Object-Oriented Programming (OOP) in Java

- July 18, 2025

Chapter 7: Object-Oriented Programming (OOP) in Java

Java is a powerful object-oriented programming language. It emphasizes designing programs using real-world entities like objects and classes. OOP helps create modular, maintainable, reusable, and scalable software systems.

🔹 Four Main Principles of OOP

Encapsulation
Inheritance
Polymorphism
Abstraction

🛡️ 1. Encapsulation in Java

Encapsulation is the concept of wrapping data (variables) and methods (code) into a single unit called a class. It helps hide the internal implementation of the object from the outside world.

Example:

class Person {
  private String name;
  private int age;

  public void setName(String name) {
    this.name = name;
  }

  public String getName() {
    return name;
  }
}

Benefits: Data hiding, code maintainability, security, and flexibility.

🧬 2. Inheritance in Java

Inheritance allows a class (child or subclass) to acquire fields and methods of another class (parent or superclass).

Example:

class Animal {
  void eat() {
    System.out.println("This animal eats food.");
  }
}

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

Types of Inheritance:

Single Inheritance
Multilevel Inheritance
Hierarchical Inheritance

Note: Java does not support multiple inheritance with classes but supports it through interfaces.

🔁 3. Polymorphism in Java

Polymorphism means "many forms." It allows methods to behave differently based on the object or parameters.

➤ Method Overloading (Compile-Time Polymorphism)

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

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

➤ Method Overriding (Run-Time Polymorphism)

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

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

🎭 4. Abstraction in Java

Abstraction is the process of hiding implementation details and showing only the essential features of an object.

➤ Using Abstract Class

abstract class Shape {
  abstract void draw();
  void display() {
    System.out.println("This is a shape.");
  }
}

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

➤ Using Interface

interface Printable {
  void print();
}

class Document implements Printable {
  public void print() {
    System.out.println("Printing document...");
  }
}

📘 Summary

Concept	Description
Encapsulation	Binding data and methods into a class
Inheritance	Acquiring properties from a parent class
Polymorphism	Multiple behaviors with the same method name
Abstraction	Hiding internal details, showing only essentials

❓ FAQs with Answers

Q1: What is encapsulation in Java?

A: It is the process of wrapping variables and methods into a single unit. It helps in data hiding and securing code.

Q2: What is inheritance in Java?

A: Inheritance allows one class to acquire properties and methods of another class, enhancing code reusability.

Q3: What is the difference between method overloading and overriding?

Overloading: Same method name, different parameters in the same class.
Overriding: Redefining the parent method in a subclass.

Q4: What is abstraction and how is it implemented?

A: Abstraction hides complexity and exposes only relevant parts. It is implemented via abstract classes and interfaces.

Q5: Interface vs Abstract Class?

Feature	Abstract Class	Interface
Methods	Abstract and Concrete	Only Abstract (till Java 7), default/static (from Java 8)
Multiple Inheritance	Not Supported	Supported
Constructor	Yes	No
Access Modifiers	Any	Public Only

Q6: Why is OOP important in Java?

A: It promotes modularity, scalability, and maintainability. It models real-world problems more naturally.

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