Creational patterns (Singleton, Factory, Builder) in Software Engineering - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Singleton Pattern Behavior

What is the main purpose of the Singleton pattern in software design?

A To ensure a class has only one instance and provide a global point of access to it.

B To create objects without specifying the exact class of object to create.

C To separate the construction of a complex object from its representation.

D To allow multiple instances of a class with different configurations.

💡 Hint

Think about controlling the number of instances of a class.

The Singleton pattern restricts a class to a single instance and provides a global access point to that instance, ensuring controlled access and resource management.

📋 Factual

intermediate

Factory Pattern Purpose

Which statement best describes the Factory pattern?

A It provides an interface for creating objects but lets subclasses decide which class to instantiate.

B It ensures only one instance of a class exists throughout the application.

C It builds complex objects step by step, allowing different representations.

D It allows an object to alter its behavior when its internal state changes.

💡 Hint

Focus on object creation and subclass responsibility.

The Factory pattern defines an interface for creating objects but lets subclasses decide which class to instantiate, promoting loose coupling.

🔍 Analysis

advanced

Builder Pattern Application

Given a scenario where you need to create different types of complex documents with many optional parts, which creational pattern is most suitable?

A Factory, because it decides which document subclass to instantiate.

B Builder, because it constructs complex objects step by step allowing different representations.

C Singleton, because it controls the number of document instances.

D Prototype, because it clones existing document instances.

💡 Hint

Think about building complex objects with many optional parts.

The Builder pattern is ideal for constructing complex objects step by step, especially when the object can have different representations or optional parts.

❓ Comparison

advanced

Comparing Singleton and Factory Patterns

Which of the following correctly contrasts the Singleton and Factory patterns?

A Singleton creates multiple objects; Factory restricts to one instance.

B Singleton and Factory both ensure only one instance exists.

C Singleton builds complex objects stepwise; Factory provides global access.

D Singleton controls instance count; Factory controls object creation type.

💡 Hint

Focus on what each pattern controls or manages.

Singleton ensures only one instance of a class exists, while Factory controls which type of object to create without specifying the exact class.

❓ Reasoning

expert

Singleton Pattern Thread Safety Challenge

Consider a Singleton implementation in a multi-threaded environment. What is the main risk if the Singleton is not implemented with thread safety?

A The Singleton will automatically become a Factory, creating different objects.

B The Singleton instance will never be created, causing null reference errors.

C Multiple instances of the Singleton class may be created, breaking the pattern's intent.

D The Singleton instance will be shared safely without any issues.

💡 Hint

Think about what happens when two threads try to create the instance at the same time.

Without thread safety, multiple threads can create separate instances simultaneously, violating the Singleton pattern's guarantee of a single instance.

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