Java’s OOPs is built on four pillars: Encapsulation<\/strong> hides your data and controls access, Inheritance<\/strong> lets classes reuse code from a parent class, Polymorphism<\/strong> allows one method to behave differently based on context, and Abstraction<\/strong> hides complexity and exposes only what’s necessary. <\/p>\n\n\n\n Direct Answer<\/strong>:<\/p>\n\n\n\n Object-Oriented Programming (OOPs) in <\/strong>Java<\/strong><\/a> is a programming paradigm that organizes code around objects<\/strong> and classes<\/strong> rather than functions and logic alone. Java is one of the most widely used OOPs languages in the world, and its entire ecosystem, from Spring Boot to Android, is built on OOPs principles.<\/p>\n\n\n\n OOPs makes your code more modular, reusable, secure, and easier to maintain<\/strong>. Instead of writing one giant block of instructions, you model your program around real-world entities (objects) that have properties (data) and behaviors (methods).<\/p>\n\n\n\n Every OOPs program in Java revolves around two core building blocks (Class and Objects<\/a>) before the four pillars even come into play.<\/p>\n\n\n\n A Class<\/strong> is a blueprint. It defines what properties and behaviors an object will have, but it doesn’t occupy memory on its own. Think of it like an architectural drawing for a house.<\/p>\n\n\n\n An Object<\/strong> is a real instance of that class, the actual house built from the blueprint. It lives in memory and has actual values.<\/p>\n\n\n\n Simple enough, right? Now let’s get to the four pillars that make OOPs truly powerful.<\/p>\n\n\n\n If you are planning on learning OOPs through a structured self-paced course, then consider enrolling in HCL GUVI\u2019s Certified Object-Oriented Programming (OOPs) Course<\/a>, which is perfect for beginners looking to master OOPs fundamentals. It covers key concepts like classes, objects, inheritance, polymorphism, and design patterns.<\/p>\n\n\n\n These four concepts are the heart of every Java interview<\/a>, every Java project<\/a>, and every Java framework you’ll ever work with. Let’s break each one down properly.<\/p>\n\n\n\n Encapsulation in OOPs is all about keeping your data safe from outside interference<\/strong>. You wrap your data (fields) and the methods that operate on that data inside a class, then restrict direct access using access modifiers. External code can only interact with your data through controlled methods, called getters and setters<\/strong>.<\/p>\n\n\n\n Think of it like your bank account. You can check your balance and withdraw money through the ATM, but you can’t directly reach into the bank’s database and change your balance. The data is encapsulated.<\/p>\n\n\n\n In Java, encapsulation is achieved by:<\/p>\n\n\n\n Why it matters:<\/strong> Encapsulation makes your code easier to maintain, debug, and test. You can change the internal implementation without breaking any code that uses the class.<\/p>\n\n\n\n Inheritance allows one class (the child<\/strong> or subclass) to inherit the properties and methods of another class (the parent<\/strong> or superclass). Instead of rewriting the same code, you build on what already exists.<\/p>\n\n\n\n This is the “IS-A” relationship<\/strong> in Java. A Dog IS-A Animal. A Tesla IS-A Car. The child gets everything the parent has, plus it can add its own unique features.<\/p>\n\n\n\n Types of Inheritance in Java:<\/strong><\/p>\n\n\n\n The word “polymorphism” literally means many forms<\/em>. In Java, it means you can use a single method name to perform different tasks depending on the context. This makes your code flexible, clean, and scalable.<\/p>\n\n\n\n There are two types you need to know:<\/p>\n\n\n\n Compile-time Polymorphism (Method Overloading)<\/strong>: The method name is the same, but the parameters differ. Java decides which version to call at compile time.