Class Basics

9.1.1 What is a Class?

In Java, a class is the blueprint for creating objects.
It’s a template that describes:

• What data the object has (fields or attributes)

• What actions it can perform (methods)

Think of a class as the design for a house — it defines the rooms, walls, and doors, but no physical house exists until you actually build one.
Once you instantiate a class, you get an object, which is the real thing you can use in your program.

A class groups related data and behavior together.
For example, a Car class might have:

• Fields: color, speed, model

• Methods: start(), accelerate(), brake()

Using classes makes your code modular, organized, and scalable — which is the foundation of object-oriented programming (OOP).

9.1.2 Why Use Classes?

Why not just write everything in main with variables and methods?

Classes solve three big problems:

1. Structure: They organize data + behavior together. Instead of tracking carColor, carSpeed, and carStart() separately, you bundle it all inside Car.

2. Reusability: You can create many objects from the same class — 10 cars, each with different colors or speeds.

3. Encapsulation: Classes hide internal details and expose only what’s needed. This keeps your data safe and your code easier to manage.

In big projects, classes become the building blocks for features.
Databases, APIs, UI elements — everything is modeled with classes.

9.1.3 Basic Class Syntax

The simplest class looks like this:

java
public class Car {
    String color;
    int speed;

    void start() {
        System.out.println("Car started!");
    }

    void accelerate() {
        speed += 10;
        System.out.println("Accelerating. Speed: " + speed);
    }
}

Key parts:

• public class Car declares the class.

• String color and int speed are fields.

• start() and accelerate() are methods that belong to the class.

At this point, this class is just a blueprint — no car exists until you create one.
You do that by instantiating the class, which we’ll cover next.