Do Java records support "with" syntax?

Question

Java 14 brings records, which are a great addition seen in many functional languages:

Java:

public record Vehicle(String brand, String licensePlate) {}

ML:

type Vehicle = 
  {
    Brand : string
    LicensePlate : string
  }

In ML languages, it is possible to "update" a record by creating a copy with a few values changed:

let u = 
  {
    Brand = "Subaru"
    LicensePlate = "ABC-DEFG"
  }

let v =
  {
    u with 
      LicensePlate = "LMN-OPQR"
  }

// Same as: 
let v = 
  {
    Brand = u.Brand
    LicensePlate = "LMN-OPQR"
  }

Is this possible in Java 14?

Go to Java 16. They have some nice changes. And the answer is no — Thorbjørn Ravn Andersen, Feb 04 '21 at 15:47
@ThorbjørnRavnAndersen Just curious, did you mean "nice changes" in relevance to the question? — Naman, Feb 04 '21 at 15:53
@naman no. Java 16 allows defining the record closer to where you need to use it. — Thorbjørn Ravn Andersen, Feb 04 '21 at 16:24
Seems like, my understanding of the `with` pattern was not correct and the precise answer to the question would be as mentioned by @ThorbjørnRavnAndersen. — Naman, Feb 04 '21 at 16:44
This is a good candidate for a [JEP](http://openjdk.java.net/jeps/0). — Arvind Kumar Avinash, Feb 04 '21 at 16:57
@texasbruce Don’t use Lombok. It does very nasty tricks under the hood — Thorbjørn Ravn Andersen, Feb 04 '21 at 18:42
There is a concept of "withers" described in this exploratory document: [Functional transformation of immutable objects](https://github.com/openjdk/amber-docs/blob/master/eg-drafts/reconstruction-records-and-classes.md) — ZhekaKozlov, Feb 05 '21 at 04:16
@texasbruce just don’t. You make your programs less future proof. — Thorbjørn Ravn Andersen, Feb 05 '21 at 09:34
Lack of this feature makes Records much less compelling and will result in yet more Lombok adoption (to be clear - I avoid Lombok but understand why people use it). I thought Records would see a major reduction in the usage of Lombok and even help drive adoption of the next LTS. Woe is me. — Shorn, Feb 16 '21 at 06:40

score 6 · Accepted Answer · edited Feb 11 '21 at 18:07

6

Unfortunately, Java does not include this functionality. Though, you could create a utility method that takes in a different license plate value:

public static Vehicle withLicensePlate(Vehicle a, String newLicensePlate) {
    return new Vehicle(a.brand, newLicensePlate);
}

Used like so:

Vehicle a = new Vehicle("Subaru", "ABC-DEFG");
Vehicle b = Vehicle.withLicensePlate(a, "LMN-OPQR");

This would give you a similar result to what you were attempting using the "with" tag. And you can use this as a way to update the record.

edited Feb 11 '21 at 18:07

sdgfsdh

33,689
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answered Feb 04 '21 at 16:52

Adrian Johnson

78
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6

The correct declaration would be `public Vehicle(Vehicle a, String newLicensePlate) { this(a.brand, newLicensePlate); }` but it’s a bad pattern anyway. The constructor’s parameter types don’t indicates whether the argument is a new brand or new license plate and supporting constructors for both elements doesn’t work at all when both have the same type. A cleaner approach would be to add a *method* to the record, like `public Vehicle withLicensePlate(String newLicensePlate) { return new Vehicle(brand, newLicensePlate); }` which can be used like `Vehicle b = a.withLicensePlate("LMN-OPQR");` – Holger Feb 04 '21 at 18:08
@Holger I am not very used to ML, and as informed by OP the `with` pattern is something that's expected out of the box of records without the introduction of any new method. I almost formulated a similar design to that of your proposal [here](https://github.com/fortybits/a-bit-of-java/blob/bits/src/main/java/edu/bit/WithPatternInRecords.java), but in my (now deleted) answer I couldn't really match up to the OP's expectation I believe. – Naman Feb 04 '21 at 18:44
@Holger suggestion is what would be achieved automatically using `with`. It's disappointing Java doesn't support this; when a PL has "records", this feature is usually assumed. – sdgfsdh Feb 04 '21 at 20:07
2

@Naman I know that the OP asked for an automatism that unfortunately doesn’t exist (yet?). I just responded to the answer which suggested a constructor whereas a method would be much better. – Holger Feb 05 '21 at 09:15
@Holger just in case you are not _yet_ tired of me pinging you for questions that simply only you can respond to (imho), [here](https://stackoverflow.com/questions/66075419/which-part-of-the-jls-specifies-that-you-cant-cast-from-list-extends-listsup) is one – Eugene Feb 08 '21 at 16:34
1

FYI - I've written a library that generates "with"ers for Java 16 records: https://github.com/Randgalt/record-builder – Randgalt Mar 16 '21 at 21:07

Do Java records support "with" syntax?

1 Answers1