nullobjectpattern-good-usage-2.kt

abstract class Workout(val duration: Duration)

class Walking(duration: Duration) : Workout(duration)
class Swimming(duration: Duration) : Workout(duration)
class Running(duration: Duration) : Workout(duration)
// null object:
object NoWorkout : Workout(Duration.ZERO) 

fun workoutService(date: LocalDate): Workout? {

    val workouts = mapOf(
        LocalDate.of(2020, 4, 25) to Walking(Duration.ofHours(2)),
        LocalDate.of(2020, 4, 23) to Swimming(Duration.ofHours(1)),
        LocalDate.of(2020, 4, 22) to Running(Duration.ofMinutes(30))
    )
    return workouts[date] ?: NoWorkout // usage of null object
}

fun main() {
    val days = listOf(
        LocalDate.of(2020, 4, 22), LocalDate.of(2020, 4, 23), LocalDate.of(2020, 4, 24), LocalDate.of(2020, 4, 25)
    )

    var sum: Long = 0
    for(day in days) {
        val workout = workoutService(day)
        sum = sum + workout!!.duration.toMillis() // yes, we are that!! sure
    }

    println("Total duration: ${Duration.ofMillis(sum)}") // Total duration: PT3H30M
}