Kotlin builder demo

Builder.kt

class PublicClassProtectedConstructor private constructor(val foo: Int) {
    fun printFoo() {
        println(foo)
    }

    class Builder private constructor() {
        var foo: Int = 0

        private fun build(): PublicClassProtectedConstructor {
            return PublicClassProtectedConstructor(foo)
        }

        companion object {
            operator fun invoke(block: Builder.() -> Unit): PublicClassProtectedConstructor {
                return Builder().apply(block).build()
            }
        }
    }
}

fun main(args: Array<String>) {
    val theThing = PublicClassProtectedConstructor.Builder {
        foo = 123
    }

    theThing.printFoo()
}