Validation and DDD - kotlin data classes

In Java I would do validation when creating constructor in domain object, but when using data class from kotlin I don't know how to make similar validation. I could do that in application service, but I want to stick to domain object and it's logic. It's better to show on example.

public class Example {

private String name;

Example(String name) {
validateName(name);
this.name = name;
}
}

In Kotlin I have just a data class is there a way to do it similarly to Java style?

data class Example(val name: String)

You can get a similar effect by using *[companion factory method][1]*
:

data class Example private constructor(val name: String) {
companion object {
operator fun invoke(name: String): Example {
//validateName
return Example(name)
}
}
}

...

val e = Example("name")
e.name //validated

[1]:

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You can put your validation code inside an [`initializer block`](

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). This will execute regardless of whether the object was instantiated via the primary constructor or via the `copy` method.

data class Example(val name: String) {
init {
require(name.isNotBlank()) { "Name is blank" }
}
}

A simple example:

fun main() {
println(Example(name = "Alice"))
println(try { Example(name = "") } catch (e: Exception) { e })
println(try { Example(name = "Bob").copy(name = "") } catch (e: Exception) { e })
}

Produces:

Example(name=Alice)
java.lang.IllegalArgumentException: Name is blank
java.lang.IllegalArgumentException: Name is blank

You may want to use the interface to hide the data class.
The amount of code will increase slightly, but I think it's more powerful.

```
interface Example {
val id: String
val name: String

companion object {
operator fun invoke(name: String): Example {
// Validate ...

return ExampleData(
id = UUID.randomUUID().toString(),
name = name
)
}
}

fun copy(name: String): Example

operator fun component1() : String
operator fun component2() : String
}

private data class ExampleData(override val id: String, override val name: String): Example {
override fun copy(name: String): Example = Example(name)
}
```