Hi folks, in case you need to create your own type...

# codereview

[George]

Hi folks, in case you need to create your own type with it's own conditions for example an Int that has range from x to y. Where would you implement the check for the condition? In the constructor or in a factory function or something else. thanks in advance for any answers !

[George]

Here is how i implemented:

public value class RegistrationId(private val value: Int) : Comparable<RegistrationId> {
    init {
        require(REGISTRATION_ID_RANGE.contains(value)) {
            "The registration id is a number between 1 and 16380 but got: $value"
        }
    }

    override fun compareTo(other: RegistrationId): Int = value.compareTo(other.toInt())

    override fun toString(): String = value.toString()

    /**
     * Adds the other value to this value.
     *
     * @throws IllegalArgumentException if the resulted [RegistrationId] is out of bounds.
     */
    public operator fun plus(other: RegistrationId): RegistrationId = RegistrationId(value.plus(other.value))

    /**
     * Adds the other value to this value.
     *
     * @throws IllegalArgumentException if the resulted [RegistrationId] is out of bounds.
     */
    public operator fun plus(other: Int): RegistrationId = RegistrationId(value.plus(other))

    /**
     * Subtracts the other value from this value.
     *
     * @throws IllegalArgumentException if the resulted [RegistrationId] is negative.
     */
    public operator fun minus(other: RegistrationId): RegistrationId = RegistrationId(value.minus(other.value))

    /** Converts this [RegistrationId] value to [Int].*/
    public fun toInt(): Int = value

    public companion object {
        private const val MIN_VALUE: Int = 1
        private const val MAX_VALUE: Int = 16380

        /**
         * A constant holding the [range][IntRange] an instance of [RegistrationId] can have.
         */
        @JvmField
        public val REGISTRATION_ID_RANGE: IntRange = IntRange(MIN_VALUE, MAX_VALUE)
    }
}

[George]

Would you say is preferable to create a factory function for this, to avoid throwing in constructor?

[Klitos Kyriacou]

I think it's fine to throw in the constructor in this case. However, I would avoid repeating the number 16380, and instead make use of the IntRange toString:

init {
        require(value in REGISTRATION_ID_RANGE) {
            "The registration id must be in the range $REGISTRATION_ID_RANGE but got: $value"
        }
    }

Also, I'm sceptical about your definitions of plus and minus. Does it really make sense to be able to add or subtract IDs?

[George]

Yes indeed, plus and minus have no value.

[Giorgos Makris]

have you considered using a contract to enforce that int values passed in this object as within the specified range?

[Giorgos Makris]

what you are looking for are called refinement types and there is this library that could help you get what you want https://github.com/ustits/krefty