Talk.kt

enum class TalkType {
    PAIR, GROUP
}

data class Talk(val id: Int, val type: TalkType, val name: String?, val users: List<String>)

sealed class TalkBuilderState

object IdOfTalk: TalkBuilderState()
object TypeOfTalk: TalkBuilderState()
object TalkNameOfGroupTalk: TalkBuilderState()
object PairTalk: TalkBuilderState()

class TalkBuilder<in S: TalkBuilderState>(
        var id: Int = 0,
        var type: TalkType = TalkType.GROUP,
        var name: String? = null) {
    companion object {
        fun create(): TalkBuilder<IdOfTalk> = TalkBuilder()
    }
}

@Suppress("UNCHECKED_CAST")
fun TalkBuilder<IdOfTalk>.id(id: Int): TalkBuilder<TypeOfTalk> =
        this.apply { this.id = id } as TalkBuilder<TypeOfTalk>

@Suppress("UNCHECKED_CAST")
fun TalkBuilder<TypeOfTalk>.pairTalk(): TalkBuilder<PairTalk> =
        this.apply { this.type = TalkType.PAIR } as TalkBuilder<PairTalk>

@Suppress("UNCHECKED_CAST")
fun TalkBuilder<TypeOfTalk>.groupTalk(): TalkBuilder<TalkNameOfGroupTalk> =
        this.apply { this.type = TalkType.GROUP } as TalkBuilder<TalkNameOfGroupTalk>

@FunctionalInterface
interface UsersToTalk  {
    fun users(vararg users: String): Talk
}

fun TalkBuilder<TalkNameOfGroupTalk>.name(name: String): UsersToTalk = object : UsersToTalk {
    override fun users(users: Array<out String>): Talk =
            Talk(this@name.id, this@name.type, name, listOf(*users))
}

fun TalkBuilder<PairTalk>.users(user1: String, user2: String): Talk =
        Talk(this.id, this.type, null, listOf(user1, user2))

fun TalkBuilder<TalkNameOfGroupTalk>.users(vararg users: String): Talk =
        Talk(this.id, this.type, this.name, listOf(*users))

TalkTest.kt

class TalkBuilderTest {

    @Test fun testGroupTalk() {
        val groupTalk = TalkBuilder.create()
                .id(10)
                .groupTalk()
                .users("石田三成", "安国寺恵瓊", "小西行長")
        println(groupTalk)
    }

    @Test fun testGroupTalkWithName() {
        val groupTalk = TalkBuilder.create()
                .id(10)
                .groupTalk()
                .name("西軍チーム")
                .users("石田三成", "安国寺恵瓊", "小西行長")
        println(groupTalk)
    }

    @Test fun testPairTalk() {
        val pairTalk = TalkBuilder.create()
                .id(20)
                .pairTalk()
                .users("徳川家康", "小早川秀秋")
        println(pairTalk)
    }
}