<\/p>\n\n\n\n Runtime Polymorphism (Method Overriding)<\/strong>: The child class provides a new version of a method already defined in the parent class. Java decides which version to call at runtime.<\/p>\n\n\n\n Overloading vs Overriding: Quick Comparison:<\/strong><\/p>\n\n\n\n Abstraction is about simplifying complexity<\/strong>. You expose only the essential features of an object and hide the internal implementation details. Users of your class don’t need to know how<\/em> something works, just that<\/em> it works.<\/p>\n\n\n\n You use an ATM every day without knowing the internal banking logic, right? That’s abstraction in real life.<\/p>\n\n\n\n In Java, abstraction is achieved through two mechanisms:<\/p>\n\n\n\n Abstract Class vs Interface: Know the Difference:<\/strong><\/p>\n\n\n\n This is one of the most commonly asked interview questions, and honestly, it trips up a lot of people. Here’s the clearest way to think about it:<\/p>\n\n\n\nWhat is OOPs (Object-Oriented Programming) in Java?<\/strong><\/h2>\n\n\n\n
Java isn’t considered 100% object-oriented; it still uses primitive data types like int, char, and boolean that aren’t objects. That’s a classic interview trap!<\/div>\n\n\n\nThe 2 Building Blocks: Object and Class<\/strong><\/h2>\n\n\n\n
![\"The](\"https:\/\/www.guvi.in\/blog\/wp-content\/uploads\/2026\/03\/1-3.webp\" "\\"\\"")
<\/figure>\n\n\n\n\/\/ Class = Blueprint\n\nclass Car {\n\n String brand;\n\n int speed;\n\n void accelerate() {\n\n System.out.println(brand + \" is accelerating at \" + speed + \" km\/h\");\n\n }\n\n}\n\n\/\/ Object = Real Instance\n\npublic class Main {\n\n public static void main(String[] args) {\n\n Car myCar = new Car(); \/\/ Creating an object\n\n myCar.brand = \"Toyota\";\n\n myCar.speed = 120;\n\n myCar.accelerate(); \/\/ Output: Toyota is accelerating at 120 km\/h\n\n }\n\n}<\/code><\/pre>\n\n\n\nThe 4 OOPs Concepts in Java: Explained with Examples<\/strong><\/h2>\n\n\n\n
1. Encapsulation: Protecting Your Data<\/strong><\/h3>\n\n\n\n
![\"Encapsulation:](\"https:\/\/www.guvi.in\/blog\/wp-content\/uploads\/2026\/03\/2-3.webp\" "\\"\\"")
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class BankAccount {\n\n private double balance; \/\/ Hidden from outside world\n\n \/\/ Controlled access through getter\n\n public double getBalance() {\n\n return balance;\n\n }\n\n \/\/ Controlled update through setter\n\n public void deposit(double amount) {\n\n if (amount > 0) {\n\n balance += amount;\n\n }\n\n }\n\n}\n\npublic class Main {\n\n public static void main(String[] args) {\n\n BankAccount account = new BankAccount();\n\n account.deposit(5000);\n\n System.out.println(\"Balance: \" + account.getBalance()); \/\/ Balance: 5000.0\n\n \/\/ account.balance = 99999; \u2190 This would cause a compile error!\n\n }\n\n}<\/code><\/pre>\n\n\n\n2. Inheritance: Don’t Repeat Yourself<\/strong><\/h3>\n\n\n\n
![\"Inheritance:](\"https:\/\/www.guvi.in\/blog\/wp-content\/uploads\/2026\/03\/3-3.webp\" "\\"\\"")
<\/figure>\n\n\n\n\/\/ Parent class\n\nclass Animal {\n\n String name;\n\n void eat() {\n\n System.out.println(name + \" is eating.\");\n\n }\n\n}\n\n\/\/ Child class inheriting from Animal\n\nclass Dog extends Animal {\n\n void bark() {\n\n System.out.println(name + \" says: Woof!\");\n\n }\n\n}\n\npublic class Main {\n\n public static void main(String[] args) {\n\n Dog dog = new Dog();\n\n dog.name = \"Bruno\";\n\n dog.eat(); \/\/ Inherited from Animal \u2192 Bruno is eating.\n\n dog.bark(); \/\/ Dog's own method \u2192 Bruno says: Woof!\n\n }\n\n}<\/code><\/pre>\n\n\n\nType<\/strong><\/td> Description<\/strong><\/td> Supported in Java?<\/strong><\/td><\/tr> Single<\/td> One child inherits from one parent<\/td> Yes<\/td><\/tr> Multilevel<\/td> Child \u2192 Parent \u2192 Grandparent chain<\/td> Yes<\/td><\/tr> Hierarchical<\/td> Multiple children from one parent<\/td> Yes<\/td><\/tr> Multiple<\/td> One child from multiple parents<\/td> Not via classes (use interfaces)<\/td><\/tr> Hybrid<\/td> Combination of the above<\/td> Not directly (use interfaces)<\/td><\/tr><\/tbody><\/table> 3. Polymorphism: One Interface, Many Forms<\/strong><\/h3>\n\n\n\n
