🧱 Object-Oriented Programming in Java – Building Real-World Models

🔖 Introduction

Java is a purely object-oriented language (except for primitive types). OOP (Object-Oriented Programming) is all about organizing code around objects – real-world entities that have state (fields) and behavior (methods).

In this lesson, you’ll learn the 4 pillars of OOP in Java and how to build your own classes and objects.

🔹 What is an Object?

An object is an instance of a class. Think of a class as a blueprint, and objects as the actual products created from that blueprint.

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🔹 Defining a Class

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

    // method
    void drive() {
        System.out.println("Driving at " + speed + " km/h");
    }
}

🔹 Creating an Object

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();     // creating object
        myCar.color = "Red";
        myCar.speed = 80;
        myCar.drive();             // call method
    }
}

🏗️ 4 Pillars of OOP in Java

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1. ✅ Encapsulation – Data Hiding

Wrap data (fields) and methods inside a class. Use private for fields and public for methods to protect data.

public class Person {
    private String name;  // private = encapsulated

    public void setName(String newName) {
        name = newName;
    }

    public String getName() {
        return name;
    }
}

2. ✅ Inheritance – Reuse Code

One class (child) inherits from another (parent). This allows code reuse and method overriding.

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}

3. ✅ Polymorphism – One Interface, Many Forms

Allows you to use the same method name but different implementations.

Animal a = new Dog();  // Polymorphism
a.sound();             // Output: Bark

4. ✅ Abstraction – Hide Complex Details

Use abstract classes or interfaces to hide implementation and only show essential features.

abstract class Shape {
    abstract void draw();
}

class Circle extends Shape {
    void draw() {
        System.out.println("Drawing Circle");
    }
}

🧠 Constructor – A Special Method

A constructor is called when an object is created. It initializes the object.

class Student {
    String name;

    Student(String n) {
        name = n;
    }
}

🧪 Practice Example

class BankAccount {
    private double balance;

    public void deposit(double amount) {
        balance += amount;
    }

    public double getBalance() {
        return balance;
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount();
        acc.deposit(1000);
        System.out.println("Balance: " + acc.getBalance());
    }
}

✅ What You Learned

• OOP revolves around classes and objects
• The 4 Pillars: Encapsulation, Inheritance, Polymorphism, Abstraction
• How to define and use classes, constructors, and access modifiers