How to implement Builder pattern in Kotlin?

Question

Hi I am a newbie in the Kotlin world. I like what I see so far and started to think to convert some of our libraries we use in our application from Java to Kotlin.

T

Answer 1

I have seen many examples that declare extra funs as builders. I personally like this approach. Save effort to write builders.

package android.zeroarst.lab.koltinlab

import kotlin.properties.Delegates

class Lab {
    companion object {
        @JvmStatic fun main(args: Array) {

            val roy = Person {
                name = "Roy"
                age = 33
                height = 173
                single = true
                car {
                    brand = "Tesla"
                    model = "Model X"
                    year = 2017
                }
                car {
                    brand = "Tesla"
                    model = "Model S"
                    year = 2018
                }
            }

            println(roy)
        }

        class Person() {
            constructor(init: Person.() -> Unit) : this() {
                this.init()
            }

            var name: String by Delegates.notNull()
            var age: Int by Delegates.notNull()
            var height: Int by Delegates.notNull()
            var single: Boolean by Delegates.notNull()
            val cars: MutableList by lazy { arrayListOf() }

            override fun toString(): String {
                return "name=$name, age=$age, " +
                        "height=$height, " +
                        "single=${when (single) {
                            true -> "looking for a girl friend T___T"
                            false -> "Happy!!"
                        }}\nCars: $cars"
            }
        }

        class Car() {

            var brand: String by Delegates.notNull()
            var model: String by Delegates.notNull()
            var year: Int by Delegates.notNull()

            override fun toString(): String {
                return "(brand=$brand, model=$model, year=$year)"
            }
        }

        fun Person.car(init: Car.() -> Unit): Unit {
            cars.add(Car().apply(init))
        }

    }
}

I have not yet found a way that can force some fields to be initialized in DSL like showing errors instead of throwing exceptions. Let me know if anyone knows.