Builder pattern in Kotlin

BuilderPattern.kt

abstract class BaseBuilder<T> {

    abstract fun buildInternal(): T
}

fun <T, R: BaseBuilder<T>> R.build(block: R.() -> Unit): T {
    block.invoke(this)
    return this.buildInternal()
}

class Rectangle(
    val x: Int,
    val y: Int,
    val width: Int,
    val height: Int
) {
    override fun toString(): String {
        return "$x $y $width $height"
    }
}

class RectangleBuilder : BaseBuilder<Rectangle>() {
    private var x: Int = 0
    private var y: Int = 0
    private var width: Int = 0
    private var height: Int = 0

    fun setX(x: Int) {
        this.x = x
    }

    fun setY(y: Int) {
        this.y = y
    }

    fun setWidth(width: Int) {
        this.width = width
    }

    fun setHeight(height: Int) {
        this.height = height
    }

    override fun build(): Rectangle {
        return Rectangle(x, y, width, height)
    }
}

fun main() {
    val rectangle = RectangleBuilder().build {
        setX(12)
        setY(24)
        setWidth(3)
        setHeight(4)
    }
    
    print(rectangle)
}

This is an example of how to use Builder pattern in Kotlin.

Using BaseBuilder is optional and a bit discouraged tbh because there is a downside to that(buildInternal is public)

This is just a reference implementation.