/solid-principles

name: solid-principles
description: 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.

SOLID 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

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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 |

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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 `import` statements 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

1. Can you describe the class purpose in one sentence without "and"? 2. Would different stakeholders request changes to this class? 3. Are there methods that don't use most of the class fields?

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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 = st