3. Classes and Objects:

• Creating classes to model real-world entities:

   class Person {
     name: string;
     age: number;

     constructor(name: string, age: number) {
       this.name = name;
       this.age = age;
     }

     sayHello(): void {
       console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
     }
   }

   const person = new Person("John Doe", 25);
   person.sayHello();

• Defining properties, methods, and constructors within classes:

   class Rectangle {
     width: number;
     height: number;

     constructor(width: number, height: number) {
       this.width = width;
       this.height = height;
     }

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

   const rectangle = new Rectangle(10, 5);
   const area = rectangle.getArea();
   console.log(`Area: ${area}`);

• Understanding access modifiers (public, private, protected) for encapsulation:

   class Car {
     private brand: string;
     protected year: number;

     constructor(brand: string, year: number) {
       this.brand = brand;
       this.year = year;
     }

     protected startEngine(): void {
       console.log("Engine started.");
     }
   }

   class Sedan extends Car {
     drive(): void {
       console.log(`Driving a ${this.brand} sedan`);
     }
   }

   const sedan = new Sedan("Honda", 2021);
   sedan.drive();

• Implementing inheritance and polymorphism using class inheritance:

   class Animal {
     makeSound(): void {
       console.log("Animal makes sound.");
     }
   }

   class Dog extends Animal {
     makeSound(): void {
       console.log("Dog barks.");
     }
   }

   const animal: Animal = new Dog();
   animal.makeSound();

• Working with abstract classes and interfaces for defining contracts:

   abstract class Shape {
     abstract getArea(): number;
     abstract getPerimeter(): number;
   }

   class Circle extends Shape {
     radius: number;

     constructor(radius: number) {
       super();
       this.radius = radius;
     }

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

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

   const circle: Shape = new Circle(5);
   const area = circle.getArea();
   const perimeter = circle.getPerimeter();