System Design (LLD)Single Responsibility Principle

Common SRP Misconceptions

LevelIntermediate

Duration60 mins

TopicSingle Responsibility Principle

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SRP Doesn't Mean One Method Per Class

The Most Dangerous Misinterpretation

Of all the misunderstandings surrounding the Single Responsibility Principle, none is more pervasive—or more destructive—than the belief that SRP means each class should have only one method.

This interpretation sounds reasonable on the surface. After all, if a class should have 'one responsibility,' wouldn't that translate to one action, and therefore one method? The logic seems airtight.

Except it's completely wrong.

This misconception has spawned countless 'enterprise-grade' codebases where simple operations are fragmented across dozens of anemic classes, each containing a single method. The result is code that's harder to understand, harder to maintain, and ironically, harder to change—the very opposite of what SRP was designed to achieve.

The Cost of Misunderstanding

Developers who misinterpret SRP as 'one method per class' often create systems with hundreds of tiny classes that are impossible to navigate. The cure becomes worse than the disease, and teams eventually abandon any attempt at principled design. This page will help you avoid that fate.

What You Will Learn

By the end of this page, you will understand the precise relationship between methods, responsibilities, and reasons to change. You'll be equipped to recognize when a class with many methods still honors SRP, and when a class with few methods violates it.

Where The Confusion Originates

To understand why this misconception is so widespread, we need to trace its origins. The confusion stems from conflating two fundamentally different concepts: methods and responsibilities.

The linguistic trap:

When we say a class should have 'one responsibility,' natural language suggests a single task—something atomic and indivisible. But Robert C. Martin's original formulation was far more nuanced:

"A class should have only one reason to change."

Notice the emphasis isn't on what the class does (its methods), but on what might cause it to change (its reason to change). These are completely different dimensions of analysis.

Methods vs. Reasons to Change
Concept	What It Describes	Quantity in Well-Designed Class
Methods	Actions the class can perform	As many as needed for cohesion
Responsibility	The conceptual purpose the class serves	One cohesive area
Reason to Change	External force that would require modification	One stakeholder or concern

The actor model clarification:

In later writings, Martin refined his definition to make it even clearer:

"A module should be responsible to one, and only one, actor."

An 'actor' is a stakeholder—a person or group who might request changes. A class that reports employee data might have methods for calculatePay(), generatePayslip(), and exportToAccounting(). If all these methods serve the Accounting department's needs, the class has one reason to change (Accounting requirements change) despite having multiple methods.

Conversely, if calculatePay() serves Accounting but reportHours() serves Operations, you have two actors—and two reasons to change—regardless of having only two methods.

The Mental Model Shift

Stop counting methods. Start identifying stakeholders. The question isn't 'How many things does this class do?' but 'Who would request changes to this class?' If multiple stakeholders with conflicting interests could request changes, you likely have an SRP violation.

Understanding Cohesive Methods

A class with a single responsibility will often have multiple methods that work together to fulfill that responsibility. These methods are cohesive—they operate on the same data, serve the same purpose, and change for the same reasons.

Consider a ShoppingCart class:

ShoppingCart.java
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public class ShoppingCart {
    private List<CartItem> items;
    private DiscountStrategy discountStrategy;
    
    // Adding and removing items - all serve the same responsibility
    public void addItem(Product product, int quantity) { ... }
    public void removeItem(String productId) { ... }
    public void updateQuantity(String productId, int newQuantity) { ... }
    public void clearCart() { ... }
    
    // Querying cart state - still same responsibility
    public List<CartItem> getItems() { ... }
    public int getItemCount() { ... }
    public boolean containsProduct(String productId) { ... }
    public CartItem findItem(String productId) { ... }
    
    // Calculating totals - same responsibility
    public Money getSubtotal() { ... }
    public Money getDiscount() { ... }
    public Money getTax() { ... }
    public Money getTotal() { ... }
    
    // Cart lifecycle - same responsibility
    public boolean isEmpty() { ... }
    public void applyDiscount(DiscountStrategy strategy) { ... }
}

This class has 17 methods, yet it adheres perfectly to SRP. Why?

Single reason to change: All methods would change only if the fundamental concept of 'what a shopping cart does' changes
Single actor: The E-commerce team owns this class; no other stakeholder would request changes
High cohesion: Every method operates on items and serves cart management
Unified abstraction: The class represents one real-world entity

Splitting this into 17 classes—CartItemAdder, CartItemRemover, CartQuantityUpdater, etc.—would be absurd. You'd destroy the cohesion that makes the code understandable.

