OOP Fundamentals

Lecture 10

Java Programming (4343203)
Diploma in ICT - Semester IV
Gujarat Technological University

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Learning Objectives

By the end of this lecture, you will be able to:

🎯 Understand the core principles of Object-Oriented Programming
🏗️ Explain the concept of classes and objects
🔒 Apply encapsulation principles in Java programs
🎭 Recognize polymorphism in different forms
📝 Implement abstraction using classes and methods
🧬 Understand inheritance relationships between classes
🛠️ Create simple classes demonstrating OOP concepts

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What is Object-Oriented Programming?

Definition

Object-Oriented Programming (OOP) is a programming paradigm that organizes software design around objects rather than functions and logic.

Real-World Analogy

Think of OOP like describing real-world entities:

Car is a class (blueprint)
Your specific car is an object (instance)
Properties: color, model, speed, fuel
Actions: start(), stop(), accelerate(), brake()

Key Characteristics

Modularity: Code organized in discrete objects
Reusability: Objects can be reused in different programs
Maintainability: Easier to modify and debug
Scalability: Easy to add new features
Real-world modeling: Mirrors how we think about problems

OOP vs Procedural Programming

Procedural Approach

// Traditional procedural style
public class CalculatorProcedural {
    public static void main(String[] args) {
        // All logic in main method
        double num1 = 10;
        double num2 = 5;
        
        double sum = num1 + num2;
        double difference = num1 - num2;
        double product = num1 * num2;
        double quotient = num1 / num2;
        
        System.out.println("Sum: " + sum);
        System.out.println("Difference: " + difference);
        System.out.println("Product: " + product);
        System.out.println("Quotient: " + quotient);
    }
}

Object-Oriented Approach

// Object-oriented style
public class Calculator {
    private double num1, num2;
    
    public Calculator(double num1, double num2) {
        this.num1 = num1;
        this.num2 = num2;
    }
    
    public double add() { return num1 + num2; }
    public double subtract() { return num1 - num2; }
    public double multiply() { return num1 * num2; }
    public double divide() { return num1 / num2; }
    
    public void displayResults() {
        System.out.println("Sum: " + add());
        System.out.println("Difference: " + subtract());
        System.out.println("Product: " + multiply());
        System.out.println("Quotient: " + divide());
    }
}

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Classes and Objects

What is a Class?

A class is a blueprint or template for creating objects. It defines:

Attributes (data/properties)
Methods (behavior/actions)

What is an Object?

An object is an instance of a class. It has:

State (actual values of attributes)
Behavior (methods that can be called)

Class Definition Syntax

public class ClassName {
    // Attributes (fields/variables)
    dataType attribute1;
    dataType attribute2;
    
    // Constructor
    public ClassName(parameters) {
        // Initialize attributes
    }
    
    // Methods
    public returnType methodName(parameters) {
        // Method body
        return value;
    }
}

Real-World Example: Student Class

public class Student {
    // Attributes
    private String name;
    private int rollNumber;
    private double marks;
    
    // Constructor
    public Student(String name, int rollNumber, double marks) {
        this.name = name;
        this.rollNumber = rollNumber;
        this.marks = marks;
    }
    
    // Methods
    public void displayInfo() {
        System.out.println("Name: " + name);
        System.out.println("Roll Number: " + rollNumber);
        System.out.println("Marks: " + marks);
    }
    
    public char getGrade() {
        if (marks >= 90) return 'A';
        else if (marks >= 80) return 'B';
        else if (marks >= 70) return 'C';
        else if (marks >= 60) return 'D';
        else return 'F';
    }
}

Object Creation and Usage

public class StudentDemo {
    public static void main(String[] args) {
        // Creating objects (instances)
        Student student1 = new Student("John Doe", 101, 85.5);
        Student student2 = new Student("Jane Smith", 102, 92.0);
        Student student3 = new Student("Bob Johnson", 103, 78.0);
        
