TennisGameKotlin.kt

import kotlin.math.abs

data class Player(val name: String, var points: Int = 0)

class TennisGameKotlin(player1Name: String, player2Name: String) : TennisGame {
    private val player1 = Player(player1Name)
    private val player2 = Player(player2Name)
    private val scores = listOf("Love", "Fifteen", "Thirty", "Forty")

    override fun wonPoint(player: String) =
        if (player === player1.name)
            player1.points += 1
        else
            player2.points += 1

    override fun getScore(): String =
        when {
            someoneHasWon() -> "Win for ${winningPlayer().name}"
            playerHasAdvantage() -> "Advantage ${winningPlayer().name}"
            gameIsDrawn() -> if (player1.points >= 3) "Deuce" else scores[player1.points] + "-All"
            else -> scores[player1.points] + "-" + scores[player2.points]
        }

    private fun someoneHasWon() = pointsDifference() >= 2 && aPlayerHasMoreThan3Points()

    private fun winningPlayer() = if (player1.points > player2.points) player1 else player2

    private fun gameIsDrawn() = player1.points == player2.points

    private fun playerHasAdvantage() = pointsDifference() == 1 && aPlayerHasMoreThan3Points()

    private fun aPlayerHasMoreThan3Points() = player1.points > 3 || player2.points > 3

    private fun pointsDifference() = abs(player1.points - player2.points)
}

Thoroughly enjoyable refactorring Kata by @emilybache - play along at home here:
https://github.com/emilybache/Tennis-Refactoring-Kata

Original source java code: https://github.com/emilybache/Tennis-Refactoring-Kata/blob/master/java/src/main/java/TennisGame2.java