Cohesion Is The Key

High cohesion means the methods of a class are strongly related—they use the same instance variables, work toward the same goal, and change together. SRP and cohesion are complementary: a class with a single responsibility should exhibit high cohesion, and highly cohesive classes naturally tend to have single responsibilities.

The Symptoms of One-Method-Per-Class Thinking

Teams that fall into the 'one method per class' trap exhibit distinctive symptoms. Recognizing these patterns can help you diagnose and correct the problem before it spreads.

Warning Signs of Over-Decomposition

•Class explosion — Simple features require dozens of new classes. Adding a discount feature creates DiscountCalculator, DiscountValidator, DiscountApplier, DiscountLogger, etc.
•Navigation nightmare — Developers can't find code because it's scattered across a labyrinth of tiny classes with mechanical names
•Shotgun surgery — Simple changes require modifications to 10+ files because functionality is atomized
•Data passing ceremonies — Classes constantly pass data to each other because they can't hold state meaningfully
•Manager/Handler proliferation — Every noun gains a Manager, Handler, Processor, or Service suffix
•Loss of domain meaning — Code no longer reflects business concepts; it reflects procedural steps

The ironic outcome:

The goal of SRP is to make code easier to change by limiting the impact of modifications. But one-method-per-class achieves the opposite:

Understanding requires more files: To comprehend a feature, you must trace through many classes
Changes ripple further: Modifying a concept touches every class involved in that concept
Testing becomes fragmented: Each tiny class needs its own tests, but the tests don't verify meaningful behavior
Refactoring is painful: Moving code means updating imports, dependencies, and wiring across the codebase

OverDecomposed.java
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// ❌ One-method-per-class nightmare
public class OrderTotalCalculator {
    public Money calculate(Order order) {
        return new OrderSubtotalCalculator().calculate(order)
            .add(new OrderTaxCalculator().calculate(order))
            .subtract(new OrderDiscountCalculator().calculate(order))
            .add(new OrderShippingCalculator().calculate(order));
    }
}
 
public class OrderSubtotalCalculator {
    public Money calculate(Order order) { ... }
}
 
public class OrderTaxCalculator {
    public Money calculate(Order order) { ... }
}
 
public class OrderDiscountCalculator {
    public Money calculate(Order order) { ... }
}
 
public class OrderShippingCalculator {
    public Money calculate(Order order) { ... }
}
 
// ✅ Cohesive approach - all pricing in one place
public class OrderPricingCalculator {
    public Money calculateTotal(Order order) {
        return calculateSubtotal(order)
            .add(calculateTax(order))
            .subtract(calculateDiscount(order))
            .add(calculateShipping(order));
    }
    
    private Money calculateSubtotal(Order order) { ... }
    private Money calculateTax(Order order) { ... }
    private Money calculateDiscount(Order order) { ... }
    private Money calculateShipping(Order order) { ... }
}

The Cohesion Test

When tempted to split a class, ask: 'Would these parts ever change for different reasons?' If they always change together—like tax, discount, and shipping calculations when pricing rules change—keep them together. Separation without cause is fragmentation, not design.

Methods as Facets of a Single Responsibility

A more productive mental model is to think of methods as facets of a single responsibility—different angles from which you interact with one cohesive concept.

Consider a UserAuthentication class:

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public class UserAuthentication {
    private final PasswordEncoder passwordEncoder;
    private final TokenGenerator tokenGenerator;
    private final UserRepository userRepository;
    
    // All methods serve the single responsibility of "authenticating users"
    
    public AuthResult login(String username, String password) {
        User user = userRepository.findByUsername(username)
            .orElseThrow(() -> new AuthenticationException("User not found"));
        
        if (!verifyPassword(user, password)) {
            recordFailedAttempt(user);
            throw new AuthenticationException("Invalid password");
        }
        
        return createAuthResult(user);
    }
    
    public void logout(String token) {
        tokenGenerator.invalidate(token);
    }
    
    public AuthResult refreshToken(String refreshToken) {
        // Validate and issue new token
    }
    
    public boolean validateToken(String token) {
        return tokenGenerator.validate(token);
    }
    
    public void changePassword(User user, String oldPassword, String newPassword) {
        if (!verifyPassword(user, oldPassword)) {
            throw new AuthenticationException("Current password incorrect");
        }
        user.setPasswordHash(passwordEncoder.encode(newPassword));
        userRepository.save(user);
    }
    
    public void resetPassword(String email) {
        // Initiate password reset flow
    }
    