![\"Polymorphism\"](\"https:\/\/www.guvi.in\/blog\/wp-content\/uploads\/2026\/03\/5-1.webp\" "\\"\\"")
<\/figure>\n\n\n\nclass Calculator {\n\n \/\/ Same method name, different parameters\n\n int add(int a, int b) {\n\n return a + b;\n\n }\n\n double add(double a, double b) {\n\n return a + b;\n\n }\n\n int add(int a, int b, int c) {\n\n return a + b + c;\n\n }\n\n}\n\npublic class Main {\n\n public static void main(String[] args) {\n\n Calculator calc = new Calculator();\n\n System.out.println(calc.add(5, 3)); \/\/ 8\n\n System.out.println(calc.add(5.5, 3.2)); \/\/ 8.7\n\n System.out.println(calc.add(1, 2, 3)); \/\/ 6\n\n }\n\n}<\/code><\/pre>\n\n\n\nclass Shape {\n\n void draw() {\n\n System.out.println(\"Drawing a shape\");\n\n }\n\n}\n\nclass Circle extends Shape {\n\n @Override\n\n void draw() {\n\n System.out.println(\"Drawing a circle\");\n\n }\n\n}\n\nclass Rectangle extends Shape {\n\n @Override\n\n void draw() {\n\n System.out.println(\"Drawing a rectangle\");\n\n }\n\n}\n\npublic class Main {\n\n public static void main(String[] args) {\n\n Shape s1 = new Circle();\n\n Shape s2 = new Rectangle();\n\n s1.draw(); \/\/ Drawing a circle\n\n s2.draw(); \/\/ Drawing a rectangle\n\n }\n\n}<\/code><\/pre>\n\n\n\nFeature<\/strong><\/td> Method Overloading<\/strong><\/td> Method Overriding<\/strong><\/td><\/tr> Where<\/td> Same class<\/td> Parent & Child class<\/td><\/tr> Parameters<\/td> Must differ<\/td> Must be same<\/td><\/tr> Return type<\/td> Can differ<\/td> Must be same (or covariant)<\/td><\/tr> Resolved at<\/td> Compile time<\/td> Runtime<\/td><\/tr><\/tbody><\/table> 4. Abstraction: Show What Matters, Hide What Doesn’t<\/strong><\/h3>\n\n\n\n
![\"Abstraction\"](\"https:\/\/www.guvi.in\/blog\/wp-content\/uploads\/2026\/03\/6.webp\" "\\"\\"")
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\/\/ Abstract class \u2014 defines the \"what\", not the \"how\"\n\nabstract class Vehicle {\n\n String brand;\n\n \/\/ Abstract method \u2014 no implementation here\n\n abstract void fuelType();\n\n \/\/ Concrete method \u2014 shared implementation\n\n void startEngine() {\n\n System.out.println(brand + \"'s engine started.\");\n\n }\n\n}\n\nclass ElectricCar extends Vehicle {\n\n @Override\n\n void fuelType() {\n\n System.out.println(brand + \" runs on electricity.\");\n\n }\n\n}\n\nclass PetrolCar extends Vehicle {\n\n @Override\n\n void fuelType() {\n\n System.out.println(brand + \" runs on petrol.\");\n\n }\n\n}\n\npublic class Main {\n\n public static void main(String[] args) {\n\n Vehicle v1 = new ElectricCar();\n\n v1.brand = \"Tesla\";\n\n v1.startEngine(); \/\/ Tesla's engine started.\n\n v1.fuelType(); \/\/ Tesla runs on electricity.\n\n }\n\n}<\/code><\/pre>\n\n\n\nFeature<\/strong><\/td> Abstract Class<\/strong><\/td> Interface<\/strong><\/td><\/tr> Methods<\/td> Abstract + Concrete<\/td> Abstract (+ default\/static)<\/td><\/tr> Variables<\/td> Any type<\/td> public static final only<\/td><\/tr> Inheritance<\/td> extends (single)<\/td> implements (multiple)<\/td><\/tr> Constructor<\/td> Can have<\/td> Cannot have<\/td><\/tr> Use when<\/td> Shared base behavior<\/td> Define a contract\/capability<\/td><\/tr><\/tbody><\/table> Abstraction vs Encapsulation: The Confusion Cleared<\/strong><\/h2>\n\n\n\n
<\/td> Abstraction<\/strong><\/td> Encapsulation<\/strong><\/td><\/tr> Focus<\/strong><\/td> Hiding complexity<\/em><\/td> Hiding data<\/em><\/td><\/tr> Goal<\/strong><\/td> Show only what’s relevant<\/td> Protect internal state<\/td><\/tr> Achieved via<\/strong><\/td> Abstract classes, Interfaces<\/td> Private fields, Getters\/Setters<\/td><\/tr> Real-world<\/strong><\/td> ATM interface hides banking logic<\/td> ATM PIN hidden from the screen<\/td><\/tr> Level<\/strong><\/td> Design level<\/td> Implementation level<\/td><\/tr><\/tbody><\/table>