readme.md

Code Smell 125 - 'IS-A' Relationship

We learned at school that inheritance represents an 'is-a' relationship. It is not.

TL;DR: Think about protocol and behavior, forget inheritance

Problems

• Bad models

• Unexpected behavior

• Subclass overrides

• Liskov substitution principle Violation

Solutions

1. Think in terms of behavior behaves-as-a

2. Prefer composition over inheritance

3. Subclassify always following 'behaves-as-a' relation

Context

IS-A relation comes from the data world.

We learned ERDs with structured design and data modeling.

Now, we need to think in terms of behavior.

Behavior is essential, data is accidental.

Sample Code

Wrong

class ComplexNumber {
    protected double realPart;
    protected double imaginaryPart;

    public ComplexNumber(double realPart, double imaginaryPart) {
        this.realPart = realPart;
        this.imaginaryPart = imaginaryPart;
    }
}

class RealNumber extends ComplexNumber {
    public RealNumber(double realPart) {
        super(realPart, 0);
    }

    public void setImaginaryPart(double imaginaryPart) {
        System.out.println("Cannot set imaginary part for a real number.");
    }
}

Right

class Number {
    protected double value;

    public Number(double value) {
        this.value = value;
    }
}

class ComplexNumber extends Number {
    protected double imaginaryPart; 
    
    public ComplexNumber(double realPart, double imaginaryPart) {
        super(realPart);
        this.imaginaryPart = imaginaryPart;
    }
}

class RealNumber extends Number { }

Detection

[X] Manual

This is a semantic smell.

Tags

• Inheritance

Conclusion

Real Number IS-A Complex number (according to math).

Integer IS-A Real number (according to math).

Real Number does not Behave-Like-A Complex number.

We cannot do real.setImaginaryPart() so it is not a Complex according to our Bijection

Relations

Code Smell 92 - Isolated Subclasses Names

Code Smell 11 - Subclassification for Code Reuse

Code Smell 37 - Protected Attributes

More Info

• Circle/Ellipse Problem

• Subtyping

• No Silver Bullet

• Bijection Principle

Credits

Photo by Joshua Rondeau on Unsplash

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DRY - Don't Repeat Yourself - Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

Andy Hunt

Software Engineering Great Quotes

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This article is part of the CodeSmell Series.

How to Find the Stinky Parts of your Code