The real world implementation of strategy pattern in Kotlin

RahmayanaDeptStore.kt

/**
 * Created by omayib on 22/05/17.
 */
fun main(args: Array<String>){
    val tShirtUbuntu = TShirt(70000,2)
    val tShirtAndroid = TShirt(50000,4)
    val tShirtCoder = TShirt(80000,3)

    val bill = Bill()
    bill.add(tShirtUbuntu)
    bill.add(tShirtAndroid)
    bill.add(tShirtCoder)

    val priceUsingDefaultCoupon = bill.calculate(DefaultCoupon())
    val priceUsingSpecialCoupon = bill.calculate(SpecialCoupon())
    val priceUsingNewYearCoupon = bill.calculate(NewYearCoupon())
    val priceUsingChristmasCoupon = bill.calculate(ChristmasCoupon())
    
    println(String.format("the total price is $priceUsingDefaultCoupon"))
    println(String.format("the total price with special discount is $priceUsingSpecialCoupon"))
    println(String.format("the total price with new year discount is $priceUsingNewYearCoupon"))
    println(String.format("the total price with new year discount is $priceUsingChristmasCoupon"))
}


/**
 * TShirt is an item used for purchase simulation
 * */
data class TShirt(val price: Int, val numberOfCloth:Int)

/**
 * The Bill will be used to hold collection of t-shirts to be purchased
 * */
class Bill{
    private var collection = ArrayList<TShirt>()

    fun add(tShirt: TShirt){
        collection.add(tShirt)
    }

    fun calculate(pricingMethod: Pricing): Int{
        val sum = collection.map { it.price * it.numberOfCloth }.sum()
        return pricingMethod.calculate(sum)
    }

}

interface Pricing{
    fun calculate(sum: Int): Int
}
/**
 * The default price calculation without any discount
 * */
class DefaultCoupon : Pricing {
    override fun calculate(sum: Int): Int {
        return sum
    }
}
/**
 * The total price is calculated by give a discount IDR 10.000 if purchase more than IDR 100.000
 * and discount IDR 25.000 if purchase more than IDR 200.000
 * */
class SpecialCoupon : Pricing{
    override fun calculate(sum: Int): Int {
        var totalPrice = sum
        if (sum in 100001..200000){
            totalPrice -=  10000
        } else if (totalPrice > 200000){
            totalPrice -= 25000
        }
        return totalPrice
    }
}
/**
 * The total price is calculated by give a discount 25% sale
 * */
class NewYearCoupon : Pricing{
    override fun calculate(sum: Int): Int {
        return sum*75/100
    }
}
/**
 * The total price is calculated by give a discount IDR 50.000
 * when total prices is equals IDR 300.000
 * */
class ChristmasCoupon : Pricing{
    override fun calculate(sum: Int): Int {
        return if (sum == 300000) sum-50000 else sum
    }
}