    // Private helper methods - implementation details of the responsibility
    private boolean verifyPassword(User user, String password) {
        return passwordEncoder.matches(password, user.getPasswordHash());
    }
    
    private void recordFailedAttempt(User user) {
        // Track failed login for security
    }
    
    private AuthResult createAuthResult(User user) {
        String accessToken = tokenGenerator.generateAccess(user);
        String refreshToken = tokenGenerator.generateRefresh(user);
        return new AuthResult(accessToken, refreshToken, user.toProfile());
    }
}

This class has 9+ methods, including public and private. Yet the responsibility is singular: user authentication.

Analyzing the facets:

Method	Facet of Authentication
`login()`	Primary authentication flow
`logout()`	Session termination
`refreshToken()`	Session extension
`validateToken()`	Session verification
`changePassword()`	Credential management
`resetPassword()`	Credential recovery

Each method represents a different aspect of the same responsibility. The Security team owns all these methods. They would all change if authentication requirements change (e.g., adding MFA). They're cohesive—operating on the same concepts (users, passwords, tokens).

The Facet Mental Model

Think of a class as a gem with multiple facets. Each method is a facet—a different way light enters the same crystal. The gem (responsibility) is singular; the facets (methods) are many. SRP doesn't limit facets; it ensures they all belong to the same gem.

When Method Count Does Matter

While method count isn't the measure of SRP compliance, it can serve as a warning signal that merits investigation. A class with 50 methods might be perfectly focused, but it's statistically more likely to harbor hidden responsibilities.

High method count as a smell (not a violation):

When to Investigate High Method Counts

•Methods serve different stakeholders — If the Marketing team cares about some methods while Finance cares about others, you likely have multiple responsibilities
•Methods don't use common state — If methodA() uses fields X and Y while methodB() uses fields Z and W with no overlap, cohesion is low
•Methods change at different times — Features in one part of the class change frequently while others remain stable for years
•Different testing concerns — Some methods require integration tests with databases while others are pure calculations
•Naming becomes strained — You can't find a name that captures what the class does without using 'Manager', 'Handler', or 'Utility'

The investigation process:

When you encounter a large class, don't immediately split it. Instead:

Map methods to stakeholders: Who would request changes to each method?
Analyze field usage: Which methods use which instance variables?
Check change history: Use git log to see if methods change together or independently
Identify clusters: Do natural groupings emerge?

If clusters emerge with distinct stakeholders, consider separation. If all methods orbit the same concepts and serve the same actors, the class may simply have a rich interface for a focused responsibility.

Signs of Hidden Responsibilities

•Methods divide into disconnected groups
•Some methods touch 'business' while others touch 'infrastructure'
•You describe the class as 'handling X and Y'
•The class name includes 'And' or is vague
•Different methods have different exception types

Signs of Unified Responsibility

•All methods operate on core class state
•Methods represent variations of one theme
•Single sentence describes the class purpose
•One stakeholder owns all methods
•Changes affect multiple methods simultaneously

Real-World Examples: Many Methods, One Responsibility

Production codebases are filled with classes that have many methods yet perfect SRP compliance. Understanding these examples builds intuition for responsible design.

Example 1: Java's ArrayList

The ArrayList class in Java has 60+ methods: add, remove, get, set, size, contains, indexOf, forEach, iterator, sort, toArray, and many more. Yet it has one responsibility: managing a dynamic, ordered collection of elements.

All methods serve List consumers
All methods operate on the internal array
All methods would change if the fundamental concept of 'list' changed
The class is owned by the JDK Collections team

ArrayListResponsibility.java
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// ArrayList has 60+ methods but ONE responsibility:
// "Managing a resizable array-backed list"
 
public class ArrayList<E> implements List<E> {
    private Object[] elementData;
    private int size;
    
    // Adding elements - facet of list management
    public boolean add(E e) { ... }
    public void add(int index, E element) { ... }
    public boolean addAll(Collection<? extends E> c) { ... }
    
    // Removing elements - facet of list management
    public E remove(int index) { ... }
    public boolean remove(Object o) { ... }
    public void clear() { ... }
    
    // Accessing elements - facet of list management
    public E get(int index) { ... }
    public E set(int index, E element) { ... }
    
