Singleton pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Singleton pattern?

The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

beginner

Why use the Singleton pattern in system design?

To control resource usage by limiting the number of instances, and to provide a single shared object accessible globally.

intermediate

How does the Singleton pattern prevent multiple instances?

By making the constructor private and providing a static method that returns the single instance, creating it only once.

intermediate

What is a common problem with Singleton in multithreaded environments?

Multiple threads may create multiple instances if access is not synchronized properly, breaking the Singleton guarantee.

intermediate

Name one way to make Singleton thread-safe.

Use synchronized access or double-checked locking to ensure only one instance is created even with multiple threads.

What does the Singleton pattern guarantee?

A Only one instance of a class exists

B Multiple instances of a class exist

C No instances of a class exist

D Instances are created on demand without limit

Singleton ensures only one instance of a class exists and provides global access to it.

How is the Singleton instance usually accessed?

A Through a public static method

B By creating new objects directly

C By subclassing the Singleton

D Through multiple constructors

Singleton provides a public static method to get the single instance.

What problem can occur if Singleton is not thread-safe?

A The instance will be destroyed

B Multiple instances may be created

C No instance will be created

D The instance will be shared incorrectly

Without thread safety, multiple threads can create multiple instances, breaking the Singleton pattern.

Which of these is NOT a typical Singleton feature?

A Private constructor

B Global access method

C Static instance variable

D Multiple public constructors

Singleton uses a private constructor to prevent multiple instances, so multiple public constructors are not allowed.

Which technique helps make Singleton thread-safe?

A Using public constructors

B Ignoring synchronization

C Double-checked locking

D Creating multiple instances

Double-checked locking ensures only one instance is created even with multiple threads.

Explain the Singleton pattern and why it is useful in system design.

Think about how you would share one resource safely across a system.

Describe how to implement a thread-safe Singleton.

Consider what happens when multiple threads try to create the instance at the same time.

Practice

1. What is the main purpose of the Singleton pattern in system design?

easy

A. To ensure only one instance of a class exists throughout the application

B. To allow multiple instances of a class with different states

C. To create a new instance every time it is requested

D. To inherit properties from multiple classes

Solution

1. Step 1: Understand Singleton pattern goal

   The Singleton pattern is designed to restrict a class to a single instance.

2. Step 2: Compare options with Singleton definition

   Only To ensure only one instance of a class exists throughout the application matches the purpose of Singleton by ensuring one instance exists.

3. Final Answer:

   To ensure only one instance of a class exists throughout the application -> Option A

4. Quick Check:

   Singleton = single instance [OK]

Hint: Singleton means one instance only, no duplicates [OK]

Common Mistakes:

• Thinking Singleton allows multiple instances
• Confusing Singleton with factory pattern
• Assuming Singleton creates new instances each call

2. Which of the following is the correct way to declare a Singleton instance method in a class?

easy

A. public static getInstance() { ... }

B. public void getInstance() { ... }

C. private static getInstance() { ... }

D. static getInstance() { ... }

Solution

1. Step 1: Identify Singleton instance method requirements

   The method to get the Singleton instance must be static and publicly accessible.

2. Step 2: Analyze options for correct access and static usage

   public static getInstance() { ... } is public and static, which is correct. public void getInstance() { ... } is not static, private static getInstance() { ... } is private, static getInstance() { ... } misses access modifier.

3. Final Answer:

   public static getInstance() { ... } -> Option A

4. Quick Check:

   Singleton instance method = public static [OK]

Hint: Singleton instance method must be public and static [OK]

Common Mistakes:

• Making getInstance() private or non-static
• Omitting access modifier
• Using void return type instead of instance

3. Given the following Singleton code snippet, what will be the output?

class Logger {
  static instance = null;
  constructor() {}
  static getInstance() {
    if (this.instance === null) {
      this.instance = new Logger();
    }
    return this.instance;
  }
}
const log1 = Logger.getInstance();
const log2 = Logger.getInstance();
console.log(log1 === log2);

medium

A. undefined

B. true

C. false

D. Error

Solution

1. Step 1: Understand Singleton instance creation

   The first call to getInstance() creates the instance; the second returns the same instance.

2. Step 2: Compare references of log1 and log2

   Both variables point to the same instance, so strict equality returns true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Same instance check = true [OK]

Hint: Singleton returns same object reference every time [OK]

Common Mistakes:

• Assuming new instance created on each call
• Confusing equality with object content
• Expecting false due to separate variables

4. Identify the error in this Singleton implementation:

class Config {
  static instance;
  constructor() {}
  static getInstance() {
    if (!instance) {
      instance = new Config();
    }
    return instance;
  }
}

medium

A. No error, code is correct

B. Constructor should be public, not private

C. Missing 'this.' before 'instance' inside getInstance method

D. Static keyword is missing before getInstance method

Solution

1. Step 1: Check static variable access inside static method

   Static variables must be accessed with 'this.' prefix inside static methods.

2. Step 2: Identify missing 'this.' usage

   The code uses 'instance' instead of 'this.instance', causing a reference error.

3. Final Answer:

   Missing 'this.' before 'instance' inside getInstance method -> Option C

4. Quick Check:

   Static variable access requires 'this.' [OK]

Hint: Use 'this.' to access static variables inside static methods [OK]

Common Mistakes:

• Ignoring 'this.' prefix for static variables
• Changing constructor visibility incorrectly
• Assuming static keyword missing on method

5. You need to design a logging system using the Singleton pattern that supports multi-threaded access safely. Which approach best ensures thread safety while maintaining Singleton behavior?

hard

A. Use a global variable outside the class to store the instance

B. Create a new instance every time getInstance() is called

C. Make the constructor public and static

D. Use double-checked locking with a private static volatile instance variable

Solution

1. Step 1: Understand thread safety in Singleton

   In multi-threaded environments, multiple threads may create multiple instances if not synchronized.

2. Step 2: Evaluate options for thread-safe Singleton

   Double-checked locking with a volatile static instance ensures only one instance is created safely.

3. Final Answer:

   Use double-checked locking with a private static volatile instance variable -> Option D

4. Quick Check:

   Thread-safe Singleton = double-checked locking [OK]

Hint: Double-checked locking ensures thread-safe Singleton [OK]

Common Mistakes:

• Ignoring thread safety in Singleton
• Making constructor public defeats Singleton
• Using global variables breaks encapsulation