Classes & Objects

Templates, instances, constructors, and methods

Classes & Objects

A class is a blueprint/template for creating objects. It defines attributes (state) and methods (behavior). An object is an instance of a class — it has its own state but shares the behavior defined in the class.

Key Concepts

Class: Template/blueprint for creating objects. Defines attributes and methods. Encapsulates related data and functions.
Object: Instance of a class. Has its own state. Shares behavior defined in the class. Created using constructors.
Constructor: Special method for initializing objects. Same name as the class. Can be overloaded. No return type.
Instance Variables: Represent object state. Unique to each instance. Access level can be controlled.
Methods: Define object behavior. Can access instance variables. Can be public/private/protected. Can be static or instance.

Class Definition Examples

java

// Java — Class Definition
public class Car {
    // Instance variables
    private String brand;
    private String model;
    private int year;

    // Constructor
    public Car(String brand, String model, int year) {
        this.brand = brand;
        this.model = model;
        this.year = year;
    }

    // Methods
    public void startEngine() {
        System.out.println("Engine started!");
    }

    public String getInfo() {
        return year + " " + brand + " " + model;
    }
}

// Creating and Using Objects
Car myCar = new Car("Toyota", "Camry", 2022);
Car anotherCar = new Car("Honda", "Civic", 2023);
myCar.startEngine();
System.out.println(myCar.getInfo());

python

// Python — Class Definition
class Car:
    # Constructor
    def __init__(self, brand: str, model: str, year: int):
        # Instance variables
        self._brand = brand
        self._model = model
        self._year = year

    # Methods
    def start_engine(self) -> None:
        print("Engine started!")

    def get_info(self) -> str:
        return f"{self._year} {self._brand} {self._model}"

# Creating and Using Objects
my_car = Car("Toyota", "Camry", 2022)
another_car = Car("Honda", "Civic", 2023)
my_car.start_engine()
print(my_car.get_info())

cpp

// C++ — Class Definition
class Car {
private:
    std::string brand;
    std::string model;
    int year;

public:
    // Constructor with initializer list
    Car(const std::string& brand, const std::string& model, int year)
        : brand(brand), model(model), year(year) {}

    void startEngine() {
        std::cout << "Engine started!" << std::endl;
    }

    std::string getInfo() const {
        return std::to_string(year) + " " + brand + " " + model;
    }
};

// Creating and Using Objects
Car myCar("Toyota", "Camry", 2022);
myCar.startEngine();
std::cout << myCar.getInfo() << std::endl;

💬 What's the difference between a class and an object?

A class is a blueprint or template that defines the structure and behavior. An object is a concrete instance of that class with its own state. Think of a class like an architectural blueprint and an object like an actual building constructed from that blueprint. You can create multiple objects from a single class.

💬 What is the 'this' keyword used for?

The 'this' keyword refers to the current instance of the class. It's used to distinguish instance variables from local variables/parameters with the same name, to pass the current object as an argument, and to call other constructors (constructor chaining).