    // Querying - facet of list management
    public int size() { ... }
    public boolean isEmpty() { ... }
    public boolean contains(Object o) { ... }
    public int indexOf(Object o) { ... }
    
    // Iteration - facet of list management
    public Iterator<E> iterator() { ... }
    public void forEach(Consumer<? super E> action) { ... }
    
    // Transformation - facet of list management
    public Object[] toArray() { ... }
    public void sort(Comparator<? super E> c) { ... }
    
    // ... and 40+ more methods
}
 
// All 60+ methods serve ONE actor: developers using the List abstraction
// All methods share ONE reason to change: List semantics evolve

Example 2: A Domain-Rich Entity

Consider an Invoice class in an accounting system:

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public class Invoice {
    private final InvoiceId id;
    private final CustomerId customerId;
    private final List<LineItem> lineItems;
    private InvoiceStatus status;
    private Money amountPaid;
    private LocalDate dueDate;
    private final LocalDate issueDate;
    
    // Lifecycle management
    public void finalize() { this.status = InvoiceStatus.FINALIZED; }
    public void send() { this.status = InvoiceStatus.SENT; }
    public void markOverdue() { this.status = InvoiceStatus.OVERDUE; }
    public void archive() { this.status = InvoiceStatus.ARCHIVED; }
    
    // Line item management
    public void addLineItem(LineItem item) { ... }
    public void removeLineItem(LineItemId itemId) { ... }
    public void updateLineItem(LineItemId itemId, LineItem updated) { ... }
    
    // Financial calculations
    public Money getSubtotal() { ... }
    public Money getTaxAmount() { ... }
    public Money getTotal() { ... }
    public Money getAmountDue() { return getTotal().subtract(amountPaid); }
    
    // Payment processing
    public void recordPayment(Money amount) { ... }
    public boolean isPaid() { return getAmountDue().isZero(); }
    public boolean isPartiallyPaid() { ... }
    
    // Status queries
    public boolean isEditable() { return status == InvoiceStatus.DRAFT; }
    public boolean isOverdue() { return !isPaid() && LocalDate.now().isAfter(dueDate); }
    public boolean canBeSent() { ... }
    
    // Presentation
    public String getFormattedTotal() { ... }
    public String getStatusDescription() { ... }
}

This Invoice class has 20+ methods covering lifecycle, line items, calculations, payments, status, and presentation. Yet the responsibility is singular: representing and managing an invoice.

The Billing/Accounting team owns this class. All methods serve the same domain concept. This is rich domain modeling, not SRP violation.

Domain-Driven Insight

In Domain-Driven Design, entities naturally accumulate methods that represent business behaviors. An Invoice that can be finalized, sent, paid, and queried is a rich domain object—not an SRP violation. The key is that all behaviors relate to the 'Invoice' aggregate concept.

The Real SRP Test: Reason to Change Analysis

Instead of counting methods, apply the Reason to Change Analysis—a systematic way to evaluate SRP compliance regardless of method count.

The Three-Question Test:

SRP Compliance Questions

•Who would request changes to this class? — Identify all stakeholders (actors) who might need modifications. If there's more than one actor with different interests, the class may have multiple responsibilities.
•Would all methods change for the same reason? — If a change in tax calculation logic affects calculateTax() but not sendReminder(), these might be different responsibilities.
•Can you describe the class's purpose in one sentence without 'and'? — If you need 'and' (handles invoices and sends emails), you likely have multiple responsibilities.

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// ❌ FAILS the Three-Question Test
public class ReportService {
    // Actor 1: Finance team
    public Report generateFinancialReport() { ... }
    public void exportToExcel(Report report) { ... }
    
    // Actor 2: Marketing team  
    public Report generateMarketingReport() { ... }
    public void exportToSlides(Report report) { ... }
    
    // Actor 3: IT Operations
    public void scheduleReport(Report report, Schedule schedule) { ... }
    public void monitorReportJobs() { ... }
}
 
// Analysis:
// Q1: Three different actors (Finance, Marketing, IT)
// Q2: Financial report changes don't affect marketing reports
// Q3: "Generates and schedules financial and marketing reports" - multiple 'and's
 
