Classes and Objects

A class is a blueprint that defines the properties (fields) and behaviors (methods) that its objects will have. An object is a concrete instance created from that blueprint, holding its own actual data.

Defining a Class

public class Car {
    // Fields (also called instance variables / attributes)
    String color;
    String model;
    int speed;

    // Method (behavior)
    void accelerate() {
        speed += 10;
        System.out.println(model + " is now going " + speed + " km/h");
    }
}

Creating Objects

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();   // creates an object of Car
        myCar.color = "Red";
        myCar.model = "Civic";
        myCar.speed = 0;

        myCar.accelerate();   // Civic is now going 10 km/h
    }
}

Multiple Objects, Independent State

Every object created from a class has its own copy of the instance variables — changing one object's fields never affects another object of the same class.

Car car1 = new Car();
car1.model = "Civic";

Car car2 = new Car();
car2.model = "Mustang";

System.out.println(car1.model); // Civic
System.out.println(car2.model); // Mustang — completely independent

Constructors

A constructor is a special method, matching the class name, that runs automatically when an object is created — used to initialize fields immediately rather than setting them one by one.

public class Car {
    String model;
    int speed;

    // Constructor
    public Car(String model) {
        this.model = model;
        this.speed = 0;
    }
}

Car myCar = new Car("Civic");  // model is set immediately
System.out.println(myCar.model); // Civic

The this Keyword

this refers to the current object — it's used to distinguish a field from a parameter that shares the same name, as seen in this.model = model; above.

Interview tip: If you don't write any constructor, Java automatically provides a no-argument "default constructor" — but as soon as you write any constructor yourself, that automatic default disappears.