Java OOP

Java OOP (Object-Oriented Programming)

Object-Oriented Programming (OOP) in Java is a programming paradigm based on objects and classes. Java is a pure OOP language, and OOP concepts make code modular, reusable, and easier to maintain.

1. Key Concepts of Java OOP

Class – Blueprint for creating objects.
Object – Instance of a class.
Encapsulation – Wrapping data (variables) and methods together.
Abstraction – Hiding complex implementation and showing only functionality.
Inheritance – One class can inherit properties and methods of another.
Polymorphism – Ability of objects to take multiple forms.
- Compile-time (Method Overloading)
- Run-time (Method Overriding)

2. Class and Object Example

// Class
class Car {
String brand;
int year;void displayInfo() {
System.out.println("Brand: " + brand + ", Year: " + year);
}
}

public class Main {
public static void main(String[] args) {
// Create Object
Car car1 = new Car();
car1.brand = "Toyota";
car1.year = 2022;
car1.displayInfo();

Car car2 = new Car();
car2.brand = "Honda";
car2.year = 2023;
car2.displayInfo();
}
}

// Class

class Car {

String brand;

int year;void displayInfo() {

System.out.println("Brand: " + brand + ", Year: " + year);

}

public class Main {

public static void main(String[] args) {

// Create Object

Car car1 = new Car();

car1.brand = "Toyota";

car1.year = 2022;

car1.displayInfo();

Car car2 = new Car();

car2.brand = "Honda";

car2.year = 2023;

car2.displayInfo();

}

📌 Output:

3. Encapsulation Example

class Person {
private String name; // private variable// Setter method
public void setName(String name) {
this.name = name;
}

// Getter method
public String getName() {
return name;
}
}

public class Main {
public static void main(String[] args) {
Person p = new Person();
p.setName("Alice");
System.out.println("Name: " + p.getName());
}
}

class Person {

private String name; // private variable// Setter method

public void setName(String name) {

this.name = name;

}

// Getter method

public String getName() {

return name;

}

public class Main {

public static void main(String[] args) {

Person p = new Person();

p.setName("Alice");

System.out.println("Name: " + p.getName());

}

📌 Output:

Encapsulation protects data by using private variables and public getter/setter methods.

4. Inheritance Example

// Parent class
class Animal {
void eat() {
System.out.println("Animal is eating");
}
}// Child class
class Dog extends Animal {
void bark() {
System.out.println("Dog is barking");
}
}

public class Main {
public static void main(String[] args) {
Dog dog = new Dog();
dog.eat(); // inherited from Animal
dog.bark(); // own method
}
}

// Parent class

class Animal {

void eat() {

System.out.println("Animal is eating");

}

}// Child class

class Dog extends Animal {

void bark() {

System.out.println("Dog is barking");

}

public class Main {

public static void main(String[] args) {

Dog dog = new Dog();

dog.eat(); // inherited from Animal

dog.bark(); // own method

}

📌 Output:

5. Polymorphism Example

a) Compile-time Polymorphism (Method Overloading)

class Calculator {
int add(int a, int b) { return a + b; }
double add(double a, double b) { return a + b; }
}public class Main {
public static void main(String[] args) {
Calculator calc = new Calculator();
System.out.println(calc.add(5, 10)); // 15
System.out.println(calc.add(3.5, 2.5)); // 6.0
}
}

class Calculator {

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

double add(double a, double b) { return a + b; }

}public class Main {

public static void main(String[] args) {

Calculator calc = new Calculator();

System.out.println(calc.add(5, 10)); // 15

System.out.println(calc.add(3.5, 2.5)); // 6.0

}

b) Run-time Polymorphism (Method Overriding)

class Animal {
void sound() {
System.out.println("Animal makes sound");
}
}class Cat extends Animal {
@Override
void sound() {
System.out.println("Cat meows");
}
}

public class Main {
public static void main(String[] args) {
Animal myCat = new Cat();
myCat.sound(); // Run-time polymorphism
}
}

class Animal {

void sound() {

System.out.println("Animal makes sound");

}

}class Cat extends Animal {

@Override

void sound() {

System.out.println("Cat meows");

}

public class Main {

public static void main(String[] args) {

Animal myCat = new Cat();

myCat.sound(); // Run-time polymorphism

}

📌 Output:

6. Abstraction Example

Option 1: Abstract Class

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

public class Main {
public static void main(String[] args) {
Shape s = new Circle();
s.draw();
}
}

abstract class Shape {

abstract void draw(); // abstract method

}class Circle extends Shape {

void draw() {

System.out.println("Drawing Circle");

}

public class Main {

public static void main(String[] args) {

Shape s = new Circle();

s.draw();

}

Option 2: Interface

interface Drawable {
void draw();
}class Rectangle implements Drawable {
public void draw() {
System.out.println("Drawing Rectangle");
}
}

public class Main {
public static void main(String[] args) {
Drawable d = new Rectangle();
d.draw();
}
}

interface Drawable {

void draw();

}class Rectangle implements Drawable {

public void draw() {

System.out.println("Drawing Rectangle");

}

public class Main {

public static void main(String[] args) {

Drawable d = new Rectangle();

d.draw();

}

🧠 Key Takeaways

OOP Concept	Description
Class & Object	Blueprint & instance
Encapsulation	Hide data, provide getter/setter
Inheritance	Reuse parent class properties
Polymorphism	Multiple forms (overloading/overriding)
Abstraction	Hide complexity, show functionality

Java OOP

Java OOP (Object-Oriented Programming)

1. Key Concepts of Java OOP

2. Class and Object Example

3. Encapsulation Example

4. Inheritance Example

5. Polymorphism Example

a) Compile-time Polymorphism (Method Overloading)

b) Run-time Polymorphism (Method Overriding)

6. Abstraction Example

🧠 Key Takeaways

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