nullobjectpattern-good-usage.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)

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]
}

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)
        if (workout == null) {
            continue
        }
        sum = sum + workout.duration.toMillis()
    }
    println("Total duration: ${Duration.ofMillis(sum)}") // Total duration: PT3H30M
}