When to use which behavioral pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Observer pattern?

The Observer pattern is used to notify multiple objects about changes in another object, promoting loose coupling between the subject and its observers.

beginner

When should you use the Strategy pattern?

Use the Strategy pattern when you want to define a family of algorithms, encapsulate each one, and make them interchangeable without changing the clients that use them.

intermediate

What problem does the Command pattern solve?

The Command pattern encapsulates a request as an object, allowing parameterization of clients with queues, requests, and operations, and supports undoable operations.

intermediate

In which scenario is the State pattern most useful?

The State pattern is useful when an object must change its behavior based on its internal state, making the state transitions explicit and easier to manage.

beginner

How does the Iterator pattern help in system design?

The Iterator pattern provides a way to access elements of a collection sequentially without exposing its underlying representation.

Which behavioral pattern is best for implementing undo functionality?

A Observer

B Command

C Strategy

D State

The Command pattern encapsulates requests and supports undo operations by storing commands.

If you want to change an object's behavior when its internal state changes, which pattern should you use?

A Strategy

B Observer

C Iterator

D State

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

Which pattern allows multiple objects to be notified automatically when one object changes?

A Observer

B State

C Command

D Strategy

The Observer pattern defines a one-to-many dependency to notify observers of changes.

When should you use the Strategy pattern?

A To encapsulate interchangeable algorithms

B To traverse a collection

C To manage object state transitions

D To notify multiple objects about changes

Strategy pattern encapsulates interchangeable algorithms for flexible use.

Which pattern helps to access elements of a collection without exposing its structure?

A State

B Command

C Iterator

D Observer

Iterator pattern provides sequential access to collection elements without exposing internal structure.

Explain when and why you would use the Observer pattern in a system design.

Think about situations where many parts of a system need to react to changes in one part.

Describe the differences between the Strategy and State patterns and when to choose each.

Focus on who controls the behavior changes and what changes.

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