Builders and data classes in kotlin

builders.kt

data class Car(
        val model: String?,
        val year: Int?,
        val required: String?
) {

    private constructor(builder: Builder) : this(builder.model, builder.year, builder.required)

    companion object {
        inline fun build(block: Builder.() -> Unit) = Builder().apply(block).build()
    }

    class Builder {
        var required: String? = null
        var model: String? = null
        var year: Int? = null


        fun build() = Car(this)
    }
}

data class Car2(
        val model: String,
        val year: Int,
        val required: String
)

data class Car3(
        val model: String?,
        val year: Int?,
        val required: String
) {

    private constructor(builder: Builder) : this(builder.model, builder.year, builder.required)

    companion object {
        inline fun build(required: String, block: Builder.() -> Unit) = Builder(required).apply(block).build()
    }

    class Builder(val required: String) {
        var model: String? = null
        var year: Int? = null


        fun build() = Car3(this)
    }
}

fun blah() {
    val car = Car.build {
        required = "Hi"
        model = "Kryspin"
        year = 2018
    }

    val car2 = Car2(
            required = "Hi",
            model = "Kryspin",
            year = 2018
    )

    val car3 = Car3.build("Hi") {
        model = "Kryspin"
        year = 2018
    }
}