Singleton pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the private static instance variable in the Singleton class.

LLD

private static [1] instance;

Drag options to blanks, or click blank then click option'

ASingleton

Bvoid

Cint

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using a primitive type instead of the class type.

Declaring the instance as public instead of private.

The Singleton instance must be of the Singleton class type and declared static to ensure only one instance exists.

2fill in blank

medium

Complete the code to make the constructor private in the Singleton class.

LLD

[1] Singleton() { }

Drag options to blanks, or click blank then click option'

Apublic

Bprivate

Cprotected

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Making the constructor public or protected.

Making the constructor static, which is not allowed.

The constructor must be private to prevent external instantiation of the Singleton class.

3fill in blank

hard

Fix the error in the getInstance method to correctly return the Singleton instance.

LLD

public static Singleton getInstance() {
    if (instance == null) {
        instance = new Singleton();
    }
    return [1];
}

Drag options to blanks, or click blank then click option'

ASingleton

Bthis

Cinstance

Dnew Singleton()

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'this' inside a static method.

Returning a new Singleton object every call.

The method should return the static instance variable, not 'this' or a new object.

4fill in blank

hard

Fill both blanks to implement thread-safe lazy initialization using synchronized block.

LLD

public static Singleton getInstance() {
    if (instance == null) {
        synchronized([1]) {
            if (instance == null) {
                instance = new [2]();
            }
        }
    }
    return instance;
}

Drag options to blanks, or click blank then click option'

ASingleton.class

Bthis

CSingleton

Dinstance

Attempts:

3 left

💡 Hint

Common Mistakes

Synchronizing on 'this' inside a static method.

Using the instance variable as the lock object.

Use the class object for synchronization and create a new Singleton instance inside the synchronized block.

5fill in blank

hard

Fill all three blanks to implement the Singleton pattern with eager initialization.

LLD

public class Singleton {
    private static final [1] instance = new [2]();

    [3] Singleton() { }

    public static [1] getInstance() {
        return instance;
    }
}

Drag options to blanks, or click blank then click option'

ASingleton

Bpublic

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Making the constructor public.

Not using static final for the instance variable.

The instance is created eagerly as a static final variable. The constructor is private to prevent external instantiation.

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