Polymorphism: subclasses instead of a type field
Let each kind answer for itself, so a new kind adds no switch case.
Watch the narrated video
Every step
Problem
Steps 1–3Refactor
Steps 4–8Payoff
Steps 9–10Recap
Steps 11–12The code: before and after
Before
class Rental(private val type: String, private val days: Int) {
fun price(): Int {
return when (type) {
"economy" -> 30 * days
"suv" -> 50 * days + 20
else -> throw IllegalArgumentException(type)
}
}
fun freeKm(): Int {
return when (type) {
"economy" -> 200 * days
"suv" -> 150 * days
else -> throw IllegalArgumentException(type)
}
}
}
fun main() {
val city = Rental("economy", 3)
val trip = Rental("suv", 3)
println("${city.price()} ${city.freeKm()}") // 90 600
println("${trip.price()} ${trip.freeKm()}") // 170 450
}After
abstract class Rental(protected val days: Int) {
abstract fun price(): Int
abstract fun freeKm(): Int
}
class EconomyRental(days: Int) : Rental(days) {
override fun price() = 30 * days
override fun freeKm() = 200 * days
}
class SuvRental(days: Int) : Rental(days) {
override fun price() = 50 * days + 20
override fun freeKm() = 150 * days
}
class VanRental(days: Int) : Rental(days) {
override fun price() = 70 * days
override fun freeKm() = 100 * days
}
fun main() {
val city = EconomyRental(3)
val trip = SuvRental(3)
val move = VanRental(3)
println("${city.price()} ${city.freeKm()}") // 90 600
println("${trip.price()} ${trip.freeKm()}") // 170 450
println("${move.price()} ${move.freeKm()}") // 210 300
}Transcript, step by step
Problem
Step 1: A car rental desk Before
At a car rental desk, a rental knows its type and how many days it runs. Price and free kilometers each switch on the type with a when, to find the rule for that car. Main rents an economy car and an SUV for three days, and prints the price and free kilometers of each.
Step 2: Vans join the fleet Code smell
The fleet now has vans, at seventy dollars a day with a hundred free kilometers a day. The quick way adds a van case to the switch in price, and then another one to the switch in free kilometers. Main rents a van for three days, and the run prints two hundred ten and three hundred.
Step 3: One string, a switch per method Code smell
A type field, which Fowler calls a type code, is a string that says what kind of thing an object is. So every method has to switch on that string before it can do its job. You can see the type field here, the switch in price here, and the switch in free kilometers here. Every new kind of car means opening Rental and editing every switch in it, so the van's rules end up spread over every method. A deposit method would add a third switch, and every later kind would edit all three.
Refactor
Step 4: Refactor first Back it out
Let's take the vans out again. We'll refactor first, so each kind of car gets its own class, and then vans will be one new class.
Step 5: One class per kind Replace Type Code with Subclasses
First, Rental becomes abstract, and each kind of car gets a subclass, a class that extends it. Kotlin classes are final by default, so the abstract keyword is also what lets the subclasses extend Rental. Economy Rental and SUV Rental are only a constructor, which passes its type string and the days up to Rental. Main now creates the subclasses, and both switches still work unchanged.
Step 6: Each kind sets its price Replace Conditional with Polymorphism
Next, each subclass overrides price with the line from its own case. Rental's price becomes abstract, so the switch in price is gone, and days is now protected so the subclasses can read it. This is polymorphism, because main asks every rental the same question, and each subclass answers it its own way. Last lesson pulled shared code up into a superclass, and this one pushes the code that differs down into subclasses.
Step 7: Each kind sets its kilometers Replace Conditional with Polymorphism
Free kilometers gets the same move, and each subclass overrides it with the line from its own case. Rental's free kilometers becomes abstract too, so the second switch is gone as well.
Step 8: The type string goes Remove Dead Code
Nothing reads the type field anymore, because no switch is left to ask it. So the property and the two strings passed up to Rental are deleted.
Payoff
Step 9: Vans, as one class Add vans
Now vans come back, and this time they are one new class, Van Rental. It overrides price with seventy dollars a day, and free kilometers with a hundred a day. Rental, Economy Rental and SUV Rental didn't change, and main only gains the two van lines.
Step 10: Each kind answers for itself Payoff
The type field used to sit here, and the two switches used to sit here and here. All the van's rules sit in one class now, so the next kind of car is one new class too. This is the Open Closed Principle, because Rental is open for extension and closed for change. It fits here because the fleet grows by kinds of car, while the questions main asks, price and free kilometers, stay the same. And if a new class leaves out free kilometers, Kotlin won't compile it, because a class that isn't abstract must override every abstract member.
Recap
Step 11: Where we started, where we ended Before / after
On the left, Rental keeps a type string, and price and free kilometers each switch on it. On the right, Rental holds only the days and declares the two questions as abstract methods. Each kind of car, like Economy Rental, answers both questions in its own class.
Step 12: When to reach for it Takeaways
Reach for it when two or more methods switch on the same type field. Skip it when an object changes kind while it lives, like a rental upgraded mid-trip, because an object can't change its class, so use State or Strategy instead. You'll spot the need by the same when or if chain on one field, repeated in several methods.