An example of a simple validator pattern

validator_example.kt

import java.util.*;

data class Error(val reason: String, val path: String)

class Address(
    val municipality: String,
    val neighbourhood: String
)

abstract class Validator<T>(val toValidate: T) {
    abstract fun validate(): List<Error?>
}

class AddressValidator(toValidate: Address) : Validator<Address>(toValidate) {
    override fun validate(): List<Error> {
         return listOf(
        	this::isMunicipalityValid,
            this::isNeighbourhoodValid
         )
         .asSequence()
         .map { it(toValidate) }
         .filter { it.isPresent() }
         .map { it.get() }
         .toList()
    }
    
    fun isMunicipalityValid(address: Address): Optional<Error> =
        if (address.municipality.isBlank()) Optional.of(Error("municipality is blank", "address.municipality"))
        else Optional.empty()
    
    fun isNeighbourhoodValid(address: Address): Optional<Error> =
        if (address.neighbourhood.isBlank()) Optional.of(Error("neighbourhood is blank", "address.neighbourhood"))
        else Optional.empty()
    
}

fun main() {
    var address = Address(
        municipality = "municipality",
        neighbourhood = "neighbourhood"
    )
    
    println(AddressValidator(address).validate())
    
    var address2 = Address(
        municipality = "",
        neighbourhood = "neighbourhood"
    )
    
    println(AddressValidator(address2).validate())
}