible and very handy to be able to
declare optional fields that would compile to null (or zero in case of
primitives)


*Please ignore the straw man syntax from here.*

record User (String name!, String email? ){}
User userWithoutEmail with { name : "name"}

This would make this feature in java to behave similar to TypeScript's
interfaces, which are used to model data

export interface User {
   name: String!
   email: String?
}


To me one of the most useful use cases of this would be for writing unitary
tests and mocks.

when writing tests sometimes you want to test the behaviour of your
contracts (the domain objects) when they carry only the bare required
fields, if one has very large json objects to try out but only a small set
of mandatory fields usually it's easier to write down the json string and
pass it to jackson or gson, with this we could use the domain object
directly.


Regular test if the domain object has many optional fields


record Message (String id, String document, String name, String
lastname, String email.... (another 20 fields)){}


var request =
"""{
  id: "XXXXXXX",
  document: "NNN-NNNNN"
}"""


var message = gson.fromJson(request, Message.class);
var res = method2Test(message);
// put your favorite assertion here//


this has the issue you don't have help from the compiler in case you
want to represent different scenarios (formatting data, invalid
fields, etc)

this is why I usually do "whitouts" for this kind of records.


record Message (String id, String document, String name, String
lastname, String email.... (another 20 fields)){

  public Static Message minimalRequest(String id, String document){

     return new Message (id, document, lastname, email, null, null...);

   // so I have to write all those nulls once

   }

}

var request = Message.minimalRequest(.....);

var res = method2Test(message);

// put your favorite assertion here//


With optional fields in records there could be changed for something like
this.

record Message (String id, String document, String name?, String
lastname?, String email? .... (another 20 nullable fields)){}

Message request  with {id: "XXXXXXXX"; document: "NNN-NNNNNN"}
var res = method2Test(message);
// put your favorite assertion here//

I hope this presented use-case it's useful

Best regards!
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