Refactoring 022 - Extract Common Ancestor

Make your class hierarchy clear and flexible

Maximiliano Contieri

Jan 23, 2025

TL;DR: Extract a common abstract class to mimic real-world structure.

Problems Addressed

Duplicate Code
Inappropriate Inheritance
Shotgun Surgery
Big Class
Feature Envy
High Coupling
Concrete classes subclassified
Parent class not abstract

Related Code Smells

Code Smell 66 - Shotgun Surgery

Code Smell 255 - Parallel Hierarchies

Code Smell 63 - Feature Envy

Code Smell 43 - Concrete Classes Subclassified

Code Smell 161 - Abstract/Final/Undefined Classes

Steps

Identify common behaviors in both classes
Create an abstract class with shared behavior and no implementation
Move common logic to the abstract class
Update subclasses to inherit from the abstract class

Sample Code

Before

class Car {
    void drive() {
        System.out.println("Driving a car");
    }
}

class Truck extends Car {
    void load() {
        System.out.println("Loading cargo");
    }

    void unload() {
        System.out.println("Unloading cargo");
    }
}

// Truck reuses driving method
// Overriding it would be another code smell
// Violating Liskov Substitution rule

After

abstract class Vehicle {
    // 2. Create an abstract class
    // with shared behavior and no implementation
    abstract void drive();
    // 1. Identify common behaviors in both classes
    // 3. Move common logic to the abstract class
}

class Car extends Vehicle {
    // 4. Update subclasses to inherit from the abstract class
    void drive() {
        System.out.println("Driving a car");
    }
}

class Truck extends Vehicle {
    // 4. Update subclasses to inherit from the abstract class
    void drive() {
        System.out.println("Driving a truck");
        // Implementation is different than the car's
    }

    void load() {
        System.out.println("Loading cargo");
    }

    void unload() {
        System.out.println("Unloading cargo");
    }
}

Type

[X] Semi-Automatic

Safety

This refactoring is safe if you identify all common behaviors correctly and move one method at a time running the tests.

Why is the Code Better?

It reduces duplication, simplifies maintenance, and makes it easier to extend functionality by adding new concrete realizations.

How Does it Improve the Bijection?

By introducing an abstract class, the code better reflects the real-world hierarchy, creating a clear relationship between the generic and specific types.

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Related Refactorings

Refactoring 013 - Remove Repeated Code

Credits

Image by Pexels on Pixabay

This article is part of the Refactoring Series.

How to Improve Your Code With Easy Refactorings

Clean Code Cookbook

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