When to use which behavioral pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the behavioral pattern used to manage communication between objects.

LLD

The [1] pattern helps objects communicate without knowing each other's details.

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

ADecorator

BSingleton

CFactory

DObserver

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing Singleton which controls instance creation, not communication.

The Observer pattern allows objects to notify others about changes, enabling loose coupling.

2fill in blank

medium

Complete the code to select the behavioral pattern that encapsulates a request as an object.

LLD

The [1] pattern allows storing and executing requests later.

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

ACommand

BAdapter

CStrategy

DPrototype

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing with Strategy which selects algorithms, not encapsulating requests.

The Command pattern wraps a request as an object, allowing parameterization and queuing.

3fill in blank

hard

Fix the error in the description of the pattern that changes an object's behavior at runtime.

LLD

The [1] pattern allows an object to change its behavior by switching its internal state.

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

AFacade

BState

CBridge

DComposite

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing Facade which simplifies interfaces but does not change behavior dynamically.

The State pattern lets an object alter its behavior when its internal state changes.

4fill in blank

hard

Fill both blanks to complete the description of the pattern that defines a family of algorithms.

LLD

The [1] pattern lets you select an algorithm at runtime, while the [2] pattern defines how objects interact to perform a task.

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

AStrategy

BObserver

CMediator

DDecorator

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up Observer and Mediator which both deal with communication but differently.

The Strategy pattern selects algorithms dynamically. The Mediator pattern manages object interactions centrally.

5fill in blank

hard

Fill all three blanks to complete the description of patterns used for managing object behavior and communication.

LLD

Use the [1] pattern to notify multiple objects of changes, the [2] pattern to encapsulate requests, and the [3] pattern to change behavior based on state.

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

AObserver

BCommand

CState

DSingleton

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Singleton with behavioral patterns; it controls instance creation.

Observer notifies objects, Command encapsulates requests, and State changes behavior dynamically.

Practice

1. Which behavioral pattern is best suited when you want multiple objects to be notified automatically when one object changes its state?

easy

A. Observer pattern

B. Strategy pattern

C. Command pattern

D. Chain of Responsibility pattern

Solution

1. Step 1: Understand the need for automatic notifications

   The problem requires multiple objects to be updated when one object changes state, which means a one-to-many dependency.

2. Step 2: Match the pattern to the problem

   The Observer pattern is designed exactly for this: it lets observers subscribe to an object and get notified on changes.

3. Final Answer:

   Observer pattern -> Option A

4. Quick Check:

   Change notification = Observer [OK]

Hint: Notifications to many? Use Observer pattern [OK]

Common Mistakes:

• Confusing Strategy with Observer
• Using Command for notifications
• Choosing Chain of Responsibility for updates

2. Which pattern allows you to change an object's behavior at runtime by switching between different algorithms or strategies?

easy

A. Observer pattern

B. Strategy pattern

C. Command pattern

D. Chain of Responsibility pattern

Solution

1. Step 1: Identify the need for interchangeable behaviors

   The question asks about changing behavior dynamically, which means selecting algorithms or methods at runtime.

2. Step 2: Select the pattern that supports behavior switching

   The Strategy pattern encapsulates algorithms and lets you swap them easily without changing the client code.

3. Final Answer:

   Strategy pattern -> Option B

4. Quick Check:

   Change behavior dynamically = Strategy [OK]

Hint: Switch algorithms easily? Use Strategy pattern [OK]

Common Mistakes:

• Mixing Strategy with Observer
• Using Command for behavior changes
• Choosing Chain of Responsibility incorrectly

3. Consider this scenario: You have a request that can be handled by multiple objects in a chain. Each object decides if it can handle the request or passes it on. Which pattern fits this design?
Request -> Handler1 -> Handler2 -> Handler3

medium

A. Command pattern

B. Strategy pattern

C. Observer pattern

D. Chain of Responsibility pattern

Solution

1. Step 1: Analyze the request handling flow

   The request passes through a chain of handlers, each deciding to handle or forward it.

2. Step 2: Identify the matching behavioral pattern

   The Chain of Responsibility pattern allows multiple objects to handle a request in sequence until one handles it.

3. Final Answer:

   Chain of Responsibility pattern -> Option D

4. Quick Check:

   Request passes chain = Chain of Responsibility [OK]

Hint: Request passes chain? Use Chain of Responsibility [OK]

Common Mistakes:

• Confusing Chain with Command
• Using Observer for request handling
• Choosing Strategy incorrectly

4. You have a system where commands need to be queued, logged, and executed later. Which behavioral pattern should you use? Identify the error in this choice:
Using Observer pattern to queue commands.

medium

A. Incorrect, use Command pattern instead

B. Incorrect, use Chain of Responsibility instead

C. Incorrect, use Strategy pattern instead

D. Correct use of Observer

Solution

1. Step 1: Understand the requirement for queuing and executing commands

   Queuing, logging, and executing commands later requires encapsulating requests as objects.

2. Step 2: Identify the pattern that encapsulates requests

   The Command pattern encapsulates requests as objects, allowing queuing and deferred execution.

3. Step 3: Identify the error in using Observer

   Observer is for notifications, not for command encapsulation or queuing.

4. Final Answer:

   Incorrect, use Command pattern instead -> Option A

5. Quick Check:

   Queue commands = Command pattern [OK]

Hint: Queue commands? Use Command, not Observer [OK]

Common Mistakes:

• Using Observer for command queuing
• Confusing Command with Strategy
• Choosing Chain of Responsibility wrongly

5. You are designing a notification system where users can subscribe to different event types, and the system should allow adding new event types without changing existing code. Which combination of behavioral patterns is best suited?

hard

A. Chain of Responsibility for subscriptions and Command for event handling

B. Command for subscriptions and Chain of Responsibility for event handling

C. Observer for subscriptions and Strategy for event handling

D. Strategy for subscriptions and Observer for event handling

Solution

1. Step 1: Identify the subscription mechanism

   Users subscribing to events fits the Observer pattern, which supports dynamic subscription and notification.

2. Step 2: Identify flexible event handling

   Strategy pattern allows interchangeable algorithms for handling different event types without changing existing code.

3. Step 3: Combine patterns for extensibility

   Using Observer for subscriptions and Strategy for event handling supports adding new event types easily and keeps code maintainable.

4. Final Answer:

   Observer for subscriptions and Strategy for event handling -> Option C

5. Quick Check:

   Subscribe = Observer, flexible handling = Strategy [OK]

Hint: Subscribe = Observer, flexible handling = Strategy [OK]

Common Mistakes:

• Mixing Command with subscriptions
• Using Chain of Responsibility for subscriptions
• Confusing Strategy with Observer roles