Java OOP (Object-Oriented Programming) Introduction

What is OOP?

• OOP = Object-Oriented Programming

• A programming approach based on classes and objects

• Used to model real-world entities

Class

• A blueprint/template to create objects

• Contains:

  • Variables (data)

  • Methods (functions)

plaintext

class Car {
    String color;
    int speed;

    void drive() {
        System.out.println("Car is running");
    }
}

Object

• An instance of a class

• Used to access class properties and methods

plaintext

Car c1 = new Car();
c1.color = "Red";
c1.drive();

Pillars of OOP

1. Encapsulation

• Wrapping data + methods into a single unit

• Use private variables + getters/setters

• Data hiding

• Secure code

java

class Student {
    private int marks;

    public void setMarks(int m) {
        marks = m;
    }

    public int getMarks() {
        return marks;
    }
}

2. Inheritance

• One class inherits another class

• Use extends keyword

• Code reuse

• Parent → Child relationship

java

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

class Dog extends Animal {
    void bark() {
        System.out.println("Barking");
    }
}

3. Polymorphism

• One method, multiple forms

Types:

• Method Overloading (compile-time)

• Method Overriding (runtime)

java

class MathUtil {
    int add(int a, int b) {
        return a + b;
    }

    int add(int a, int b, int c) {
        return a + b + c;
    }
}

4. Abstraction

• Hiding implementation details

• Show only important features

Key Points:

• Use abstract class or interface

• Reduce complexity

java

abstract class Vehicle {
    abstract void start();
}

class Car extends Vehicle {
    void start() {
        System.out.println("Start with key");
    }
}

Advantages of OOP

• ✔ Code reusability

• ✔ Easy maintenance

• ✔ Data security

• ✔ Real-world modeling

Quick Short

• Class → Blueprint

• Object → Instance

• Encapsulation → Data hiding

• Inheritance → Code reuse

• Polymorphism → Many forms

• Abstraction → Hide details