        // Using objects
        System.out.println("=== Student 1 ===");
        student1.displayInfo();
        System.out.println("Grade: " + student1.getGrade());
        
        System.out.println("\n=== Student 2 ===");
        student2.displayInfo();
        System.out.println("Grade: " + student2.getGrade());
        
        System.out.println("\n=== Student 3 ===");
        student3.displayInfo();
        System.out.println("Grade: " + student3.getGrade());
    }
}

Memory Representation

// When you create objects:
Student student1 = new Student("John", 101, 85.5);
Student student2 = new Student("Jane", 102, 92.0);

/*
Memory Layout:
+-----------+     +------------------+
| student1  |---->| name: "John"     |
+-----------+     | rollNumber: 101  |
                  | marks: 85.5      |
                  +------------------+

+-----------+     +------------------+
| student2  |---->| name: "Jane"     |
+-----------+     | rollNumber: 102  |
                  | marks: 92.0      |
                  +------------------+
*/

Key Points

One class, many objects: Multiple objects can be created from one class
Independent state: Each object has its own copy of attributes
Shared behavior: All objects share the same methods
Memory efficiency: Methods are stored once, shared by all objects

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The Four Pillars of OOP

1. Encapsulation 🔒

Definition: Bundling data and methods together, hiding internal details.

Benefits:

Data Protection: Prevent unauthorized access
Controlled Access: Use getters and setters
Code Maintenance: Changes don't affect other code
Validation: Ensure data integrity

Example: Bank Account

public class BankAccount {
    private double balance;  // Private - hidden from outside
    private String accountNumber;
    
    public BankAccount(String accountNumber, double initialBalance) {
        this.accountNumber = accountNumber;
        if (initialBalance >= 0) {
            this.balance = initialBalance;
        } else {
            this.balance = 0;
        }
    }
    
    // Controlled access through methods
    public double getBalance() {
        return balance;
    }
    
    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
            System.out.println("Deposited: $" + amount);
        } else {
            System.out.println("Invalid deposit amount");
        }
    }
    
    public void withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            System.out.println("Withdrawn: $" + amount);
        } else {
            System.out.println("Invalid withdrawal amount");
        }
    }
}

2. Inheritance 🧬

Definition: Creating new classes based on existing classes.

Benefits:

Code Reusability: Reuse existing code
Hierarchical Organization: Natural classification
Extensibility: Add new features to existing classes
Polymorphism Support: Foundation for polymorphic behavior

Example: Vehicle Hierarchy

// Parent class (Base class)
public class Vehicle {
    protected String brand;
    protected int speed;
    
    public Vehicle(String brand) {
        this.brand = brand;
        this.speed = 0;
    }
    
    public void start() {
        System.out.println(brand + " vehicle started");
    }
    
    public void accelerate(int increment) {
        speed += increment;
        System.out.println("Speed increased to: " + speed);
    }
}

// Child class (Derived class)
public class Car extends Vehicle {
    private int numberOfDoors;
    
    public Car(String brand, int doors) {
        super(brand);  // Call parent constructor
        this.numberOfDoors = doors;
    }
    
    // New method specific to Car
    public void honk() {
        System.out.println(brand + " car is honking!");
    }
    
    // Override parent method for specific behavior
    @Override
    public void start() {
        System.out.println(brand + " car engine started");
    }
}

// Usage
public class VehicleDemo {
    public static void main(String[] args) {
        Car myCar = new Car("Toyota", 4);
        myCar.start();        // Calls overridden method
        myCar.accelerate(50); // Inherited from Vehicle
        myCar.honk();         // Car-specific method
    }
}

Inheritance Types:

Single: One parent, one child
Multilevel: Chain of inheritance
Hierarchical: Multiple children from one parent
Multiple: Not directly supported in Java (use interfaces)

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Polymorphism and Abstraction

3. Polymorphism 🎭

Definition: "Many forms" - same interface, different implementations.