// ✅ PASSES the Three-Question Test
public class FinancialReportGenerator {
    public Report generate(DateRange period, ReportOptions options) { ... }
    public Report generateYTD() { ... }
    public Report generateQuarterly(Quarter quarter) { ... }
    public Summary generateExecutiveSummary() { ... }
    public void exportToExcel(Report report, OutputStream out) { ... }
    public void exportToPDF(Report report, OutputStream out) { ... }
}
 
// Analysis:
// Q1: One actor (Finance team)
// Q2: All methods change when financial reporting requirements change
// Q3: "Generates financial reports in various formats" - one sentence, no 'and'

The Method-Responsibility Disconnect

The failing example has 6 methods; the passing example also has 6 methods. Method count is identical, but SRP compliance is opposite. This proves definitively that SRP is not about counting methods—it's about analyzing cohesion and change drivers.

Summary: Methods ≠ Responsibilities

We've dismantled one of the most pervasive SRP misconceptions. Let's consolidate the key insights:

Key Takeaways

•SRP limits reasons to change, not method count — A class can have many methods yet only one reason to change
•Methods are facets of a responsibility — Multiple methods can represent different aspects of the same cohesive purpose
•One-method-per-class creates worse problems — It fragments code, obscures domain concepts, and makes changes harder
•Cohesion is the companion principle — Methods that use the same data and serve the same actor belong together
•Use the Three-Question Test — Identify actors, analyze change patterns, and articulate purpose in one sentence
•Method count is a smell, not a violation — High counts warrant investigation, but don't automatically indicate SRP problems

What's next:

Having cleared up the 'one method per class' myth, we'll tackle another common misconception: the belief that SRP requires tiny classes everywhere. As we'll see, this interpretation leads to a different but equally problematic form of over-engineering.

Page Complete

You now understand that SRP is about cohesion and change drivers, not method counting. The goal is focused classes that change for one reason—regardless of how many methods that focus requires.

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Loading learning content...

System Design (LLD)Single Responsibility Principle

Common SRP Misconceptions

LevelIntermediate

Duration60 mins

TopicSingle Responsibility Principle

1 / 4

SRP Doesn't Mean One Method Per Class

The Most Dangerous Misinterpretation

Of all the misunderstandings surrounding the Single Responsibility Principle, none is more pervasive—or more destructive—than the belief that SRP means each class should have only one method.

Except it's completely wrong.

The Cost of Misunderstanding

What You Will Learn

Where The Confusion Originates

To understand why this misconception is so widespread, we need to trace its origins. The confusion stems from conflating two fundamentally different concepts: methods and responsibilities.

The linguistic trap:

When we say a class should have 'one responsibility,' natural language suggests a single task—something atomic and indivisible. But Robert C. Martin's original formulation was far more nuanced:

"A class should have only one reason to change."

Notice the emphasis isn't on what the class does (its methods), but on what might cause it to change (its reason to change). These are completely different dimensions of analysis.

Methods vs. Reasons to Change
Concept	What It Describes	Quantity in Well-Designed Class
Methods	Actions the class can perform	As many as needed for cohesion
Responsibility	The conceptual purpose the class serves	One cohesive area
Reason to Change	External force that would require modification	One stakeholder or concern

The actor model clarification:

In later writings, Martin refined his definition to make it even clearer:

"A module should be responsible to one, and only one, actor."

Conversely, if calculatePay() serves Accounting but reportHours() serves Operations, you have two actors—and two reasons to change—regardless of having only two methods.

The Mental Model Shift

Understanding Cohesive Methods

Consider a ShoppingCart class:

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public class ShoppingCart {
    private List<CartItem> items;
    private DiscountStrategy discountStrategy;
    
    // Adding and removing items - all serve the same responsibility
    public void addItem(Product product, int quantity) { ... }
    public void removeItem(String productId) { ... }
    public void updateQuantity(String productId, int newQuantity) { ... }
    public void clearCart() { ... }
    
    // Querying cart state - still same responsibility
    public List<CartItem> getItems() { ... }
    public int getItemCount() { ... }
    public boolean containsProduct(String productId) { ... }
    public CartItem findItem(String productId) { ... }
    
    // Calculating totals - same responsibility
    public Money getSubtotal() { ... }
    public Money getDiscount() { ... }
    public Money getTax() { ... }
    public Money getTotal() { ... }
    
    // Cart lifecycle - same responsibility
    public boolean isEmpty() { ... }
    public void applyDiscount(DiscountStrategy strategy) { ... }
}

This class has 17 methods, yet it adheres perfectly to SRP. Why?

