LESSON 10.3

Code smells

Identify symptoms in source code that indicate deeper design problems: Long Methods, Feature Envy, Primitive Obsession, and Shotgun Surgery.

6 min read

A Code Smell is a surface indication that usually corresponds to a deeper design defect in the system. Smells do not prevent code from functioning, but they indicate code that is brittle, error-prone, and difficult to extend.

Common Code Smells & Patterns That Fix Them

Smell	Description	Target Pattern / Refactoring
Primitive Obsession	Using primitives (`string`, `number`) for domain concepts (Money, Email, ZipCode)	Value Object / Class extraction
Feature Envy	A method queries another object's fields repeatedly to do work	Move Method into the target class
Long Method / God Class	Methods exceeding 50 lines or classes exceeding 500 lines doing many things	Extract Class / Method, Facade, SRP
Shotgun Surgery	A single change requires editing 10 different files	Consolidate logic into one module / Strategy
Data Clumps	The same 3 variables (`street`, `city`, `zip`) always passed together	Introduce Parameter Object (Address)

Laying out the diagram…

Refactoring code smells: Primitive Obsession transforms to Value Objects. Feature Envy moves to Domain Methods.

In code

When to refactor

Follow the Boy Scout Rule: Always leave the code cleaner than you found it. When touching a file to add a feature or fix a bug, take 10 minutes to extract a primitive obsession or eliminate a feature envy. Over time, code quality improves steadily without requiring massive, risky "rewrite everything" projects.

Primitive Obsession and Value Object: Value Objects are immutable representations with validated domain rules.
Data Clumps and Parameter Object: Group related parameters into coherent domain objects.

Quick check

What is the difference between Divergent Change and Shotgun Surgery?

Try it

Ticket Booking LLD

Refactor a ticket booking service plagued with primitive obsession by introducing SeatNumber and Money value objects.

Check your understanding

1.A method in OrderService makes 10 consecutive getter calls on a Customer object to calculate discounts. What code smell is this?
2.Using raw strings for currency, phone numbers, and email addresses throughout a system without dedicated validation types is an example of what smell?
3.What is Shotgun Surgery?

Practice it

Library Management LLDModel titles, physical copies, loans and reservations without double lending.Ticket Booking LLDReserve seats with expiring holds and reconcile payment without overselling.

Your notes

1 // ❌ CODE SMELLS: Primitive Obsession + Feature Envy 2 class User { 3 constructor(name, email, balanceAmount, currencyCode) { 4 this.name = name; 5 this.email = email; // Raw string: could be invalid "not-an-email" 6 this.balanceAmount = balanceAmount; 7 this.currencyCode = currencyCode; 8 } 9 } 10 11 class PaymentService { 12 // Feature Envy: reads user fields and does user domain math 13 canAfford(user, price) { 14 if (user.currencyCode !== "USD") throw new Error("Only USD supported"); 15 return user.balanceAmount >= price; 16 } 17 } 18 19 // ✔️ REFACTORED: Value Objects + Encapsulation 20 class Money { 21 #cents; 22 #currency; 23 24 constructor(cents, currency = "USD") { 25 if (cents < 0) throw new Error("Money cannot be negative"); 26 this.#cents = Math.round(cents); 27 this.#currency = currency; 28 } 29 30 isGreaterThanOrEqualTo(other) { 31 if (this.#currency !== other.#currency) throw new Error("Currency mismatch"); 32 return this.#cents >= other.#cents; 33 } 34 35 get cents() { return this.#cents; } 36 } 37 38 class RichUser { 39 #name; 40 #balance; 41 42 constructor(name, balance) { 43 this.#name = name; 44 this.#balance = balance; 45 } 46 47 // Feature Envy eliminated: method belongs on the data owner 48 canAfford(price) { 49 return this.#balance.isGreaterThanOrEqualTo(price); 50 } 51 } 52 53 const user = new RichUser("Bob", new Money(5000)); 54 console.assert(user.canAfford(new Money(3000)) === true); 55 console.assert(user.canAfford(new Money(8000)) === false); 56 console.log("Smells eliminated. Affordability check:", user.canAfford(new Money(3000)));

1 public class Example { 2 static final class Money { 3 private final int cents; 4 private final String currency; 5 6 Money(int cents, String currency) { 7 if (cents < 0) throw new IllegalArgumentException("Negative amount"); 8 this.cents = cents; 9 this.currency = currency; 10 } 11 12 public boolean gte(Money other) { 13 if (!this.currency.equals(other.currency)) throw new IllegalArgumentException("Currency mismatch"); 14 return this.cents >= other.cents; 15 } 16 } 17 18 static final class Customer { 19 private final String name; 20 private final Money balance; 21 22 Customer(String name, Money balance) { 23 this.name = name; 24 this.balance = balance; 25 } 26 27 public boolean canAfford(Money price) { 28 return balance.gte(price); 29 } 30 } 31 32 public static void main(String[] args) { 33 var customer = new Customer("Bob", new Money(5000, "USD")); 34 if (!customer.canAfford(new Money(3000, "USD"))) throw new AssertionError(); 35 if (customer.canAfford(new Money(8000, "USD"))) throw new AssertionError(); 36 System.out.println("Code smells refactored to Value Objects successfully."); 37 } 38 }

Common Code Smells & Patterns That Fix Them

In code

When to refactor

Related patterns

Check your understanding

Practice it

Common Code Smells & Patterns That Fix Them

In code

When to refactor

Related patterns

Check your understanding

Practice it