Types of Polymorphism:

A. Compile-time Polymorphism (Method Overloading)

public class Calculator {
    // Same method name, different parameters
    public int add(int a, int b) {
        return a + b;
    }
    
    public double add(double a, double b) {
        return a + b;
    }
    
    public int add(int a, int b, int c) {
        return a + b + c;
    }
    
    public String add(String a, String b) {
        return a + b;
    }
}

// Usage
Calculator calc = new Calculator();
System.out.println(calc.add(5, 3));        // Calls int version
System.out.println(calc.add(5.5, 3.2));    // Calls double version  
System.out.println(calc.add(1, 2, 3));     // Calls three-parameter version
System.out.println(calc.add("Hello", "World")); // Calls String version

B. Runtime Polymorphism (Method Overriding)

// Base class
public class Animal {
    public void makeSound() {
        System.out.println("Animal makes a sound");
    }
}

// Derived classes
public class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Dog barks: Woof!");
    }
}

public class Cat extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Cat meows: Meow!");
    }
}

// Polymorphic behavior
public class AnimalDemo {
    public static void main(String[] args) {
        Animal[] animals = {
            new Dog(),
            new Cat(),
            new Animal()
        };
        
        // Same method call, different behaviors
        for (Animal animal : animals) {
            animal.makeSound(); // Polymorphic call
        }
    }
}

4. Abstraction 📝

Definition: Hiding complex implementation details, showing only essential features.

Ways to Achieve Abstraction:

A. Abstract Classes

// Abstract class - cannot be instantiated
public abstract class Shape {
    protected String color;
    
    public Shape(String color) {
        this.color = color;
    }
    
    // Concrete method - has implementation
    public void displayColor() {
        System.out.println("Color: " + color);
    }
    
    // Abstract method - must be implemented by subclasses
    public abstract double calculateArea();
    public abstract double calculatePerimeter();
}

// Concrete class implementing abstract methods
public class Rectangle extends Shape {
    private double width, height;
    
    public Rectangle(String color, double width, double height) {
        super(color);
        this.width = width;
        this.height = height;
    }
    
    @Override
    public double calculateArea() {
        return width * height;
    }
    
    @Override
    public double calculatePerimeter() {
        return 2 * (width + height);
    }
}

public class Circle extends Shape {
    private double radius;
    
    public Circle(String color, double radius) {
        super(color);
        this.radius = radius;
    }
    
    @Override
    public double calculateArea() {
        return Math.PI * radius * radius;
    }
    
    @Override
    public double calculatePerimeter() {
        return 2 * Math.PI * radius;
    }
}

B. Interface-based Abstraction

// Interface - pure abstraction
public interface Drawable {
    void draw();
    void resize(double factor);
}

public class Circle implements Drawable {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
    
    @Override
    public void resize(double factor) {
        System.out.println("Resizing circle by factor: " + factor);
    }
}

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Hands-on Exercise 1: Student Class Creation

Exercise: Create Student Management System

Create a Student class with the following requirements:

Requirements:

Attributes: enrollmentNo (String), name (String), marks (double)
Constructor: Initialize all attributes
Methods:
- displayInfo(): Show student details
- getGrade(): Return grade based on marks
- updateMarks(double newMarks): Update student marks
- isPass(): Return true if marks >= 35

Grading System:

A: 90-100
B: 80-89
C: 70-79
D: 60-69
E: 35-59
F: Below 35

Test Cases:

Create 3 student objects and demonstrate all methods.