Single reason to change: All methods would change only if the fundamental concept of 'what a shopping cart does' changes
Single actor: The E-commerce team owns this class; no other stakeholder would request changes
High cohesion: Every method operates on items and serves cart management
Unified abstraction: The class represents one real-world entity

Splitting this into 17 classes—CartItemAdder, CartItemRemover, CartQuantityUpdater, etc.—would be absurd. You'd destroy the cohesion that makes the code understandable.

Cohesion Is The Key

The Symptoms of One-Method-Per-Class Thinking

Teams that fall into the 'one method per class' trap exhibit distinctive symptoms. Recognizing these patterns can help you diagnose and correct the problem before it spreads.

Warning Signs of Over-Decomposition

•Class explosion — Simple features require dozens of new classes. Adding a discount feature creates DiscountCalculator, DiscountValidator, DiscountApplier, DiscountLogger, etc.
•Navigation nightmare — Developers can't find code because it's scattered across a labyrinth of tiny classes with mechanical names
•Shotgun surgery — Simple changes require modifications to 10+ files because functionality is atomized
•Data passing ceremonies — Classes constantly pass data to each other because they can't hold state meaningfully
•Manager/Handler proliferation — Every noun gains a Manager, Handler, Processor, or Service suffix
•Loss of domain meaning — Code no longer reflects business concepts; it reflects procedural steps

The ironic outcome:

The goal of SRP is to make code easier to change by limiting the impact of modifications. But one-method-per-class achieves the opposite:

Understanding requires more files: To comprehend a feature, you must trace through many classes
Changes ripple further: Modifying a concept touches every class involved in that concept
Testing becomes fragmented: Each tiny class needs its own tests, but the tests don't verify meaningful behavior
Refactoring is painful: Moving code means updating imports, dependencies, and wiring across the codebase

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// ❌ One-method-per-class nightmare
public class OrderTotalCalculator {
    public Money calculate(Order order) {
        return new OrderSubtotalCalculator().calculate(order)
            .add(new OrderTaxCalculator().calculate(order))
            .subtract(new OrderDiscountCalculator().calculate(order))
            .add(new OrderShippingCalculator().calculate(order));
    }
}
 
public class OrderSubtotalCalculator {
    public Money calculate(Order order) { ... }
}
 
public class OrderTaxCalculator {
    public Money calculate(Order order) { ... }
}
 
public class OrderDiscountCalculator {
    public Money calculate(Order order) { ... }
}
 
public class OrderShippingCalculator {
    public Money calculate(Order order) { ... }
}
 
// ✅ Cohesive approach - all pricing in one place
public class OrderPricingCalculator {
    public Money calculateTotal(Order order) {
        return calculateSubtotal(order)
            .add(calculateTax(order))
            .subtract(calculateDiscount(order))
            .add(calculateShipping(order));
    }
    
    private Money calculateSubtotal(Order order) { ... }
    private Money calculateTax(Order order) { ... }
    private Money calculateDiscount(Order order) { ... }
    private Money calculateShipping(Order order) { ... }
}

The Cohesion Test

Methods as Facets of a Single Responsibility

A more productive mental model is to think of methods as facets of a single responsibility—different angles from which you interact with one cohesive concept.

Consider a UserAuthentication class:

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public class UserAuthentication {
    private final PasswordEncoder passwordEncoder;
    private final TokenGenerator tokenGenerator;
    private final UserRepository userRepository;
    
    // All methods serve the single responsibility of "authenticating users"
    
    public AuthResult login(String username, String password) {
        User user = userRepository.findByUsername(username)
            .orElseThrow(() -> new AuthenticationException("User not found"));
        
        if (!verifyPassword(user, password)) {
            recordFailedAttempt(user);
            throw new AuthenticationException("Invalid password");
        }
        
        return createAuthResult(user);
    }
    
    public void logout(String token) {
        tokenGenerator.invalidate(token);
    }
    
    public AuthResult refreshToken(String refreshToken) {
        // Validate and issue new token
    }
    
    public boolean validateToken(String token) {
        return tokenGenerator.validate(token);
    }
    
    public void changePassword(User user, String oldPassword, String newPassword) {
        if (!verifyPassword(user, oldPassword)) {
            throw new AuthenticationException("Current password incorrect");
        }
        user.setPasswordHash(passwordEncoder.encode(newPassword));
        userRepository.save(user);
    }
    
    public void resetPassword(String email) {
        // Initiate password reset flow
    }
    
