Kotlin Design Pattern: Strategy

The strategy pattern is used to create an interchangeable family of algorithms from which the required process is chosen at run-time.

Example 1:

// Step 3: Client usage:
fun main(args: Array) {
    val add = ArithmeticOperation(addOperation)
    val subtract = ArithmeticOperation(subOperation)
    val times = ArithmeticOperation(timesOperation)

    println("5 + 10 = ${add.calculate(5, 10)}")
    println("5 - 10 = ${subtract.calculate(5, 10)}")
    println("5 * 10 = ${times.calculate(5, 10)}")

}

// Step 1: Create a strategy class

class ArithmeticOperation(val operation: (Int, Int) -> Int){
    fun calculate(x: Int, y: Int): Int{
       return operation.invoke(x, y)
    }
}

// Step 2: Define some different behaviors:

val addOperation: (Int, Int) -> Int = {x: Int, y: Int -> x+y}
val subOperation = {x: Int, y: Int -> x-y}
val timesOperation = {x: Int, y: Int -> x*y}

/*
prints

5 + 10 = 15
5 - 10 = -5
5 * 10 = 50
 */

Example 2

// Step 4: Client usage:
fun main(args: Array) {
    val byAir = GoodsTrans(ByAir())
    byAir.execute("Crabs from Australia are transported")

    val byWater = GoodsTrans(ByWater())
    byWater.execute("Petroleum from Arab is transported")

    val byTrain = GoodsTrans(ByTrain())
    byTrain.execute("Visitors from Shanghai are transported")

}

// Step 1: Create an strategy interface

interface Strategy{
    fun vehicle()
}

// Step 2: Implement the Strategy for different strategies

class ByAir: Strategy{
    override fun vehicle() {
        println(" by air.")
    }
}

class ByTrain: Strategy{
    override fun vehicle() {
        println(" by train.")
    }
}

class ByWater: Strategy{
    override fun vehicle() {
        println(" by water.")
    }
}

// Step 3: Create a context class which use Strategies

class GoodsTrans(val strategy: Strategy){
    fun execute(msg: String){
        print(msg)
        strategy.vehicle()
    }
}


/*
prints:

Crabs from Australia are transported by air.
Petroleum from Arab is transported by water.
Visitors from Shanghai are transported by train.
 */