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Principles

Polymorphism: Subclasses Instead of a Type Field

Polymorphism lets a caller ask every object the same question while each subclass answers it its own way. These lessons take a car rental class that switches on a type string in every method, give each kind of car its own subclass, and then add vans as one new class.

Pick your language

Polymorphism: subclasses instead of a type field in Java, step 2: Vans join the fleetJava · ~4 minPolymorphism in JavaPolymorphism: subclasses instead of a type field in Kotlin, step 2: Vans join the fleetKotlin · ~4 minPolymorphism in KotlinPolymorphism: subclasses instead of a type field in C#, step 2: Vans join the fleetC# · ~4 minPolymorphism in C#Polymorphism: subclasses instead of a type field in C++, step 2: Vans join the fleetC++ · ~4 minPolymorphism in C++Polymorphism: subclasses instead of a type field in Python, step 2: Vans join the fleetPython · ~4 minPolymorphism in PythonPolymorphism: subclasses instead of a type field in Ruby, step 2: Vans join the fleetRuby · ~4 minPolymorphism in RubyPolymorphism: subclasses instead of a type field in PHP, step 2: Vans join the fleetPHP · ~5 minPolymorphism in PHPPolymorphism: subclasses instead of a type field in TypeScript, step 2: Vans join the fleetTypeScript · ~4 minPolymorphism in TypeScriptPolymorphism: subclasses instead of a type field in Go, step 2: Vans join the fleetGo · ~4 minPolymorphism in GoPolymorphism: subclasses instead of a type field in Swift, step 2: Vans join the fleetSwift · ~4 minPolymorphism in Swift

When to use it

Use 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. It can't change class, so use State or Strategy.

Spot it by

The same switch or if chain on one field, repeated in several methods.