Day 10: Inheritance and Abstract Classes

Building on Foundations

Welcome to Day 10! Today we unlock the real power of OOP: Inheritance.

Inheritance (extends)

Inheritance allows a class (Child) to acquire properties and methods from another class (Parent).

class Animal {
  move(distance: number = 0) {
    console.log(`Animal moved ${distance}m.`);
  }
}

class Dog extends Animal {
  bark() {
    console.log('Woof! Woof!');
  }
}

const dog = new Dog();
dog.bark(); // "Woof! Woof!"
dog.move(10); // "Animal moved 10m." (Inherited!)

Overriding Methods

A child class can replace a parent’s method. You can still access the parent’s method using super.

class Snake extends Animal {
  move(distance: number = 5) {
    console.log("Slithering...");
    super.move(distance);
  }
}

Protected Members

Remember protected from yesterday?

• private: Only visible in the class.
• protected: Visible in the class and subclasses.

class Person {
  protected name: string;
  constructor(name: string) { this.name = name; }
}

class Employee extends Person {
  private department: string;

  constructor(name: string, dept: string) {
    super(name); // Must call super() first!
    this.department = dept;
  }

  public getElevatorPitch() {
    return `Hello, my name is ${this.name} and I work in ${this.department}.`;
  }
}

Abstract Classes

An abstract class is a base class that cannot be instantiated directly. It serves as a blueprint for other classes.

abstract class Department {
  constructor(public name: string) {}

  printName(): void {
    console.log("Department name: " + this.name);
  }

  // Abstract methods MUST be implemented by derived classes
  abstract printMeeting(): void;
}

// const d = new Department(); // Error: Cannot create an instance of an abstract class.

class AccountingDepartment extends Department {
  printMeeting(): void {
    console.log("The Accounting Department meets each Monday at 10am.");
  }
}

Challenge for Today

1. Create an abstract class Shape with an abstract method getArea(): number.
2. Create two classes Rectangle and Circle that extend Shape.
3. Implement getArea for both (Rectangle: width * height, Circle: PI * r^2).
4. Create a function printArea(shape: Shape) that works with either.

See you on Day 11 for Static Members and Accessors!