Kotlin : implenting an immutable class through the data class method but making sure the input values are clean

I agree with the second link. If you look at the comments on the Baeldung article, you'll see even they were convinced and pledged to revise the article.

You can throw exceptions in an init block but if these are exceptions that are not caused by programmer error, it would be more Kotlin-idiomatic to expose a single constructor-like function that returns a wrapper or just null for invalid input.

Examples:

data class Person(val name: String, val age: Int = 0) {
    init {
        if (age < 0) {
            throw IllegalArgumentException("Age $age is less than 0.")
        }
    }
}

If you want to return a wrapper or nullable, a data class isn't suitable for preventing invalid input because the generated copy() function will always return a fully constructed object. Sadly, Kotlin does not support overriding the generated copy() function.

sealed class Result<T>
data class Success<T>(val value: T): Result<T>()
data class Failure<T>(val reason: String): Result<T>()

class Person private constructor(val name: String, val age: Int = 0) {
    companion object {
        fun build(name: String, age: Int = 0): Result<Person> {
            return when {
                age < 0 -> Failure("Age $age is less than 0.")
                else -> Success(Person(name, age))
            }
        }
    }
    
    fun buildCopy(name: String = this.name, age: Int = this.age) = build(name, age)
}