Assessment.kt

enum class Assessment {
    A, B, C, D, E, F
}

Candidate.kt

data class CandidateEntity(val id: String, val fullName: String, val contactNumbers: Collection<PhoneNumberEntity> = mutableListOf(), val grade: Assessment?) {

    init {
        if (fullName.split(' ').size < 2) throw NotFullNameException()
    }

    fun addContactNumber(contactNumber: PhoneNumberEntity) {
        (contactNumbers as MutableCollection).add(contactNumber)
    }
}

PhoneNumber.kt

data class PhoneNumberEntity(val phoneNumber: String) {
    init {
        if (phoneNumber.matches(Regex("^\\+\\d{11}\$")).not())
            throw IncorrectPhoneNumberException()
    }
}