Abstract classes in Typescript - Mini Project: Build & Apply

Abstract Classes in TypeScript

📖 Scenario: You are creating a simple program to represent different types of vehicles. Each vehicle has a method to describe its sound, but the exact sound depends on the type of vehicle.

🎯 Goal: Build a TypeScript program using an abstract class called Vehicle with an abstract method makeSound(). Then create two classes Car and Motorcycle that extend Vehicle and implement the makeSound() method. Finally, create instances and print their sounds.

📋 What You'll Learn

Create an abstract class called Vehicle with an abstract method makeSound() that returns a string.

Create a class Car that extends Vehicle and implements makeSound() to return the string 'Vroom'.

Create a class Motorcycle that extends Vehicle and implements makeSound() to return the string 'Braaap'.

Create one instance of Car called myCar and one instance of Motorcycle called myMotorcycle.

Print the result of calling makeSound() on myCar and myMotorcycle.

💡 Why This Matters

🌍 Real World

Abstract classes help organize code when you have a general concept with shared behavior but different specific details, like different types of vehicles.

💼 Career

Understanding abstract classes is important for designing clean, reusable code in many software development jobs, especially when working with object-oriented programming.

1

Create the abstract class Vehicle

Create an abstract class called Vehicle with an abstract method makeSound() that returns a string.

Typescript

// Your code here

Hint

Use the abstract keyword before the class and the method. The method should have no body.

2

Create Car and Motorcycle classes

Create a class Car that extends Vehicle and implements makeSound() to return 'Vroom'. Also create a class Motorcycle that extends Vehicle and implements makeSound() to return 'Braaap'.

Typescript

abstract class Vehicle {
    abstract makeSound(): string;
}

// Your code here

Hint

Use extends Vehicle to inherit. Implement makeSound() with the correct return strings.

3

Create instances of Car and Motorcycle

Create one instance of Car called myCar and one instance of Motorcycle called myMotorcycle.

Typescript

abstract class Vehicle {
    abstract makeSound(): string;
}

class Car extends Vehicle {
    makeSound(): string {
        return 'Vroom';
    }
}

class Motorcycle extends Vehicle {
    makeSound(): string {
        return 'Braaap';
    }
}

// Your code here

Hint

Use new Car() and new Motorcycle() to create instances.

4

Print the sounds of the vehicles

Print the result of calling makeSound() on myCar and myMotorcycle.

Typescript

abstract class Vehicle {
    abstract makeSound(): string;
}

class Car extends Vehicle {
    makeSound(): string {
        return 'Vroom';
    }
}

class Motorcycle extends Vehicle {
    makeSound(): string {
        return 'Braaap';
    }
}

const myCar = new Car();
const myMotorcycle = new Motorcycle();

// Your code here

Hint

Use console.log() to print the sounds.

Practice

1. What is true about an abstract class in TypeScript?

easy

A. Abstract classes are the same as interfaces.

B. Abstract classes cannot have any methods implemented.

C. You cannot create an instance of an abstract class directly.

D. You must use the new keyword to instantiate an abstract class.

Solution

1. Step 1: Understand abstract class instantiation rules

   Abstract classes cannot be instantiated directly, so you cannot create objects from them using new.

2. Step 2: Compare other options

   Abstract classes can have implemented methods, are different from interfaces, and cannot be instantiated with new.

3. Final Answer:

   You cannot create an instance of an abstract class directly. -> Option C

4. Quick Check:

   Abstract class instantiation = not allowed [OK]

Hint: Remember: abstract classes can't be directly instantiated [OK]

Common Mistakes:

• Thinking abstract classes are interfaces
• Assuming abstract classes can't have implemented methods
• Trying to instantiate abstract classes directly

2. Which of the following is the correct syntax to declare an abstract class in TypeScript?

easy

A. abstract class Vehicle {}

B. class abstract Vehicle {}

C. abstract Vehicle class {}

D. class Vehicle abstract {}

Solution

1. Step 1: Recall TypeScript abstract class syntax

   The correct syntax places the keyword abstract before class, followed by the class name.

