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 let type: String
let days: Int
init(type: String, days: Int) {
self.type = type
self.days = days
}
func price() -> Int {
switch type {
case "economy": return 30 * days
case "suv": return 50 * days + 20
default: fatalError("unknown type \(type)")
}
}
func freeKm() -> Int {
switch type {
case "economy": return 200 * days
case "suv": return 150 * days
default: fatalError("unknown type \(type)")
}
}
}
func desk() {
let city = Rental(type: "economy", days: 3)
let trip = Rental(type: "suv", days: 3)
print(city.price(), city.freeKm()) // 90 600
print(trip.price(), trip.freeKm()) // 170 450
}
desk()After
class Rental {
let days: Int
init(days: Int) {
self.days = days
}
func price() -> Int {
fatalError("override price")
}
func freeKm() -> Int {
fatalError("override freeKm")
}
}
class EconomyRental: Rental {
override func price() -> Int { 30 * days }
override func freeKm() -> Int { 200 * days }
}
class SuvRental: Rental {
override func price() -> Int { 50 * days + 20 }
override func freeKm() -> Int { 150 * days }
}
class VanRental: Rental {
override func price() -> Int { 70 * days }
override func freeKm() -> Int { 100 * days }
}
func desk() {
let city = EconomyRental(days: 3)
let trip = SuvRental(days: 3)
let move = VanRental(days: 3)
print(city.price(), city.freeKm()) // 90 600
print(trip.price(), trip.freeKm()) // 170 450
print(move.price(), move.freeKm()) // 210 300
}
desk()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 to find the rule for that car. The desk function 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 price switch, and then another one to the free kilometers switch. The desk 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, each kind of car gets a subclass, a class that extends Rental, which now plays the superclass. Economy Rental and SUV Rental have only an initializer, which passes its type string and the days up to Rental. The desk 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. Swift has no abstract methods, so Rental's price calls fatal error, and the price switch is gone. This is polymorphism, because the desk 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 calls fatal error now 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 field and its parameter are deleted, and the subclass initializers go too, because each one only passed a string up. Swift hands Rental's initializer down to each subclass, because they add no stored properties of their own, so the desk keeps calling them the same way.
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 the desk 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 the desk asks, price and free kilometers, stay the same. And if a new class leaves out free kilometers, Swift still compiles it, and the fatal error stops the app only when someone calls it.
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 leaves the two questions to its subclasses. 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 keeps the class it was created with, so use State or Strategy instead. You'll spot the need by the same switch or if chain on one field, repeated in several methods.