SOLID

Single Responsibility Principle (SRP)

A class should have only one reason to change.

class UserManager {
    public void saveUser(User user) {
        // Save user to database
        db.save(user);
    }
    
    public void emailUser(User user) {
        // Send email
        emailService.send(user.getEmail(), "Welcome!", "Welcome to our platform!");
    }
    
    public void generateUserReport(User user) {
        // Generate PDF report
        PDFGenerator.create(user.getData());
    }
}

Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification.

class PaymentProcessor {
    public void processPayment(String type, double amount) {
        if (type.equals("credit")) {
            // Process credit payment
        } else if (type.equals("debit")) {
            // Process debit payment
        }
        // Adding new payment type requires modifying this class
    }
}

Liskov Substitution Principle (LSP)

Objects of a superclass should be replaceable with objects of its subclasses without breaking the application.

class Bird {
    public void fly() {
        // Flying implementation
    }
}

class Penguin extends Bird {
    @Override
    public void fly() {
        throw new UnsupportedOperationException("Penguins can't fly!");
    }
}

Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

interface Worker {
    void work();
    void eat();
    void sleep();
}

class Robot implements Worker {
    public void work() { /* */ }
    public void eat() { throw new UnsupportedOperationException(); }
    public void sleep() { throw new UnsupportedOperationException(); }
}

Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

class EmailService {
    private MySQLDatabase database;
    
    public EmailService() {
        this.database = new MySQLDatabase();
    }
    
    public void sendEmail(String message) {
        database.save(message);
        // Send email
    }
}