Solid Principles Skill for Claude
SOLID principles checklist with Java examples. Use when reviewing classes, refactoring code, or when user asks about Single Responsibility, Open/Closed, Liskov, Interface Segregation, or Dependency Inversion.
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32 sections, as written by the authorSOLID Principles Skill
Review and apply SOLID principles in Java code.
When to Use
- User says "check SOLID" / "SOLID review" / "is this class doing too much?"
- Reviewing class design
- Refactoring large classes
- Code review focusing on design
Quick Reference
| Letter | Principle | One-liner |
|--------|-----------|-----------|
| S | Single Responsibility | One class = one reason to change |
| O | Open/Closed | Open for extension, closed for modification |
| L | Liskov Substitution | Subtypes must be substitutable for base types |
| I | Interface Segregation | Many specific interfaces > one general interface |
| D | Dependency Inversion | Depend on abstractions, not concretions |
S - Single Responsibility Principle (SRP)
> "A class should have only one reason to change."
Violation
// ❌ BAD: UserService does too much
public class UserService {
public User createUser(String name, String email) {
// validation logic
if (email == null || !email.contains("@")) {
throw new IllegalArgumentException("Invalid email");
}
// persistence logic
User user = new User(name, email);
entityManager.persist(user);
// notification logic
String subject = "Welcome!";
String body = "Hello " + name;
emailClient.send(email, subject, body);
// audit logic
auditLog.log("User created: " + email);
return user;
}
}
Problems:
- Validation changes? Modify UserService
- Email template changes? Modify UserService
- Audit format changes? Modify UserService
- Hard to test each concern separately
Refactored
// ✅ GOOD: Each class has one responsibility
public class UserValidator {
public void validate(String name, String email) {
if (email == null || !email.contains("@")) {
throw new ValidationException("Invalid email");
}
}
}
public class UserRepository {
public User save(User user) {
entityManager.persist(user);
return user;
}
}
public class WelcomeEmailSender {
public void sendWelcome(User user) {
String subject = "Welcome!";
String body = "Hello " + user.getName();
emailClient.send(user.getEmail(), subject, body);
}
}
public class UserAuditLogger {
public void logCreation(User user) {
auditLog.log("User created: " + user.getEmail());
}
}
public class UserService {
private final UserValidator validator;
private final UserRepository repository;
private final WelcomeEmailSender emailSender;
private final UserAuditLogger auditLogger;
public User createUser(String name, String email) {
validator.validate(name, email);
User user = repository.save(new User(name, email));
emailSender.sendWelcome(user);
auditLogger.logCreation(user);
return user;
}
}
How to Detect SRP Violations
- Class has many
importstatements from different domains - Class name contains "And" or "Manager" or "Handler" (often)
- Methods operate on unrelated data
- Changes in one area require touching unrelated methods
- Hard to name the class concisely
Quick Check Questions
- Can you describe the class purpose in one sentence without "and"?
- Would different stakeholders request changes to this class?
- Are there methods that don't use most of the class fields?
O - Open/Closed Principle (OCP)
> "Software entities should be open for extension, but closed for modification."
Violation
// ❌ BAD: Must modify class to add new discount type
public class DiscountCalculator {
public double calculate(Order order, String discountType) {
if (discountType.equals("PERCENTAGE")) {
return order.getTotal() * 0.1;
} else if (discountType.equals("FIXED")) {
return 50.0;
} else if (discountType.equals("LOYALTY")) {
return order.getTotal() * order.getCustomer().getLoyaltyRate();
}
// Every new discount type = modify this class
return 0;
}
}
Refactored
// ✅ GOOD: Add new discounts without modifying existing code
public interface DiscountStrategy {
double calculate(Order order);
boolean supports(String discountType);
}
public class PercentageDiscount implements DiscountStrategy {
@Override
public double calculate(Order order) {
return order.getTotal() * 0.1;
}
@Override
public boolean supports(String discountType) {
return "PERCENTAGE".equals(discountType);
}
}
public class FixedDiscount implements DiscountStrategy {
@Override
public double calculate(Order order) {
return 50.0;
}
@Override
public boolean supports(String discountType) {
return "FIXED".equals(discountType);
}
}
public class LoyaltyDiscount implements DiscountStrategy {
@Override
public double calculate(Order order) {
return order.getTotal() * order.getCustomer().getLoyaltyRate();
}
@Override
public boolean supports(String discountType) {
return "LOYALTY".equals(discountType);
}
}
// New discount? Just add new class, no modification needed
public class SeasonalDiscount implements DiscountStrategy {
@Override
public double calculate(Order order) {
return order.getTotal() * 0.2;
}
@Override
public boolean supports(String discountType) {
return "SEASONAL".equals(discountType);
}
}
public class DiscountCalculator {
private final List<DiscountStrategy> strategies;
public DiscountCalculator(List<DiscountStrategy> strategies) {
this.strategies = strategies;
}
public double calculate(Order order, String discountType) {
return strategies.stream()
.filter(s -> s.supports(discountType))
.findFirst()
.map(s -> s.calculate(order))
.orElse(0.0);
}
}
How to Detect OCP Violations
if/elseorswitchon type/status that grows over time- Enum-based dispatching with frequent new values
- Changes require modifying core classes
Common OCP Patterns
| Pattern | Use When |
|---------|----------|
| Strategy | Multiple algorithms for same operation |
| Template Method | Same structure, different steps |
| Decorator | Add behavior dynamically |
| Factory | Create objects without specifying class |
L - Liskov Substitution Principle (LSP)
> "Subtypes must be substitutable for their base types."