    // Private helper methods - implementation details of the responsibility
    private boolean verifyPassword(User user, String password) {
        return passwordEncoder.matches(password, user.getPasswordHash());
    }
    
    private void recordFailedAttempt(User user) {
        // Track failed login for security
    }
    
    private AuthResult createAuthResult(User user) {
        String accessToken = tokenGenerator.generateAccess(user);
        String refreshToken = tokenGenerator.generateRefresh(user);
        return new AuthResult(accessToken, refreshToken, user.toProfile());
    }
}

This class has 9+ methods, including public and private. Yet the responsibility is singular: user authentication.

Analyzing the facets:

Method	Facet of Authentication
`login()`	Primary authentication flow
`logout()`	Session termination
`refreshToken()`	Session extension
`validateToken()`	Session verification
`changePassword()`	Credential management
`resetPassword()`	Credential recovery

The Facet Mental Model

When Method Count Does Matter

High method count as a smell (not a violation):

When to Investigate High Method Counts

•Methods serve different stakeholders — If the Marketing team cares about some methods while Finance cares about others, you likely have multiple responsibilities
•Methods don't use common state — If methodA() uses fields X and Y while methodB() uses fields Z and W with no overlap, cohesion is low
•Methods change at different times — Features in one part of the class change frequently while others remain stable for years
•Different testing concerns — Some methods require integration tests with databases while others are pure calculations
•Naming becomes strained — You can't find a name that captures what the class does without using 'Manager', 'Handler', or 'Utility'

The investigation process:

When you encounter a large class, don't immediately split it. Instead:

Map methods to stakeholders: Who would request changes to each method?
Analyze field usage: Which methods use which instance variables?
Check change history: Use git log to see if methods change together or independently
Identify clusters: Do natural groupings emerge?

Signs of Hidden Responsibilities

•Methods divide into disconnected groups
•Some methods touch 'business' while others touch 'infrastructure'
•You describe the class as 'handling X and Y'
•The class name includes 'And' or is vague
•Different methods have different exception types

Signs of Unified Responsibility

•All methods operate on core class state
•Methods represent variations of one theme
•Single sentence describes the class purpose
•One stakeholder owns all methods
•Changes affect multiple methods simultaneously

Real-World Examples: Many Methods, One Responsibility

Production codebases are filled with classes that have many methods yet perfect SRP compliance. Understanding these examples builds intuition for responsible design.

Example 1: Java's ArrayList

All methods serve List consumers
All methods operate on the internal array
All methods would change if the fundamental concept of 'list' changed
The class is owned by the JDK Collections team

ArrayListResponsibility.java
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// ArrayList has 60+ methods but ONE responsibility:
// "Managing a resizable array-backed list"
 
public class ArrayList<E> implements List<E> {
    private Object[] elementData;
    private int size;
    
    // Adding elements - facet of list management
    public boolean add(E e) { ... }
    public void add(int index, E element) { ... }
    public boolean addAll(Collection<? extends E> c) { ... }
    
    // Removing elements - facet of list management
    public E remove(int index) { ... }
    public boolean remove(Object o) { ... }
    public void clear() { ... }
    
    // Accessing elements - facet of list management
    public E get(int index) { ... }
    public E set(int index, E element) { ... }
    
    // Querying - facet of list management
    public int size() { ... }
    public boolean isEmpty() { ... }
    public boolean contains(Object o) { ... }
    public int indexOf(Object o) { ... }
    
    // Iteration - facet of list management
    public Iterator<E> iterator() { ... }
    public void forEach(Consumer<? super E> action) { ... }
    
    // Transformation - facet of list management
    public Object[] toArray() { ... }
    public void sort(Comparator<? super E> c) { ... }
    
    // ... and 40+ more methods
}
 
// All 60+ methods serve ONE actor: developers using the List abstraction
// All methods share ONE reason to change: List semantics evolve

Example 2: A Domain-Rich Entity

Consider an Invoice class in an accounting system:

Invoice.java
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public class Invoice {
    private final InvoiceId id;
    private final CustomerId customerId;
    private final List<LineItem> lineItems;
    private InvoiceStatus status;
    private Money amountPaid;
    private LocalDate dueDate;
    private final LocalDate issueDate;
    
