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Principles

Composition over Inheritance: End the Subclass Explosion

Composition over inheritance means an object holds other objects as fields and asks them for their part of the work, instead of getting that work from a parent class. When a class varies in two independent ways, inheritance needs one subclass for every pair. These lessons take a car rental desk with a subclass per car kind and cover, move car and cover into fields, and then add vans as one small class.

Pick your language

Composition over inheritance: end the subclass explosion in Java, step 2: Vans join the fleetJava · ~6 minComposition over Inheritance in JavaComposition over inheritance: end the subclass explosion in Kotlin, step 2: Vans join the fleetKotlin · ~6 minComposition over Inheritance in KotlinComposition over inheritance: end the subclass explosion in C#, step 2: Vans join the fleetC# · ~6 minComposition over Inheritance in C#Composition over inheritance: end the subclass explosion in C++, step 2: Vans join the fleetC++ · ~6 minComposition over Inheritance in C++Composition over inheritance: end the subclass explosion in Python, step 2: Vans join the fleetPython · ~6 minComposition over Inheritance in PythonComposition over inheritance: end the subclass explosion in Ruby, step 2: Vans join the fleetRuby · ~6 minComposition over Inheritance in RubyComposition over inheritance: end the subclass explosion in PHP, step 2: Vans join the fleetPHP · ~6 minComposition over Inheritance in PHPComposition over inheritance: end the subclass explosion in TypeScript, step 2: Vans join the fleetTypeScript · ~6 minComposition over Inheritance in TypeScriptComposition over inheritance: end the subclass explosion in Swift, step 2: Vans join the fleetSwift · ~6 minComposition over Inheritance in Swift

When to use it

Use it when

A class varies in two or more independent ways, like car kind and cover.

Skip it when

There is only one way to vary. Nothing multiplies, so a delegate only adds an object and a call.

Spot it by

Class names that join words from two lists, like InsuredSuv, and the same override copied across siblings.