Violation
// ❌ BAD: Square violates Rectangle contract
public class Rectangle {
protected int width;
protected int height;
public void setWidth(int width) {
this.width = width;
}
public void setHeight(int height) {
this.height = height;
}
public int getArea() {
return width * height;
}
}
public class Square extends Rectangle {
@Override
public void setWidth(int width) {
this.width = width;
this.height = width; // Violates expected behavior!
}
@Override
public void setHeight(int height) {
this.width = height; // Violates expected behavior!
this.height = height;
}
}
// This test fails for Square!
void testRectangle(Rectangle r) {
r.setWidth(5);
r.setHeight(4);
assert r.getArea() == 20; // Square returns 16!
}
Refactored
// ✅ GOOD: Separate abstractions
public interface Shape {
int getArea();
}
public class Rectangle implements Shape {
private final int width;
private final int height;
public Rectangle(int width, int height) {
this.width = width;
this.height = height;
}
@Override
public int getArea() {
return width * height;
}
}
public class Square implements Shape {
private final int side;
public Square(int side) {
this.side = side;
}
@Override
public int getArea() {
return side * side;
}
}
LSP Rules
| Rule | Meaning |
|------|---------|
| Preconditions | Subclass cannot strengthen (require more) |
| Postconditions | Subclass cannot weaken (promise less) |
| Invariants | Subclass must maintain parent's invariants |
| History | Subclass cannot modify inherited state unexpectedly |
How to Detect LSP Violations
- Subclass throws exception parent doesn't
- Subclass returns null where parent returns object
- Subclass ignores or overrides parent behavior unexpectedly
instanceofchecks before calling methods- Empty or throwing implementations of interface methods
Quick Check
// If you see this, LSP might be violated
if (bird instanceof Penguin) {
// don't call fly()
} else {
bird.fly();
}
I - Interface Segregation Principle (ISP)
> "Clients should not be forced to depend on interfaces they do not use."
Violation
// ❌ BAD: Fat interface forces unnecessary implementations
public interface Worker {
void work();
void eat();
void sleep();
void attendMeeting();
void writeReport();
}
// Robot can't eat or sleep!
public class Robot implements Worker {
@Override public void work() { /* OK */ }
@Override public void eat() { /* Can't eat! */ }
@Override public void sleep() { /* Can't sleep! */ }
@Override public void attendMeeting() { /* OK */ }
@Override public void writeReport() { /* Maybe */ }
}
// Intern doesn't attend meetings or write reports
public class Intern implements Worker {
@Override public void work() { /* OK */ }
@Override public void eat() { /* OK */ }
@Override public void sleep() { /* OK */ }
@Override public void attendMeeting() { /* Not allowed! */ }
@Override public void writeReport() { /* Not expected! */ }
}
Refactored
// ✅ GOOD: Segregated interfaces
public interface Workable {
void work();
}
public interface Feedable {
void eat();
void sleep();
}
public interface Manageable {
void attendMeeting();
void writeReport();
}
// Combine what you need
public class Employee implements Workable, Feedable, Manageable {
@Override public void work() { /* ... */ }
@Override public void eat() { /* ... */ }
@Override public void sleep() { /* ... */ }
@Override public void attendMeeting() { /* ... */ }
@Override public void writeReport() { /* ... */ }
}
public class Robot implements Workable {
@Override public void work() { /* ... */ }
// No unnecessary methods!
}
public class Intern implements Workable, Feedable {
@Override public void work() { /* ... */ }
@Override public void eat() { /* ... */ }
@Override public void sleep() { /* ... */ }
// No meeting/report methods!
}
How to Detect ISP Violations
- Implementations with empty methods or
throw new UnsupportedOperationException() - Interface has 10+ methods
- Different clients use completely different subsets of methods
- Changes to interface affect unrelated implementations
Java Standard Library Violations
// java.util.List has many methods - but this is acceptable for collections
// However, be careful with your own interfaces!
