Creational patterns (Singleton, Factory, Builder) in Software Engineering - Time & Space Complexity

Time Complexity: Creational patterns (Singleton, Factory, Builder)

O(n)

Understanding Time Complexity

When using creational patterns like Singleton, Factory, and Builder, it's important to understand how the time to create objects grows as the program runs.

We want to know how the cost of creating objects changes when we create many of them.

Scenario Under Consideration

Analyze the time complexity of creating multiple objects using a Factory pattern.

class Product {
  constructor(name) {
    this.name = name;
  }
}

class ProductFactory {
  createProduct(name) {
    return new Product(name);
  }
}

const factory = new ProductFactory();
for (let i = 0; i < n; i++) {
  factory.createProduct(`Product${i}`);
}
    

This code creates n products using a Factory. Each product is a simple object with a name.

Identify Repeating Operations

Look at what repeats when creating many objects.

• Primary operation: Calling createProduct to make a new object.
• How many times: Exactly n times, once for each product.

How Execution Grows With Input

Each new product takes about the same time to create, so total time grows as we make more products.

Input Size (n)	Approx. Operations
10	10 object creations
100	100 object creations
1000	1000 object creations

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

Final Time Complexity

Time Complexity: O(n)

This means if you double the number of objects, the time to create them roughly doubles too.

Common Mistake

[X] Wrong: "Using a Factory or Builder makes object creation instant or constant time regardless of how many objects."

[OK] Correct: Each object still needs to be created one by one, so total time grows with the number of objects, even if the pattern helps organize the code.

Interview Connect

Understanding how object creation scales helps you explain design choices clearly and shows you grasp both design and performance, a useful skill in real projects.

Self-Check

What if the Factory cached and reused objects instead of creating new ones each time? How would the time complexity change?

Practice

1. Which creational pattern ensures that only one instance of a class is created throughout the application?

easy

A. Singleton

B. Factory

C. Builder

D. Prototype

Solution

1. Step 1: Understand the Singleton pattern purpose

   The Singleton pattern restricts the instantiation of a class to one object only.

2. Step 2: Compare with other patterns

   Factory creates objects but does not limit the number; Builder constructs complex objects step-by-step; Prototype clones objects.

3. Final Answer:

   Singleton -> Option A

4. Quick Check:

   Only one instance = Singleton [OK]

Hint: Singleton = single instance only [OK]

Common Mistakes:

• Confusing Factory with Singleton
• Thinking Builder limits instances
• Mixing Prototype with Singleton

2. Which of the following is the correct way to define a Singleton class in most object-oriented languages?

easy

A. Allow multiple instances but share data between them

B. Build the object step-by-step using a builder

C. Make the constructor private and provide a static method to get the instance

D. Use a factory method to create new instances every time

Solution

1. Step 1: Recall Singleton implementation details

   Singleton classes hide their constructor (make it private) to prevent direct instantiation.

2. Step 2: Provide controlled access

   A static method returns the single instance, creating it if it doesn't exist yet.

3. Final Answer:

   Make the constructor private and provide a static method to get the instance -> Option C

4. Quick Check:

   Private constructor + static getter = Singleton [OK]

Hint: Singleton hides constructor, uses static getter [OK]

Common Mistakes:

• Allowing public constructors in Singleton
• Using factory method to create multiple instances
• Confusing Builder with Singleton syntax

3. Consider the following code snippet using a Factory pattern:

class AnimalFactory {
  createAnimal(type) {
    switch(type) {
      case 'dog': return new Dog();
      case 'cat': return new Cat();
      default: return null;
    }
  }
}

const factory = new AnimalFactory();
const pet = factory.createAnimal('dog');
console.log(pet.sound());

What will be the output if Dog.sound() returns "Woof" and Cat.sound() returns "Meow"?

medium

A. Error: createAnimal is not a function

B. "Meow"

C. null

D. "Woof"

Solution

1. Step 1: Understand the Factory method call

   The factory creates an object based on the string 'dog', so it returns a new Dog instance.

2. Step 2: Call the sound method on the Dog instance

   Since Dog.sound() returns "Woof", the console.log outputs "Woof".

3. Final Answer:

   "Woof" -> Option D

4. Quick Check:

   Factory creates Dog, Dog.sound() = "Woof" [OK]

Hint: Factory returns object by type, call its method [OK]

Common Mistakes:

• Assuming factory returns null for known types
• Confusing Dog and Cat outputs
• Thinking createAnimal is undefined

4. A developer tries to implement a Singleton but writes this code:

class Logger {
  constructor() {
    if (!Logger.instance) {
      Logger.instance = this;
    }
    return Logger.instance;
  }
}

const log1 = new Logger();
const log2 = new Logger();
console.log(log1 === log2);

What is the output and what is the issue with this Singleton implementation?

medium

A. false; constructor allows multiple instances

B. true; but constructor is public which is not ideal

C. true; works perfectly as Singleton

D. Error; cannot assign to Logger.instance

Solution

1. Step 1: Analyze the constructor logic

   The constructor checks if Logger.instance exists; if not, assigns this to it, then returns Logger.instance.

2. Step 2: Check the instance equality

   Both log1 and log2 refer to the same Logger.instance, so log1 === log2 is true.

3. Step 3: Identify the issue

   The constructor is still public, so new Logger() can be called multiple times, which is not ideal for Singleton pattern.

4. Final Answer:

   true; but constructor is public which is not ideal -> Option B

5. Quick Check:

   Same instance but public constructor = A [OK]

Hint: Singleton needs private constructor to prevent multiple calls [OK]

Common Mistakes:

• Assuming public constructor is fine for Singleton
• Thinking output is false due to new calls
• Expecting runtime error on instance assignment

5. You need to create a complex computer object with many optional parts (CPU, RAM, GPU, Storage). Which creational pattern is best suited to build this object step-by-step without confusing the client code?

hard

A. Builder

B. Factory

C. Singleton

D. Prototype

Solution

1. Step 1: Understand the problem requirements

   The object is complex with many optional parts, so building it step-by-step is needed.

2. Step 2: Match with creational patterns

   Builder pattern is designed to construct complex objects step-by-step, allowing optional parts without confusing client code.

3. Step 3: Exclude other patterns

   Singleton limits instances, Factory creates objects but not stepwise, Prototype clones existing objects.

4. Final Answer:

   Builder -> Option A

5. Quick Check:

   Step-by-step complex object = Builder [OK]

Hint: Builder builds complex objects stepwise clearly [OK]

Common Mistakes:

• Choosing Singleton for complex object creation
• Using Factory when stepwise building is needed
• Confusing Prototype cloning with building