Validation and DDD - kotlin data classes - validation

Question

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)

Answer 1

You can put your validation code inside an initializer block. 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

Answer 2

You can get a similar effect by using companion factory method
:
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

Answer 3

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)
}