// Your implementation here
public class Student {
    // Add attributes
    
    // Add constructor
    
    // Add methods
}

public class StudentTest {
    public static void main(String[] args) {
        // Create objects and test methods
    }
}

Solution:

public class Student {
    private String enrollmentNo;
    private String name;
    private double marks;
    
    // Constructor
    public Student(String enrollmentNo, String name, double marks) {
        this.enrollmentNo = enrollmentNo;
        this.name = name;
        this.marks = marks;
    }
    
    // Display student information
    public void displayInfo() {
        System.out.println("=== Student Information ===");
        System.out.println("Enrollment No: " + enrollmentNo);
        System.out.println("Name: " + name);
        System.out.println("Marks: " + marks);
        System.out.println("Grade: " + getGrade());
        System.out.println("Status: " + (isPass() ? "PASS" : "FAIL"));
    }
    
    // Calculate grade based on marks
    public char getGrade() {
        if (marks >= 90) return 'A';
        else if (marks >= 80) return 'B';
        else if (marks >= 70) return 'C';
        else if (marks >= 60) return 'D';
        else if (marks >= 35) return 'E';
        else return 'F';
    }
    
    // Update student marks
    public void updateMarks(double newMarks) {
        if (newMarks >= 0 && newMarks <= 100) {
            this.marks = newMarks;
            System.out.println("Marks updated to: " + newMarks);
        } else {
            System.out.println("Invalid marks! Must be between 0-100");
        }
    }
    
    // Check if student has passed
    public boolean isPass() {
        return marks >= 35;
    }
    
    // Getter methods
    public String getEnrollmentNo() { return enrollmentNo; }
    public String getName() { return name; }
    public double getMarks() { return marks; }
}

Test Class:

public class StudentTest {
    public static void main(String[] args) {
        // Create three students
        Student student1 = new Student("201901001", "John Doe", 85.5);
        Student student2 = new Student("201901002", "Jane Smith", 92.0);
        Student student3 = new Student("201901003", "Bob Johnson", 32.5);
        
        // Display initial information
        student1.displayInfo();
        student2.displayInfo();
        student3.displayInfo();
        
        // Update marks and show changes
        System.out.println("\n=== Updating Marks ===");
        student3.updateMarks(65.0);
        student3.displayInfo();
        
        // Demonstrate pass/fail checking
        System.out.println("\n=== Pass/Fail Status ===");
        System.out.println(student1.getName() + ": " + 
                         (student1.isPass() ? "PASSED" : "FAILED"));
        System.out.println(student2.getName() + ": " + 
                         (student2.isPass() ? "PASSED" : "FAILED"));
        System.out.println(student3.getName() + ": " + 
                         (student3.isPass() ? "PASSED" : "FAILED"));
    }
}

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Hands-on Exercise 2: Vehicle Hierarchy

Exercise: Create Vehicle Inheritance System

Requirements:

Create a vehicle hierarchy demonstrating inheritance:

Base Class: Vehicle

Attributes: brand (String), maxSpeed (int), currentSpeed (int)
Methods:
- start(): Display start message
- accelerate(int speed): Increase current speed
- brake(int speed): Decrease current speed
- displayInfo(): Show vehicle details

Derived Class: Car extends Vehicle

Additional Attributes: numberOfDoors (int), fuelType (String)
Additional Methods:
- honk(): Car-specific sound
- openTrunk(): Car-specific action
Override: start() method with car-specific message

Derived Class: Motorcycle extends Vehicle

Additional Attributes: hasCarrier (boolean), engineType (String)
Additional Methods:
- wheelie(): Motorcycle-specific action
- putOnHelmet(): Safety method
Override: start() method with motorcycle-specific message

Test Requirements:

Create objects of Car and Motorcycle
Demonstrate inheritance (calling parent methods)
Demonstrate method overriding
Show polymorphic behavior

Solution:

// Base class
public class Vehicle {
    protected String brand;
    protected int maxSpeed;
    protected int currentSpeed;
    
    public Vehicle(String brand, int maxSpeed) {
        this.brand = brand;
        this.maxSpeed = maxSpeed;
        this.currentSpeed = 0;
    }
    
    public void start() {
        System.out.println(brand + " vehicle is starting...");
        currentSpeed = 0;
    }
    
    public void accelerate(int speed) {
        if (currentSpeed + speed <= maxSpeed) {
            currentSpeed += speed;
            System.out.println("Accelerated to: " + currentSpeed + " km/h");
        } else {
            System.out.println("Cannot exceed max speed of " + maxSpeed + " km/h");
        }
    }
    
    public void brake(int speed) {
        currentSpeed = Math.max(0, currentSpeed - speed);
        System.out.println("Speed reduced to: " + currentSpeed + " km/h");
    }
    
    public void displayInfo() {
        System.out.println("Brand: " + brand);
        System.out.println("Max Speed: " + maxSpeed + " km/h");
        System.out.println("Current Speed: " + currentSpeed + " km/h");
    }
}

