Classes & Objects (OOP)

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Java is an object-oriented language, which means programs are built around classes and objects. This is one of the most important ideas in the whole language, so let's build it up slowly with a real example.

Class vs Object

Term	What it is
Class	A blueprint or template. It describes what properties and behaviors something will have, but isn't a real thing by itself.
Object	An instance of a class — a real, concrete thing created from that blueprint, with its own actual values.

Think of a class like an architect's blueprint for a house, and each object like an actual house built from that blueprint. You can build many houses (objects) from one blueprint (class), and each one can be painted a different color.

Defining a Class

A class groups together fields (also called properties — the data it holds) and methods (the behavior it can perform).

public class Car {
  // fields (properties)
  String brand;
  int speed;

  // method (behavior)
  void drive() {
    System.out.println(brand + " is driving at " + speed + " km/h");
  }
}

Creating an Object with new

The new keyword creates an actual object from a class, allocating memory for it. You then access its fields and methods using dot notation.

public class Main {
  public static void main(String[] args) {
    Car myCar = new Car();  // create an object

    myCar.brand = "Toyota";  // set a field
    myCar.speed = 120;

    myCar.drive();  // call a method: Toyota is driving at 120 km/h
  }
}

Breaking it down:

Car myCar = new Car(); — Car is the type, myCar is the variable name, and new Car() actually builds the object.
myCar.brand and myCar.speed — dot notation accesses fields on a specific object.
myCar.drive() — dot notation also calls methods on a specific object.

Objects Are Independent

Each object created from a class has its own copy of the fields. Changing one object's fields has no effect on any other object made from the same class.

Car car1 = new Car();
car1.brand = "Toyota";
car1.speed = 120;

Car car2 = new Car();
car2.brand = "Honda";
car2.speed = 100;

car1.drive();  // Toyota is driving at 120 km/h
car2.drive();  // Honda is driving at 100 km/h - unaffected by car1

Note: car1 and car2 are two completely separate objects in memory. They share the same blueprint (Car) but hold different data.

Next up: Head to Constructors to learn a cleaner way to set up an object's initial values the moment it's created.

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