students and freshers learning programming

OOP Concepts — the four pillars, explained with code

Object-oriented programming (OOP) organises code around 'objects' that bundle data and behaviour together. Its four core pillars each solve a different problem in how code is structured and reused.

Encapsulation — bundling and protecting data

Encapsulation keeps an object's internal data private and exposes only controlled methods to interact with it. A BankAccount class might keep 'balance' private and only allow changes through a deposit()/withdraw() method, preventing invalid direct edits like balance = -500.

Inheritance — reusing behaviour

Inheritance lets a new class (subclass) reuse and extend an existing class's (superclass) code. A SavingsAccount class can inherit from BankAccount and reuse deposit()/withdraw(), while adding its own interest-calculation method.

Polymorphism — one interface, different behaviour

Polymorphism lets different classes respond to the same method call in their own way. Calling makeSound() on a Dog object and a Cat object runs different code in each, even though the calling code looks identical.

Abstraction — hiding unnecessary detail

Abstraction exposes only what's necessary to use an object, hiding internal implementation complexity. When you call car.start(), you don't need to know the internal ignition sequence — that complexity is hidden behind a simple method.

Examples

class Animal { makeSound() { return 'some sound'; } } class Dog extends Animal { makeSound() { return 'Woof'; } } — this is inheritance + polymorphism together.
A private balance field with public deposit()/withdraw() methods is encapsulation in action.
An abstract Shape class with an area() method that every subclass (Circle, Square) must implement differently is abstraction + polymorphism.

Common mistakes

Confusing inheritance (reusing a class's structure) with polymorphism (one method behaving differently per class).
Making all fields public 'for convenience', which defeats the purpose of encapsulation.
Overusing inheritance for code reuse when composition (an object containing another) would be simpler and more flexible.

Try it yourself

Which OOP pillar is demonstrated when a Circle class and a Square class both implement their own version of a calculateArea() method?

Show answer

Polymorphism — the same method name behaves differently depending on the actual object's class.

Frequently asked questions

Can Learn2Plus AI explain OOP in my preferred language?

Yes — tell Learn2Plus AI your language (Java, Python, C++, etc.) and every code example will use that language's syntax.

Is this asked in placement interviews?

Yes — OOP concepts are among the most commonly asked technical-fundamentals questions in placement interviews across almost every software role.

Can I get a small coding practice task on this?

Yes — ask for a small OOP practice task and you'll get one to attempt, with feedback after you try it.

Practice OOP with Learn2Plus AI

Try 2 lessons free — no card required.