Abstract methods in Typescript - Time & Space Complexity

Time Complexity: Abstract methods

O(n)

Understanding Time Complexity

We want to understand how using abstract methods affects the time it takes for a program to run.

Specifically, we ask: how does calling abstract methods impact the number of steps as input grows?

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; }
}

const squares: Square[] = [new Square(2), new Square(3), new Square(4)];
for (const sq of squares) {
  console.log(sq.area());
}
    

This code defines an abstract method area and calls it on several objects in a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling the area method on each object in the array.
• How many times: Once for each object in the squares array.

How Execution Grows With Input

Each time we add one more square, we call area one more time.

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

Pattern observation: The number of operations grows directly with the number of objects.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line as the number of objects increases.

Common Mistake

[X] Wrong: "Abstract methods slow down the program because they add extra steps."

[OK] Correct: Calling an abstract method is just like calling any other method; the time depends on how many times you call it, not on it being abstract.

Interview Connect

Understanding how abstract methods affect performance helps you explain design choices clearly and confidently in interviews.

Self-Check

"What if the area method itself contained a loop over a large array? How would the time complexity change?"

Practice

1. What is true about abstract methods in TypeScript?

easy

A. They have no body and must be implemented by subclasses.

B. They can be called directly without implementation.

C. They can only be used in interfaces, not classes.

D. They are optional to implement in child classes.

Solution

1. Step 1: Understand abstract method definition

   Abstract methods declare a method signature without a body inside an abstract class.

2. Step 2: Check subclass requirements

   Child classes must provide an implementation for all abstract methods.

3. Final Answer:

   They have no body and must be implemented by subclasses. -> Option A

4. Quick Check:

   Abstract methods require implementation [OK]

Hint: Abstract methods have no body and force subclass implementation [OK]

Common Mistakes:

• Thinking abstract methods can be called directly
• Confusing abstract methods with interface methods
• Assuming abstract methods are optional to implement

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

easy

A. abstract myMethod() { }

B. abstract myMethod(): void;

C. myMethod(): void;

D. abstract function myMethod() {}

Solution

1. Step 1: Recall abstract method syntax

   Abstract methods are declared with the abstract keyword and no body, ending with a semicolon.

2. Step 2: Analyze options

   abstract myMethod(): void; matches the correct syntax. Syntaxes that incorrectly include a body are invalid. myMethod(): void; lacks the abstract keyword.

3. Final Answer:

   abstract myMethod(): void; -> Option B

4. Quick Check:

   Abstract method syntax = abstract myMethod(): void; [OK]

Hint: Abstract methods have no body and end with a semicolon [OK]

Common Mistakes:

• Adding method body to abstract methods
• Omitting the abstract keyword
• Using function keyword inside class

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

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

class Dog extends Animal {
  makeSound(): string {
    return "Woof!";
  }
}

const dog = new Dog();
console.log(dog.makeSound());

medium

A. "Woof!"

B. Error: Cannot instantiate abstract class

C. undefined

D. Error: makeSound method not implemented

Solution

1. Step 1: Understand class hierarchy

   Animal is abstract with an abstract method makeSound. Dog extends Animal and implements makeSound returning "Woof!".

2. Step 2: Check instantiation and method call

   Dog is instantiated (allowed), and calling makeSound() returns "Woof!".

3. Final Answer:

   "Woof!" -> Option A

4. Quick Check:

   Implemented abstract method returns "Woof!" [OK]

Hint: Abstract class can't instantiate, but subclass can [OK]

Common Mistakes:

• Trying to instantiate abstract class directly
• Forgetting to implement abstract method
• Expecting undefined instead of returned string

4. Identify the error in the following TypeScript code:

abstract class Vehicle {
  abstract startEngine(): void;
}

class Car extends Vehicle {
  startEngine() {
    console.log("Engine started");
  }
}

const myCar = new Vehicle();

medium

A. startEngine method must be static.

B. Missing return type in startEngine method.

C. Cannot instantiate abstract class Vehicle.

D. Car class must be abstract.

Solution

1. Step 1: Check instantiation of abstract class

   The code tries to create an instance of Vehicle, which is abstract and cannot be instantiated.

2. Step 2: Verify method implementation and class declarations

   The Car class correctly implements startEngine. The method return type is optional in implementation. No need for static method or abstract Car class.

3. Final Answer:

   Cannot instantiate abstract class Vehicle. -> Option C

4. Quick Check:

   Abstract classes cannot be instantiated [OK]

Hint: Abstract classes cannot be instantiated directly [OK]

Common Mistakes:

• Trying to create instance of abstract class
• Thinking method must be static
• Confusing missing return type as error

5. Given the abstract class below, which implementation correctly uses abstract methods to calculate area?

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

// Choose the correct subclass implementation:

hard

A. class Square extends Shape {
constructor(private side: number) { super(); }
area(): string { return `${this.side}`; }
}

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

C. class Square extends Shape {
area(): number { return 0; }
}

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

Solution

1. Step 1: Check constructor and super call

   class Square extends Shape { constructor(private side: number) { super(); } area(): number { return this.side * this.side; } } correctly calls super() in constructor, required for subclass of abstract class.

2. Step 2: Verify method signature and implementation

   class Square extends Shape { constructor(private side: number) { super(); } area(): number { return this.side * this.side; } } implements area() returning a number, matching abstract method signature. Other implementations have errors: one misses super() and uses an undefined variable, one returns 0 ignoring side, one returns string instead of number.

3. Final Answer:

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

4. Quick Check:

   Correct subclass calls super and matches method signature [OK]

Hint: Subclass must call super() and match abstract method signature [OK]

Common Mistakes:

• Omitting super() call in constructor
• Mismatching return types in method
• Using undefined variables inside method