Kotlin builder pattern

An example of a builder pattern in Kotlin.

Table of contents for design patterns in Kotlin

The builder pattern is a design pattern designed to provide a flexible solution to various object creation problems in object-oriented programming. The intent of the Builder design pattern is to separate the construction of a complex object from its representation. It is one of the Gang of Four design patterns. Wikipedia

Example:

data class Order private constructor(
    val uid: String,
    val date: LocalDateTime,
    val total: BigDecimal
) {

    class Builder() {
        private var uid: String = ""
        private var date: LocalDateTime = LocalDateTime.now()
        private var total: BigDecimal = BigDecimal.ZERO

        constructor(initBuilder: Builder.() -> Unit) : this() {
            initBuilder()
        }

        fun uid(uid: String): Builder = apply { this.uid = uid }

        fun date(date: String): Builder = date(LocalDateTime.parse(date))

        fun date(date: LocalDateTime): Builder = apply { this.date = date }

        fun total(total: Double): Builder = total(BigDecimal.valueOf(total))

        fun total(total: BigDecimal): Builder = apply { this.total = total }

        fun build() = Order(
            uid = this.uid,
            date = this.date,
            total = this.total
        )
    }

    companion object {
        fun order(init: Builder.() -> Unit): Order = Builder(init).build()
    }
}

Usage:

fun main() {
    val order = order {
        uid("ed37eae6-c32e-49ee-887b-52d829455c70")
        date("2018-10-23T17:19:33")
        total(1024.00)
    }
    print(order)
}

Result

Order(uid=ed37eae6-c32e-49ee-887b-52d829455c70, date=2018-10-23T17:19:33, total=1024.0)

kotlin pattern