2. Step 2: Check each option

   Only abstract class Vehicle {} correctly uses abstract class Vehicle {}. Others have incorrect keyword order.

3. Final Answer:

   abstract class Vehicle {} -> Option A

4. Quick Check:

   Abstract class syntax = abstract class Name {} [OK]

Hint: Put 'abstract' before 'class' keyword [OK]

Common Mistakes:

• Placing 'abstract' after 'class'
• Mixing keyword order
• Omitting 'abstract' keyword

3. What will be the output of the following TypeScript code?

abstract class Animal {
  abstract makeSound(): void;
  move(): void {
    console.log('Moving');
  }
}

class Dog extends Animal {
  makeSound(): void {
    console.log('Bark');
  }
}

const dog = new Dog();
dog.makeSound();
dog.move();

medium

A. Moving\nBark

B. No output

C. Error: Cannot create instance of abstract class

D. Bark\nMoving

Solution

1. Step 1: Understand class inheritance and method calls

   The class Dog extends Animal and implements makeSound(). Calling dog.makeSound() prints 'Bark'.

2. Step 2: Check the move() method call

   The move() method is implemented in Animal and prints 'Moving'. Calling dog.move() prints 'Moving'.

3. Final Answer:

   Bark Moving -> Option D

4. Quick Check:

   Subclass methods run, abstract class methods inherited [OK]

Hint: Abstract methods must be implemented; inherited methods run normally [OK]

Common Mistakes:

• Thinking abstract class instance is created
• Confusing method call order
• Expecting errors on calling inherited methods

4. Identify the error in the following TypeScript code:

abstract class Shape {
  abstract area(): number;
}

class Circle extends Shape {
  radius: number;
  constructor(radius: number) {
    super();
    this.radius = radius;
  }
}

medium

A. Circle must implement the abstract method area()

B. Abstract class cannot have abstract methods

C. radius property should be private

D. Missing call to super() in Circle constructor

Solution

1. Step 1: Check abstract method implementation

   The abstract class Shape declares an abstract method area(). Any subclass must implement this method.

2. Step 2: Verify Circle class methods

   The Circle class does not implement area(), so this causes a compile-time error.

3. Final Answer:

   Circle must implement the abstract method area() -> Option A

4. Quick Check:

   Abstract methods require subclass implementation [OK]

Hint: All abstract methods must be implemented in subclasses [OK]

Common Mistakes:

• Forgetting to implement abstract methods
• Thinking super() call is not mandatory when base has no constructor
• Confusing abstract methods with optional methods

5. Given the abstract class below, which implementation correctly uses it to calculate the area of a rectangle?

abstract class Shape {
  abstract area(): number;
}

// Choose the correct Rectangle class implementation

hard

A. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
this.width = width;
this.height = height;
}
area(): number {
return this.width + this.height;
}
}

B. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
super();
this.width = width;
this.height = height;
}
area(): number {
return this.width * this.height;
}
}

C. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
super();
this.width = width;
this.height = height;
}
area(): number {
return this.width + this.height;
}
}

D. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
super();
this.width = width;
this.height = height;
}
}

Solution

1. Step 1: Check constructor and super() call

   Since Shape has no constructor, calling super() is required before using this in the subclass constructor.

2. Step 2: Verify area() method correctness

   class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } area(): number { return this.width * this.height; } } correctly implements area() returning width * height, which is the rectangle's area.

3. Step 3: Check other options for errors

   class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { this.width = width; this.height = height; } area(): number { return this.width + this.height; } } return width + height, which is incorrect for area. class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } } lacks area() implementation, causing error.

4. Final Answer:

   class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } area(): number { return this.width * this.height; } } -> Option B

5. Quick Check:

   Correct area method and constructor = class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } area(): number { return this.width * this.height; } } [OK]

Hint: Area of rectangle = width * height; implement abstract method [OK]

Common Mistakes:

• Returning sum instead of product for area
• Omitting abstract method implementation
• Forgetting to assign constructor parameters