project/
├── src/
│   ├── main/
│   │   ├── java/
│   │   │   └── com/example/
│   │   │       ├── model/
│   │   │       ├── service/
│   │   │       ├── repository/
│   │   │       ├── controller/
│   │   │       └── util/
│   │   └── resources/
│   │       └── application.properties
│   └── test/
│       └── java/
│           └── com/example/
├── pom.xml (or build.gradle)
└── README.md

// This class violates SRP: it handles user data, validation, persistence, and email
public class User {
    private String email;
    private String password;

    // Responsibility 1: Data validation
    public boolean isValid() {
        return email != null && email.contains("@")
            && password != null && password.length() >= 8;
    }

    // Responsibility 2: Database operations
    public void save() {
        Connection conn = DriverManager.getConnection("jdbc:...");
        PreparedStatement ps = conn.prepareStatement("INSERT INTO users...");
        ps.setString(1, email);
        ps.setString(2, password);
        ps.executeUpdate();
    }

    // Responsibility 3: Email operations
    public void sendWelcomeEmail() {
        EmailService.send(email, "Welcome!", "Welcome to our app");
    }

    // Responsibility 4: Password encryption
    public void encryptPassword() {
        this.password = BCrypt.hashpw(password, BCrypt.gensalt());
    }
}

// Responsibility: Hold user data
public class User {
    private final String email;
    private final String passwordHash;

    public User(String email, String passwordHash) {
        this.email = email;
        this.passwordHash = passwordHash;
    }

    public String getEmail() { return email; }
    public String getPasswordHash() { return passwordHash; }
}

// Responsibility: Validate user data
public class UserValidator {
    public ValidationResult validate(String email, String password) {
        List<String> errors = new ArrayList<>();

        if (email == null || !email.contains("@")) {
            errors.add("Invalid email format");
        }
        if (password == null || password.length() < 8) {
            errors.add("Password must be at least 8 characters");
        }

        return new ValidationResult(errors.isEmpty(), errors);
    }
}

// Responsibility: Persist user data
public class UserRepository {
    private final DataSource dataSource;

    public UserRepository(DataSource dataSource) {
        this.dataSource = dataSource;
    }

    public void save(User user) {
        try (Connection conn = dataSource.getConnection();
             PreparedStatement ps = conn.prepareStatement(
                 "INSERT INTO users (email, password_hash) VALUES (?, ?)")) {
            ps.setString(1, user.getEmail());
            ps.setString(2, user.getPasswordHash());
            ps.executeUpdate();
        } catch (SQLException e) {
            throw new DataAccessException("Failed to save user", e);
        }
    }
}

// Responsibility: Send emails
public class EmailService {
    public void sendWelcomeEmail(User user) {
        send(user.getEmail(), "Welcome!", "Welcome to our app");
    }

    private void send(String to, String subject, String body) {
        // Email sending logic
    }
}

// Responsibility: Hash passwords
public class PasswordEncoder {
    public String encode(String rawPassword) {
        return BCrypt.hashpw(rawPassword, BCrypt.gensalt());
    }

    public boolean matches(String rawPassword, String encodedPassword) {
        return BCrypt.checkpw(rawPassword, encodedPassword);
    }
}

public class PaymentProcessor {
    public void processPayment(String paymentType, double amount) {
        if (paymentType.equals("CREDIT_CARD")) {
            // Process credit card payment
            System.out.println("Processing credit card payment: $" + amount);
        } else if (paymentType.equals("PAYPAL")) {
            // Process PayPal payment
            System.out.println("Processing PayPal payment: $" + amount);
        } else if (paymentType.equals("BITCOIN")) {
            // Process Bitcoin payment
            System.out.println("Processing Bitcoin payment: $" + amount);
        }
        // Adding new payment method requires modifying this class!
    }
}

// Abstract payment interface
public interface PaymentMethod {
    void process(double amount);
}

// Concrete implementations
public class CreditCardPayment implements PaymentMethod {
    @Override
    public void process(double amount) {
        System.out.println("Processing credit card payment: $" + amount);
        // Credit card specific logic
    }
}

public class PayPalPayment implements PaymentMethod {
    @Override
    public void process(double amount) {
        System.out.println("Processing PayPal payment: $" + amount);
        // PayPal specific logic
    }
}

public class BitcoinPayment implements PaymentMethod {
    @Override
    public void process(double amount) {
        System.out.println("Processing Bitcoin payment: $" + amount);
        // Bitcoin specific logic
    }
}