// ❌ This interface is too fat for most use cases
public interface Repository<T> {
T findById(Long id);
List<T> findAll();
T save(T entity);
void delete(T entity);
void deleteById(Long id);
List<T> findByExample(T example);
Page<T> findAll(Pageable pageable);
List<T> findAllById(Iterable<Long> ids);
long count();
boolean existsById(Long id);
// ... 20 more methods
}
// ✅ Better: Split by use case
public interface ReadRepository<T> {
Optional<T> findById(Long id);
List<T> findAll();
}
public interface WriteRepository<T> {
T save(T entity);
void delete(T entity);
}
D - Dependency Inversion Principle (DIP)
> "High-level modules should not depend on low-level modules. Both should depend on abstractions."
Violation
// ❌ BAD: High-level depends on low-level directly
public class OrderService {
private MySqlOrderRepository repository; // Concrete class!
private SmtpEmailSender emailSender; // Concrete class!
public OrderService() {
this.repository = new MySqlOrderRepository(); // Hard dependency
this.emailSender = new SmtpEmailSender(); // Hard dependency
}
public void createOrder(Order order) {
repository.save(order);
emailSender.send(order.getCustomerEmail(), "Order confirmed");
}
}
Problems:
- Cannot test without real MySQL database
- Cannot swap email provider
- OrderService knows about MySQL, SMTP details
Refactored
// ✅ GOOD: Depend on abstractions
// Abstractions (interfaces)
public interface OrderRepository {
void save(Order order);
Optional<Order> findById(Long id);
}
public interface NotificationSender {
void send(String recipient, String message);
}
// High-level module depends on abstractions
public class OrderService {
private final OrderRepository repository;
private final NotificationSender notificationSender;
// Dependencies injected
public OrderService(OrderRepository repository,
NotificationSender notificationSender) {
this.repository = repository;
this.notificationSender = notificationSender;
}
public void createOrder(Order order) {
repository.save(order);
notificationSender.send(order.getCustomerEmail(), "Order confirmed");
}
}
// Low-level modules implement abstractions
public class MySqlOrderRepository implements OrderRepository {
@Override
public void save(Order order) { /* MySQL specific */ }
@Override
public Optional<Order> findById(Long id) { /* MySQL specific */ }
}
public class SmtpEmailSender implements NotificationSender {
@Override
public void send(String recipient, String message) { /* SMTP specific */ }
}
// Easy to test with mocks!
public class InMemoryOrderRepository implements OrderRepository {
private Map<Long, Order> orders = new HashMap<>();
@Override
public void save(Order order) {
orders.put(order.getId(), order);
}
@Override
public Optional<Order> findById(Long id) {
return Optional.ofNullable(orders.get(id));
}
}
DIP with Spring
// Spring handles dependency injection automatically
@Service
public class OrderService {
private final OrderRepository repository;
private final NotificationSender notificationSender;
// Constructor injection (recommended)
public OrderService(OrderRepository repository,
NotificationSender notificationSender) {
this.repository = repository;
this.notificationSender = notificationSender;
}
}
@Repository
public class JpaOrderRepository implements OrderRepository {
// Spring provides implementation
}
@Component
@Profile("production")
public class SmtpEmailSender implements NotificationSender { }
@Component
@Profile("test")
public class MockEmailSender implements NotificationSender { }
How to Detect DIP Violations
new ConcreteClass()inside business logic- Import statements include implementation packages (e.g.,
com.mysql,org.apache.http) - Cannot easily swap implementations
- Tests require real infrastructure (database, network)
SOLID Review Checklist
When reviewing code, check:
| Principle | Question |
|-----------|----------|
| SRP | Does this class have more than one reason to change? |
| OCP | Will adding a new type/feature require modifying this class? |
| LSP | Can subclasses be used wherever parent is expected? |
| ISP | Are there empty or throwing method implementations? |
| DIP | Does high-level code depend on concrete implementations? |
Common Refactoring Patterns
| Violation | Refactoring |
|-----------|-------------|
| SRP - God class | Extract Class, Move Method |
| OCP - Type switching | Strategy Pattern, Factory |
| LSP - Broken inheritance | Composition over Inheritance, Extract Interface |
| ISP - Fat interface | Split Interface, Role Interface |
| DIP - Hard dependencies | Dependency Injection, Abstract Factory |
Related Skills
design-patterns- Implementation patterns (Factory, Strategy, Observer, etc.)clean-code- Code-level principles (DRY, KISS, naming)java-code-review- Comprehensive review checklist
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