What Is OOP?

How OOP Works

A Dog class has properties (name, breed) and methods (bark, fetch). Each Dog object holds its own data. Encapsulation hides internal state — you call dog.feed() without knowing how hunger is tracked. Inheritance lets GoldenRetriever extend Dog, inheriting all behavior and adding its own.

Modern OOP favors composition over inheritance: instead of class GoldenRetriever extends Dog extends Animal, compose behaviors: a Dog has a Walker, a Swimmer, and a Fetcher. This is more flexible and avoids deep inheritance hierarchies.

Why Developers Use OOP

OOP is the foundation of Java, C#, Python, TypeScript, and most enterprise software. Understanding classes, interfaces, and SOLID principles is essential for working in any object-oriented codebase.

Key Concepts

• Encapsulation — Bundle data and methods together, hide internal state — only expose what's needed
• Inheritance — Create new classes based on existing ones — reuse and extend behavior
• Polymorphism — Different objects respond to the same method call in different ways — enables flexible, extensible code
• Abstraction — Define interfaces without implementation details — focus on what, not how
• Composition over Inheritance — Build complex objects from simple components — more flexible than deep class hierarchies
• SOLID Principles — Five design principles for maintainable OOP code — Single Responsibility, Open/Closed, etc.

OOP Example in TypeScript

interface Shape {
  area(): number;
  perimeter(): number;
}

class Circle implements Shape {
  constructor(private radius: number) {}

  area(): number {
    return Math.PI * this.radius ** 2;
  }

  perimeter(): number {
    return 2 * Math.PI * this.radius;
  }
}

class Rectangle implements Shape {
  constructor(private width: number, private height: number) {}

  area(): number {
    return this.width * this.height;
  }

  perimeter(): number {
    return 2 * (this.width + this.height);
  }
}

// Polymorphism: same function works with any Shape
function printInfo(shape: Shape) {
  console.log(`Area: ${shape.area().toFixed(2)}`);
  console.log(`Perimeter: ${shape.perimeter().toFixed(2)}`);
}

Frequently Asked Questions