// Car class
public class Car extends Vehicle {
    private int numberOfDoors;
    private String fuelType;
    
    public Car(String brand, int maxSpeed, int doors, String fuelType) {
        super(brand, maxSpeed);
        this.numberOfDoors = doors;
        this.fuelType = fuelType;
    }
    
    @Override
    public void start() {
        System.out.println(brand + " car engine started with a gentle hum");
        currentSpeed = 0;
    }
    
    public void honk() {
        System.out.println(brand + " car: BEEP BEEP!");
    }
    
    public void openTrunk() {
        System.out.println("Opening " + brand + " car trunk");
    }
    
    @Override
    public void displayInfo() {
        super.displayInfo();
        System.out.println("Number of Doors: " + numberOfDoors);
        System.out.println("Fuel Type: " + fuelType);
    }
}

Motorcycle Class:

public class Motorcycle extends Vehicle {
    private boolean hasCarrier;
    private String engineType;
    
    public Motorcycle(String brand, int maxSpeed, boolean hasCarrier, String engineType) {
        super(brand, maxSpeed);
        this.hasCarrier = hasCarrier;
        this.engineType = engineType;
    }
    
    @Override
    public void start() {
        System.out.println(brand + " motorcycle roars to life!");
        currentSpeed = 0;
    }
    
    public void wheelie() {
        if (currentSpeed > 20) {
            System.out.println("Performing wheelie on " + brand + " motorcycle!");
        } else {
            System.out.println("Need more speed for wheelie");
        }
    }
    
    public void putOnHelmet() {
        System.out.println("Safety first! Helmet is on.");
    }
    
    @Override
    public void displayInfo() {
        super.displayInfo();
        System.out.println("Has Carrier: " + (hasCarrier ? "Yes" : "No"));
        System.out.println("Engine Type: " + engineType);
    }
}

Test Class:

public class VehicleTest {
    public static void main(String[] args) {
        // Create objects
        Car myCar = new Car("Toyota", 180, 4, "Petrol");
        Motorcycle myBike = new Motorcycle("Honda", 220, true, "4-Stroke");
        
        System.out.println("=== CAR DEMONSTRATION ===");
        myCar.start();           // Overridden method
        myCar.accelerate(50);    // Inherited method
        myCar.honk();           // Car-specific method
        myCar.displayInfo();    // Overridden method
        myCar.openTrunk();      // Car-specific method
        
        System.out.println("\n=== MOTORCYCLE DEMONSTRATION ===");
        myBike.putOnHelmet();   // Bike-specific method
        myBike.start();         // Overridden method
        myBike.accelerate(30);  // Inherited method
        myBike.wheelie();       // Bike-specific method
        myBike.displayInfo();   // Overridden method
        
        System.out.println("\n=== POLYMORPHISM DEMONSTRATION ===");
        Vehicle[] vehicles = {myCar, myBike};
        
        for (Vehicle vehicle : vehicles) {
            vehicle.start();        // Calls appropriate overridden method
            vehicle.accelerate(25); // Inherited method
            vehicle.displayInfo();  // Calls appropriate overridden method
            System.out.println();
        }
    }
}

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Benefits of Object-Oriented Programming

Code Organization Benefits

1. Modularity

// Each class is a separate module
public class BankAccount {
    // Account-related functionality only
}

public class Customer {
    // Customer-related functionality only  
}

public class Transaction {
    // Transaction-related functionality only
}

public class BankingSystem {
    // Coordinates different modules
    private List<Customer> customers;
    private List<BankAccount> accounts;
    private List<Transaction> transactions;
}