// Processor delegates to payment method
public class PaymentProcessor {
    public void processPayment(PaymentMethod paymentMethod, double amount) {
        paymentMethod.process(amount);
    }
}

// Usage
PaymentProcessor processor = new PaymentProcessor();
processor.processPayment(new CreditCardPayment(), 100.0);
processor.processPayment(new PayPalPayment(), 50.0);

// Adding new payment method: just create new class, no modification needed!
public class ApplePayPayment implements PaymentMethod {
    @Override
    public void process(double amount) {
        System.out.println("Processing Apple Pay payment: $" + amount);
    }
}

// Strategy interface
public interface DiscountStrategy {
    double applyDiscount(double price);
}

// Concrete strategies
public class NoDiscount implements DiscountStrategy {
    @Override
    public double applyDiscount(double price) {
        return price;
    }
}

public class PercentageDiscount implements DiscountStrategy {
    private final double percentage;

    public PercentageDiscount(double percentage) {
        this.percentage = percentage;
    }

    @Override
    public double applyDiscount(double price) {
        return price * (1 - percentage / 100);
    }
}

public class FixedAmountDiscount implements DiscountStrategy {
    private final double amount;

    public FixedAmountDiscount(double amount) {
        this.amount = amount;
    }

    @Override
    public double applyDiscount(double price) {
        return Math.max(0, price - amount);
    }
}

// Context uses strategy
public class PriceCalculator {
    private final DiscountStrategy discountStrategy;

    public PriceCalculator(DiscountStrategy discountStrategy) {
        this.discountStrategy = discountStrategy;
    }

    public double calculateFinalPrice(double originalPrice) {
        return discountStrategy.applyDiscount(originalPrice);
    }
}

// Usage
PriceCalculator calc1 = new PriceCalculator(new PercentageDiscount(10));
double price1 = calc1.calculateFinalPrice(100); // 90.0

PriceCalculator calc2 = new PriceCalculator(new FixedAmountDiscount(15));
double price2 = calc2.calculateFinalPrice(100); // 85.0

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;
    }
}

// Square violates LSP because it changes behavior of setters
public class Square extends Rectangle {
    @Override
    public void setWidth(int width) {
        this.width = width;
        this.height = width; // Side effect!
    }

    @Override
    public void setHeight(int height) {
        this.width = height; // Side effect!
        this.height = height;
    }
}

// This test works for Rectangle but fails for Square
public void testRectangle(Rectangle rect) {
    rect.setWidth(5);
    rect.setHeight(4);
    assertEquals(20, rect.getArea()); // Fails for Square! (25 instead of 20)
}

// Common interface for shapes
public interface Shape {
    int getArea();
}

// Rectangle implementation
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 int getWidth() { return width; }
    public int getHeight() { return height; }
}

// Square implementation (no inheritance from Rectangle)
public class Square implements Shape {
    private final int side;

    public Square(int side) {
        this.side = side;
    }

    @Override
    public int getArea() {
        return side * side;
    }

    public int getSide() { return side; }
}

// Works for any Shape
public int calculateTotalArea(List<Shape> shapes) {
    return shapes.stream()
                 .mapToInt(Shape::getArea)
                 .sum();
}

public interface BankAccount {
    // Precondition: amount > 0
    // Postcondition: balance increased by amount
    void deposit(double amount);

    // Precondition: amount > 0 and amount <= balance
    // Postcondition: balance decreased by amount
    void withdraw(double amount) throws InsufficientFundsException;

    double getBalance();
}

public class SavingsAccount implements BankAccount {
    private double balance;

    @Override
    public void deposit(double amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException("Amount must be positive");
        }
        balance += amount;
    }

    @Override
    public void withdraw(double amount) throws InsufficientFundsException {
        if (amount <= 0) {
            throw new IllegalArgumentException("Amount must be positive");
        }
        if (amount > balance) {
            throw new InsufficientFundsException();
        }
        balance -= amount;
    }

