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 {
constructor(
private readonly type: string,
private readonly days: number,
) {}
price(): number {
switch (this.type) {
case "economy":
return 30 * this.days;
case "suv":
return 50 * this.days + 20;
default:
throw new Error(this.type);
}
}
freeKm(): number {
switch (this.type) {
case "economy":
return 200 * this.days;
case "suv":
return 150 * this.days;
default:
throw new Error(this.type);
}
}
}
function desk(): string[] {
const city = new Rental("economy", 3);
const trip = new Rental("suv", 3);
return [
`${city.price()} ${city.freeKm()}`, // 90 600
`${trip.price()} ${trip.freeKm()}`, // 170 450
];
}After
abstract class Rental {
constructor(protected readonly days: number) {}
abstract price(): number;
abstract freeKm(): number;
}
class EconomyRental extends Rental {
override price(): number {
return 30 * this.days;
}
override freeKm(): number {
return 200 * this.days;
}
}
class SuvRental extends Rental {
override price(): number {
return 50 * this.days + 20;
}
override freeKm(): number {
return 150 * this.days;
}
}
class VanRental extends Rental {
override price(): number {
return 70 * this.days;
}
override freeKm(): number {
return 100 * this.days;
}
}
function desk(): string[] {
const city = new EconomyRental(3);
const trip = new SuvRental(3);
const move = new VanRental(3);
return [
`${city.price()} ${city.freeKm()}`, // 90 600
`${trip.price()} ${trip.freeKm()}`, // 170 450
`${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 to find the rule for that car. The desk rents an economy car and an SUV for three days, and returns 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 gets back 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. Economy Rental and SUV Rental have only a constructor, 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. Rental's price becomes abstract, so the price switch is gone, and days is now protected so the subclasses can read it. 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 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 field and its constructor parameter are deleted, and Rental's constructor shrinks to the days. The two subclass constructors only passed a string up, and a class without a constructor inherits Rental's, so they go too.
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, TypeScript won't compile it, because it must implement every abstract method.
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 switch or if chain on one field, repeated in several methods.