2. Reusability

// Base Employee class can be reused
public class Employee {
    protected String name, id;
    protected double salary;
    
    public void displayInfo() {
        System.out.println("ID: " + id + ", Name: " + name);
    }
}

// Reused in different contexts
public class Manager extends Employee {
    private int teamSize;
}

public class Developer extends Employee {
    private String programmingLanguage;
}

public class SalesRepresentative extends Employee {
    private double commissionRate;
}

3. Maintainability

// Changes in one class don't affect others
public class EmailService {
    public void sendEmail(String to, String subject, String body) {
        // If email logic changes, only this class needs modification
        // Other classes using this remain unchanged
    }
}

public class NotificationManager {
    private EmailService emailService = new EmailService();
    
    public void notifyUser(String email, String message) {
        emailService.sendEmail(email, "Notification", message);
        // This code remains the same even if EmailService changes internally
    }
}

Design Benefits

4. Flexibility and Extensibility

// Easy to add new shapes without changing existing code
public abstract class Shape {
    public abstract double getArea();
}

public class Rectangle extends Shape {
    public double getArea() { return width * height; }
}

public class Circle extends Shape {
    public double getArea() { return Math.PI * radius * radius; }
}

// Adding new shape doesn't break existing code
public class Triangle extends Shape {
    public double getArea() { return 0.5 * base * height; }
}

// Client code works with any shape
public class AreaCalculator {
    public double getTotalArea(Shape[] shapes) {
        double total = 0;
        for (Shape shape : shapes) {
            total += shape.getArea(); // Polymorphism in action
        }
        return total;
    }
}

5. Problem-Solving Approach

// Models real-world entities naturally
public class Library {
    private List<Book> books;
    private List<Member> members;
    
    // Real-world operations
    public void issueBook(Member member, Book book) {
        if (book.isAvailable() && member.canBorrowMore()) {
            book.setIssued(true, member, new Date());
            member.addBorrowedBook(book);
        }
    }
    
    public void returnBook(Member member, Book book) {
        book.setAvailable();
        member.removeBorrowedBook(book);
        // Calculate fine if overdue
    }
}

6. Team Development

// Different developers can work on different classes simultaneously

// Developer A works on User Management
public class User {
    // User-related functionality
}

// Developer B works on Product Management  
public class Product {
    // Product-related functionality
}

// Developer C works on Order Management
public class Order {
    // Order-related functionality
}

// All can be integrated later through interfaces
public interface OrderProcessor {
    void processOrder(User user, List<Product> products);
}

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Common OOP Mistakes and Best Practices

Common Mistakes to Avoid

1. Breaking Encapsulation

// BAD: Public fields
public class BadStudent {
    public String name;      // Anyone can modify
    public double marks;     // No validation possible
    
    public void setMarks(double marks) {
        this.marks = marks;  // Direct assignment, no validation
    }
}

// GOOD: Proper encapsulation
public class GoodStudent {
    private String name;     // Protected from direct access
    private double marks;    // Controlled access only
    
    public void setMarks(double marks) {
        if (marks >= 0 && marks <= 100) {
            this.marks = marks;
        } else {
            throw new IllegalArgumentException("Marks must be between 0-100");
        }
    }
    
    public double getMarks() {
        return marks;
    }
}

2. Poor Inheritance Design

// BAD: Inheritance for code reuse only
public class Animal {
    public void eat() { /* eating logic */ }
    public void sleep() { /* sleeping logic */ }
    public void fly() { /* flying logic - not all animals fly! */ }
}

public class Dog extends Animal {
    // Dog inherits fly() method which doesn't make sense
}

// GOOD: Proper inheritance hierarchy
public class Animal {
    public void eat() { /* eating logic */ }
    public void sleep() { /* sleeping logic */ }
}

public class Bird extends Animal {
    public void fly() { /* flying logic specific to birds */ }
}

public class Dog extends Animal {
    public void bark() { /* dog-specific behavior */ }
}