    // Lifecycle management
    public void finalize() { this.status = InvoiceStatus.FINALIZED; }
    public void send() { this.status = InvoiceStatus.SENT; }
    public void markOverdue() { this.status = InvoiceStatus.OVERDUE; }
    public void archive() { this.status = InvoiceStatus.ARCHIVED; }
    
    // Line item management
    public void addLineItem(LineItem item) { ... }
    public void removeLineItem(LineItemId itemId) { ... }
    public void updateLineItem(LineItemId itemId, LineItem updated) { ... }
    
    // Financial calculations
    public Money getSubtotal() { ... }
    public Money getTaxAmount() { ... }
    public Money getTotal() { ... }
    public Money getAmountDue() { return getTotal().subtract(amountPaid); }
    
    // Payment processing
    public void recordPayment(Money amount) { ... }
    public boolean isPaid() { return getAmountDue().isZero(); }
    public boolean isPartiallyPaid() { ... }
    
    // Status queries
    public boolean isEditable() { return status == InvoiceStatus.DRAFT; }
    public boolean isOverdue() { return !isPaid() && LocalDate.now().isAfter(dueDate); }
    public boolean canBeSent() { ... }
    
    // Presentation
    public String getFormattedTotal() { ... }
    public String getStatusDescription() { ... }
}

This Invoice class has 20+ methods covering lifecycle, line items, calculations, payments, status, and presentation. Yet the responsibility is singular: representing and managing an invoice.

The Billing/Accounting team owns this class. All methods serve the same domain concept. This is rich domain modeling, not SRP violation.

Domain-Driven Insight

The Real SRP Test: Reason to Change Analysis

Instead of counting methods, apply the Reason to Change Analysis—a systematic way to evaluate SRP compliance regardless of method count.

The Three-Question Test:

SRP Compliance Questions

•Who would request changes to this class? — Identify all stakeholders (actors) who might need modifications. If there's more than one actor with different interests, the class may have multiple responsibilities.
•Would all methods change for the same reason? — If a change in tax calculation logic affects calculateTax() but not sendReminder(), these might be different responsibilities.
•Can you describe the class's purpose in one sentence without 'and'? — If you need 'and' (handles invoices and sends emails), you likely have multiple responsibilities.

SRPAnalysis.java
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// ❌ FAILS the Three-Question Test
public class ReportService {
    // Actor 1: Finance team
    public Report generateFinancialReport() { ... }
    public void exportToExcel(Report report) { ... }
    
    // Actor 2: Marketing team  
    public Report generateMarketingReport() { ... }
    public void exportToSlides(Report report) { ... }
    
    // Actor 3: IT Operations
    public void scheduleReport(Report report, Schedule schedule) { ... }
    public void monitorReportJobs() { ... }
}
 
// Analysis:
// Q1: Three different actors (Finance, Marketing, IT)
// Q2: Financial report changes don't affect marketing reports
// Q3: "Generates and schedules financial and marketing reports" - multiple 'and's
 
// ✅ PASSES the Three-Question Test
public class FinancialReportGenerator {
    public Report generate(DateRange period, ReportOptions options) { ... }
    public Report generateYTD() { ... }
    public Report generateQuarterly(Quarter quarter) { ... }
    public Summary generateExecutiveSummary() { ... }
    public void exportToExcel(Report report, OutputStream out) { ... }
    public void exportToPDF(Report report, OutputStream out) { ... }
}
 
// Analysis:
// Q1: One actor (Finance team)
// Q2: All methods change when financial reporting requirements change
// Q3: "Generates financial reports in various formats" - one sentence, no 'and'

The Method-Responsibility Disconnect

Summary: Methods ≠ Responsibilities

We've dismantled one of the most pervasive SRP misconceptions. Let's consolidate the key insights:

Key Takeaways

•SRP limits reasons to change, not method count — A class can have many methods yet only one reason to change
•Methods are facets of a responsibility — Multiple methods can represent different aspects of the same cohesive purpose
•One-method-per-class creates worse problems — It fragments code, obscures domain concepts, and makes changes harder
•Cohesion is the companion principle — Methods that use the same data and serve the same actor belong together
•Use the Three-Question Test — Identify actors, analyze change patterns, and articulate purpose in one sentence
•Method count is a smell, not a violation — High counts warrant investigation, but don't automatically indicate SRP problems

What's next:

Page Complete

You now understand that SRP is about cohesion and change drivers, not method counting. The goal is focused classes that change for one reason—regardless of how many methods that focus requires.

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