Classes and Objects

public class Main { public static void main(String[] args) { // Creating objects (instances) from the Car class Car myCar = new Car("Toyota", "Camry", 2023); Car friendsCar = new Car("Honda", "Civic", 2022);

// Each object has its own state myCar.startEngine(); // Toyota Camry engine started! myCar.drive(50); // Drove 50 km. Fuel remaining: 95.0%

friendsCar.startEngine(); // Honda Civic engine started! friendsCar.drive(100); // Drove 100 km. Fuel remaining: 90.0%

// myCar's fuel is unaffected by friendsCar's driving System.out.println(myCar.fuelLevel); // 95.0 System.out.println(friendsCar.fuelLevel); // 90.0 } }

CLASS (Blueprint) OBJECTS (Instances) ┌─────────────────┐ ┌─────────────────┐ │ Car │ │ myCar │ │ │ ──────> │ make: "Toyota" │ │ - make │ │ model: "Camry" │ │ - model │ │ year: 2023 │ │ - year │ │ fuel: 95.0 │ │ - fuelLevel │ └─────────────────┘ │ │ │ + startEngine() │ ──────> ┌─────────────────┐ │ + drive() │ │ friendsCar │ │ + stopEngine() │ │ make: "Honda" │ └─────────────────┘ │ model: "Civic" │ │ year: 2022 │ │ fuel: 90.0 │ └─────────────────┘

class BankAccount: """A simple bank account class"""

def __init__(self, owner, balance=0): """Constructor — called when creating a new account""" self.owner = owner self.balance = balance self.transactions = []

def deposit(self, amount): """Add money to the account""" if amount <= 0: print("Deposit amount must be positive") return self.balance += amount self.transactions.append(f"+${amount}") print(f"Deposited ${amount}. New balance: ${self.balance}")

def withdraw(self, amount): """Remove money from the account""" if amount <= 0: print("Withdrawal amount must be positive") return if amount > self.balance: print(f"Insufficient funds. Balance: ${self.balance}") return self.balance -= amount self.transactions.append(f"-${amount}") print(f"Withdrew ${amount}. New balance: ${self.balance}")

def get_statement(self): """Print transaction history""" print(f"\n--- Statement for {self.owner} ---") for t in self.transactions: print(f" {t}") print(f" Current balance: ${self.balance}") print("---")

alice_account = BankAccount("Alice", 1000) bob_account = BankAccount("Bob", 500)

Using objects

alice_account.deposit(200) # Deposited $200. New balance: $1200 alice_account.withdraw(50) # Withdrew $50. New balance: $1150 bob_account.deposit(300) # Deposited $300. New balance: $800 alice_account.get_statement()

#include <iostream> #include <string> #include <vector>

class Student { private: std::string name; int id; std::vector<double> grades;

public: // Constructor Student(std::string name, int id) : name(name), id(id) {}

void addGrade(double grade) { if (grade >= 0 && grade <= 100) { grades.push_back(grade); } }

double getGPA() const { if (grades.empty()) return 0.0; double sum = 0; for (double g : grades) sum += g; return sum / grades.size(); }

void display() const { std::cout << "Student: " << name << " (ID: " << id << ")" << " GPA: " << getGPA() << std::endl; } };

int main() { Student s1("Alice", 1001); Student s2("Bob", 1002);

s1.addGrade(95); s1.addGrade(87); s2.addGrade(72); s2.addGrade(85);

s1.display(); // Student: Alice (ID: 1001) GPA: 91 s2.display(); // Student: Bob (ID: 1002) GPA: 78.5 return 0; }

Car car1 = new Car("Toyota", "Camry", 2023); Car car2 = new Car("Toyota", "Camry", 2023);

// Same state, but different identity System.out.println(car1 == car2); // false — different objects in memory System.out.println(car1.equals(car2)); // depends on equals() implementation