Best Practices

1. Follow Naming Conventions

// GOOD naming conventions
public class BankAccount {           // PascalCase for classes
    private double accountBalance;   // camelCase for variables
    private static final int MAX_WITHDRAWAL_LIMIT = 10000; // UPPER_CASE for constants
    
    public void depositMoney(double amount) {  // camelCase for methods
        // Method implementation
    }
    
    public double getAccountBalance() {        // Descriptive method names
        return accountBalance;
    }
}

2. Design for Single Responsibility

// GOOD: Each class has single responsibility
public class Customer {
    // Only customer data and basic operations
    private String customerId;
    private String name;
    private String email;
}

public class CustomerValidator {
    // Only validation logic
    public boolean isValidEmail(String email) {
        return email.contains("@") && email.contains(".");
    }
}

public class CustomerDatabase {
    // Only database operations
    public void saveCustomer(Customer customer) {
        // Database save logic
    }
    
    public Customer findCustomer(String id) {
        // Database search logic
        return null;
    }
}

public class CustomerService {
    // Coordinates between different components
    private CustomerValidator validator;
    private CustomerDatabase database;
    
    public void createCustomer(String name, String email) {
        if (validator.isValidEmail(email)) {
            Customer customer = new Customer(name, email);
            database.saveCustomer(customer);
        }
    }
}

3. Use Composition Over Inheritance When Appropriate

// Instead of inheritance
public class Car {
    private Engine engine;      // HAS-A relationship
    private Transmission transmission;
    private GPS gpsSystem;
    
    public void start() {
        engine.start();         // Delegate to composed objects
    }
    
    public void navigate(String destination) {
        gpsSystem.findRoute(destination);
    }
}

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Real-world Applications

1. E-commerce System Design

// Product catalog management
public class Product {
    private String productId;
    private String name;
    private double price;
    private int stockQuantity;
    private Category category;
    
    public boolean isInStock() {
        return stockQuantity > 0;
    }
    
    public void reduceStock(int quantity) {
        if (quantity <= stockQuantity) {
            stockQuantity -= quantity;
        } else {
            throw new IllegalArgumentException("Insufficient stock");
        }
    }
}

// Shopping cart functionality
public class ShoppingCart {
    private Map<Product, Integer> items;
    private Customer customer;
    
    public void addItem(Product product, int quantity) {
        if (product.isInStock() && quantity > 0) {
            items.put(product, items.getOrDefault(product, 0) + quantity);
        }
    }
    
    public double calculateTotal() {
        return items.entrySet().stream()
                   .mapToDouble(entry -> entry.getKey().getPrice() * entry.getValue())
                   .sum();
    }
    
    public Order checkout() {
        // Create order and process payment
        return new Order(customer, new ArrayList<>(items.entrySet()));
    }
}

// Order management
public class Order {
    private String orderId;
    private Customer customer;
    private List<OrderItem> items;
    private OrderStatus status;
    private Date orderDate;
    
    public void processOrder() {
        status = OrderStatus.PROCESSING;
        // Process payment, update inventory, send confirmation
    }
}

2. Student Information System

// Academic management
public class Course {
    private String courseCode;
    private String courseName;
    private int credits;
    private Professor professor;
    
    public void enrollStudent(Student student) {
        if (student.canEnroll(this)) {
            student.addCourse(this);
        }
    }
}

public class Student {
    private String enrollmentNo;
    private String name;
    private List<Course> enrolledCourses;
    private Map<Course, Grade> grades;
    
    public double calculateGPA() {
        double totalPoints = 0;
        int totalCredits = 0;
        
        for (Map.Entry<Course, Grade> entry : grades.entrySet()) {
            Course course = entry.getKey();
            Grade grade = entry.getValue();
            totalPoints += grade.getPoints() * course.getCredits();
            totalCredits += course.getCredits();
        }
        
        return totalCredits > 0 ? totalPoints / totalCredits : 0.0;
    }
    
    public List<Course> getEligibleCourses() {
        // Return courses student can enroll in based on prerequisites
        return new ArrayList<>();
    }
}

