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Principles

Inheritance: Pull Up Duplicated Behavior

Inheritance lets a subclass get the fields and methods of its superclass, so code that the subclasses share is written once. These lessons take a courier's two shipment classes that each repeat the tracking code, the weight and the label, pull the fields, the constructor body and the label up into Shipment, and then add a fragile flag in one place.

Pick your language

Inheritance: pull up duplicated behavior in Java, step 2: Fragile parcelsJava · ~4 minInheritance in JavaInheritance: pull up duplicated behavior in Kotlin, step 2: Fragile parcelsKotlin · ~4 minInheritance in KotlinInheritance: pull up duplicated behavior in C#, step 2: Fragile parcelsC# · ~4 minInheritance in C#Inheritance: pull up duplicated behavior in C++, step 2: Fragile parcelsC++ · ~4 minInheritance in C++Inheritance: pull up duplicated behavior in Python, step 2: Fragile parcelsPython · ~4 minInheritance in PythonInheritance: pull up duplicated behavior in Ruby, step 2: Fragile parcelsRuby · ~4 minInheritance in RubyInheritance: pull up duplicated behavior in PHP, step 2: Fragile parcelsPHP · ~4 minInheritance in PHPInheritance: pull up duplicated behavior in TypeScript, step 2: Fragile parcelsTypeScript · ~4 minInheritance in TypeScriptInheritance: pull up duplicated behavior in Swift, step 2: Fragile parcelsSwift · ~4 minInheritance in Swift

When to use it

Use it when

Two or more subclasses of one superclass repeat the same fields or the same method body.

Skip it when

Two classes look alike today but aren't the same kind of thing. Extract Class for the shared part and delegate to it.

Spot it by

The same method copied into sibling subclasses, and the same fields and constructor lines in each.