Hello at all. I've got some java objects that I'd ...

# getting-started

[Wesley Acheson]

Hello at all. I've got some java objects that I'd need to convert to kotlin. Is there an idiomatic way of specifying constraints on data classes. Like the following Java:

public class TotallyMadeUpClass{
    private String username
    public String getUsername(){ return this.username }
    public void setUsername(String username) {
        if (username.length > 10)
        throw new IllegalArgumentException()
        this.username = username}
}

[Wesley Acheson]

it doesn't need to be strings, it doesn't need the illegal argument exception. I just want a way to specify the constraint in the language.

[Ruckus]

class TotallyMadeUpClass(userName: String) {
  var userName = userName
    set(value) {
      require(value.length <= 10)
      field = value
    }
}

[Wesley Acheson]

Okay so vars not vals. I assume also that this cannot be used as a constructor parameter? I'm also crossposting to stack overflow.

[Ruckus]

Edited to show using constructor param.

For future note, if you're asking on SO, it's common practice to post the question there and then just post a link here (with some context) so people can go check it out if they want. That way you don't split the discussion.

[Wesley Acheson]

Thank you

[Wesley Acheson]

I've put it on StackOverflow anway. As maybe will help someone else find the answer. Feel free to answer or I can copy your text into an answer.

https://stackoverflow.com/questions/62283491/kotlin-data-class-enforce-value-constraints

[Wesley Acheson]

I'll remember to do that in the future.

[Ruckus]

Not a big deal, and it's not a hard requirement or anything, just something that has become a common practice of sorts.

[Philip Puffinburger]

If you do want to use a data class and val’s you should be able to do:

data class TotallyMadeUpClass(val username: String) {
    init {
        require(username.length > 10)
    }
}

That should validate whether you set it from the constructor or the copy method.

[Wesley Acheson]

@Philip Puffinburger Perfect thats what I'm looking for. It would be even cooler if it had TypeScript like constraints but this is what I needed. I was already trying to see if I could force @Ruckus example to something like this.

[Wesley Acheson]

Thank you both I didn't know the require keyword either.

fun main() {
    println(TotallyMadeUpClass("OK"))
    println(TotallyMadeUpClass("This username should be rejected as its too long"))
}

data class TotallyMadeUpClass(val username:String) {
    init {
        require(username.length < 10) {"Testing to see if I can get a constraint message"}
    }
}

TotallyMadeUpClass(username=OK)
Exception in thread "main" java.lang.IllegalArgumentException: Testing to see if I can get a constraint message
at com.blah.TotallyMadeUpClass.<init>(ConstraintTest.kt:10)
at com.blah.merchantapi.ConstraintTestKt.main(ConstraintTest.kt:5)
at com.blah.merchantapi.ConstraintTestKt.main(ConstraintTest.kt)

[Wesley Acheson]

sorry about formatting

[Matteo Mirk]

Here is a little variation to show how it could be done in case of a standard class, having a constructor parameter that doesn’t correspond to a property:

class Username(username: String) {
    val user = "@${username.toLowerCase()}"
    init {
        require(username.length < 10) { "user name must be shorter than 10 characters" }
    }
}

// usage
val u = Username("Joe")
u.user // @joe

notice the absence of val/var in the constructor argument. The read-only property user is initialized only after the init check passes