// Academic record management
public class Transcript {
    private Student student;
    private List<AcademicRecord> records;
    
    public void generateTranscript() {
        // Generate official transcript
        System.out.println("Transcript for: " + student.getName());
        System.out.println("GPA: " + student.calculateGPA());
        
        for (AcademicRecord record : records) {
            System.out.println(record.getCourse().getCourseName() + 
                             " - " + record.getGrade());
        }
    }
}

3. Banking System Architecture

// Account hierarchy
public abstract class Account {
    protected String accountNumber;
    protected double balance;
    protected Customer owner;
    
    public abstract void withdraw(double amount);
    public abstract double calculateInterest();
}

public class SavingsAccount extends Account {
    private double interestRate;
    private int freeTransactionsPerMonth;
    
    @Override
    public void withdraw(double amount) {
        if (balance - amount >= 1000) { // Minimum balance
            balance -= amount;
        } else {
            throw new InsufficientFundsException("Minimum balance required");
        }
    }
    
    @Override
    public double calculateInterest() {
        return balance * interestRate / 100 / 12; // Monthly interest
    }
}

public class CurrentAccount extends Account {
    private double overdraftLimit;
    
    @Override
    public void withdraw(double amount) {
        if (balance + overdraftLimit >= amount) {
            balance -= amount;
        } else {
            throw new InsufficientFundsException("Overdraft limit exceeded");
        }
    }
    
    @Override
    public double calculateInterest() {
        return 0; // No interest for current accounts
    }
}

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Summary and Key Takeaways

What We Learned

🎯 OOP Fundamentals: Understanding classes, objects, and their relationships
🔒 Encapsulation: Data hiding and controlled access through methods
🧬 Inheritance: Code reusability through parent-child relationships
🎭 Polymorphism: Same interface, multiple implementations
📝 Abstraction: Hiding complexity, showing only essential features
🛠️ Practical Implementation: Creating real-world applications using OOP

Core Concepts Review

Classes and Objects

Class: Blueprint for creating objects
Object: Instance of a class with specific data
Constructor: Special method for object initialization
Methods: Define object behavior

Four Pillars of OOP

Encapsulation: Data protection and controlled access
Inheritance: Building new classes from existing ones
Polymorphism: Multiple forms of the same interface
Abstraction: Simplifying complex systems

Benefits Achieved

Code Organization: Better structure and modularity
Reusability: Write once, use multiple times
Maintainability: Easier to modify and debug
Scalability: Easy to extend functionality
Team Development: Multiple developers can work simultaneously

Best Practices Learned

Naming Conventions: Use meaningful, consistent names
Single Responsibility: Each class should have one purpose
Encapsulation: Keep data private, provide public methods
Composition over Inheritance: Prefer HAS-A over IS-A when appropriate
Method Design: Small, focused methods with clear purposes
Validation: Always validate input parameters
Documentation: Comment your classes and complex methods

Real-world Applications

Systems We Can Now Design

Student Management System: Academic records and enrollment
Banking System: Accounts, transactions, and customer management
E-commerce Platform: Products, shopping cart, and orders
Library Management: Books, members, and borrowing system
Inventory Management: Products, stock, and suppliers

Next Steps

In upcoming lectures, we will:

Deep dive into Classes and Objects (Lecture 11)
Master Access Modifiers (Lecture 12)
Explore Java Keywords (Lecture 13)
Work with Constructors (Lecture 14)
Implement Method Overloading (Lecture 15)

Preparation for Next Class

Review today's examples
Practice creating simple classes
Think about real-world objects and their properties
Prepare questions about OOP concepts

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Thank You!

OOP Fundamentals Complete

Lecture 10 Successfully Completed!
You now understand the foundation of Object-Oriented Programming in Java

Ready to dive deeper into classes and objects!