Three builder method in Kotlin

README.md

Introduction

Three builder pattern ways in Kotlin.

1. Normal Builder
2. Kotlin Block Builder 1
3. Kotlin Block Builder 2

builder1.kt

class Car(  //add private constructor if necessary
    val model: String?,
    val year: Int
) {
    private constructor(builder: Builder) : this(builder.model, builder.year)

    class Builder {
        var model: String? = null
            private set
        var year: Int = 0
            private set

        fun model(model: String): Builder {
            this.model = model
            return this
        }

        fun year(year: Int): Builder {
            this.year = year
            return this
        }

        fun build() = Car(this)
    }
}


// Usage
val car = Car.Builder().model("BMW").build()

builder2.kt

class Car (  //add private constructor if necessary
    val model: String?,
    val year: Int
) {

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

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

    class Builder {
        var model: String? = null
        var year: Int = 0

        fun build() = Car(this)
    }
}

// Usage
val car = Car.build { model = "BMW" }

builder3.kt

class Car (  //add private constructor if necessary
        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 = 0

        fun build() = Car(this)
    }
}

// Usage
val car = Car.build(required = "value") { model = "BMW" }