Abstract classes in Typescript - Time & Space Complexity

Time Complexity: Abstract classes

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run code with abstract classes changes as the input grows.

Specifically, how does using abstract classes affect the number of steps the program takes?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

class Square extends Shape {
  constructor(private side: number) { super(); }
  area() { return this.side * this.side; }
}

function totalArea(shapes: Shape[]): number {
  let sum = 0;
  for (const shape of shapes) {
    sum += shape.area();
  }
  return sum;
}
    

This code defines an abstract class and calculates the total area of many shapes by calling their area method.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the array of shapes and calling area() on each.
• How many times: Once for each shape in the input array.

How Execution Grows With Input

Each shape requires one call to area(), so the total steps grow directly with the number of shapes.

Input Size (n)	Approx. Operations
10	10 calls to area() and 10 loop steps
100	100 calls to area() and 100 loop steps
1000	1000 calls to area() and 1000 loop steps

Pattern observation: The work grows evenly as the number of shapes increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to calculate total area grows in a straight line with the number of shapes.

Common Mistake

[X] Wrong: "Using abstract classes makes the code slower because of extra overhead."

[OK] Correct: The abstract class itself does not add extra loops or repeated work; it just defines a structure. The main cost depends on how many shapes you process.

Interview Connect

Understanding how abstract classes affect time helps you explain design choices clearly and shows you know how structure relates to performance.

Self-Check

"What if each shape's area() method did a complex calculation with its own loops? How would the time complexity change?"

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