    @Override
    public double getBalance() {
        return balance;
    }
}

// Subclass maintains contracts (LSP)
public class CheckingAccount implements BankAccount {
    private double balance;
    private final double overdraftLimit;

    public CheckingAccount(double overdraftLimit) {
        this.overdraftLimit = overdraftLimit;
    }

    @Override
    public void deposit(double amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException("Amount must be positive");
        }
        balance += amount; // Same postcondition
    }

    @Override
    public void withdraw(double amount) throws InsufficientFundsException {
        if (amount <= 0) {
            throw new IllegalArgumentException("Amount must be positive");
        }
        // Can weaken precondition (allow overdraft) but not strengthen
        if (amount > balance + overdraftLimit) {
            throw new InsufficientFundsException();
        }
        balance -= amount; // Same postcondition
    }

    @Override
    public double getBalance() {
        return balance;
    }
}

// Fat interface forces all implementations to provide all methods
public interface Worker {
    void work();
    void eat();
    void sleep();
    void getSalary();
    void attendMeeting();
}

// Robot doesn't eat or sleep but is forced to implement these methods
public class RobotWorker implements Worker {
    @Override
    public void work() {
        System.out.println("Robot working");
    }

    @Override
    public void eat() {
        // Doesn't make sense for robots!
        throw new UnsupportedOperationException("Robots don't eat");
    }

    @Override
    public void sleep() {
        // Doesn't make sense for robots!
        throw new UnsupportedOperationException("Robots don't sleep");
    }

    @Override
    public void getSalary() {
        throw new UnsupportedOperationException("Robots don't get paid");
    }

    @Override
    public void attendMeeting() {
        System.out.println("Robot attending meeting");
    }
}

// Segregated interfaces - clients depend only on what they need
public interface Workable {
    void work();
}

public interface Eatable {
    void eat();
}

public interface Sleepable {
    void sleep();
}

public interface Payable {
    void getSalary();
}

public interface MeetingAttendee {
    void attendMeeting();
}

// Human implements relevant interfaces
public class HumanWorker implements Workable, Eatable, Sleepable, Payable, MeetingAttendee {
    @Override
    public void work() {
        System.out.println("Human working");
    }

    @Override
    public void eat() {
        System.out.println("Human eating");
    }

    @Override
    public void sleep() {
        System.out.println("Human sleeping");
    }

    @Override
    public void getSalary() {
        System.out.println("Human receiving salary");
    }

    @Override
    public void attendMeeting() {
        System.out.println("Human attending meeting");
    }
}

// Robot only implements relevant interfaces
public class RobotWorker implements Workable, MeetingAttendee {
    @Override
    public void work() {
        System.out.println("Robot working");
    }

    @Override
    public void attendMeeting() {
        System.out.println("Robot attending meeting");
    }
}

// Low-level module
public class MySQLDatabase {
    public void save(String data) {
        System.out.println("Saving to MySQL: " + data);
    }
}

// High-level module depends on concrete low-level module
public class UserService {
    private MySQLDatabase database; // Concrete dependency!

    public UserService() {
        this.database = new MySQLDatabase(); // Tight coupling!
    }

    public void createUser(String userData) {
        // Business logic
        database.save(userData);
    }
}

// Abstraction
public interface Database {
    void save(String data);
}

// Low-level modules depend on abstraction
public class MySQLDatabase implements Database {
    @Override
    public void save(String data) {
        System.out.println("Saving to MySQL: " + data);
    }
}

public class PostgreSQLDatabase implements Database {
    @Override
    public void save(String data) {
        System.out.println("Saving to PostgreSQL: " + data);
    }
}

public class MongoDatabase implements Database {
    @Override
    public void save(String data) {
        System.out.println("Saving to MongoDB: " + data);
    }
}

// High-level module depends on abstraction
public class UserService {
    private final Database database; // Abstraction!

    // Dependency injected through constructor
    public UserService(Database database) {
        this.database = database;
    }

    public void createUser(String userData) {
        // Business logic
        database.save(userData);
    }
}

// Usage - client chooses implementation
Database db = new MySQLDatabase();
UserService service = new UserService(db);
service.createUser("John Doe");

// Easy to switch implementations
Database postgresDb = new PostgreSQLDatabase();
UserService postgresService = new UserService(postgresDb);

// Easy to test with mock
Database mockDb = mock(Database.class);
UserService testService = new UserService(mockDb);