Observer pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Observer Pattern Roles

In the Observer pattern, which component is responsible for notifying all registered observers about a change?

A The Client

B The Observer

C The Concrete Observer

D The Subject

💡 Hint

Think about who holds the list of observers and triggers updates.

The Subject maintains a list of observers and notifies them when its state changes.

❓ Architecture

intermediate

Choosing the Right Data Structure for Observers

Which data structure is best suited for storing observers in the Subject to efficiently add, remove, and notify them?

A ArrayList (dynamic array)

B HashSet (unordered set)

C LinkedList

D Stack

💡 Hint

Consider the need to avoid duplicate observers and fast removal.

A HashSet prevents duplicates and allows fast add/remove operations, making it ideal for observer storage.

❓ scaling

advanced

Scaling Notifications in Observer Pattern

When the number of observers grows very large, what is a common technique to improve notification performance?

A Batch notifications and send asynchronously using a thread pool

B Use recursion to notify observers

C Remove half of the observers to reduce load

D Notify observers synchronously in a single thread

💡 Hint

Think about how to avoid blocking the Subject during notifications.

Batching notifications and sending them asynchronously with a thread pool improves scalability and responsiveness.

❓ tradeoff

advanced

Tradeoffs of Using Push vs Pull Model in Observer Pattern

What is a key tradeoff when choosing between the push and pull models for notifying observers?

A Push model sends all data, increasing bandwidth; pull model requires observers to query for data, increasing complexity

B Push model does not notify observers; pull model does

C Push model is slower; pull model is always faster

D Push model requires observers to query data; pull model sends all data automatically

💡 Hint

Consider who controls the data flow and how much data is sent.

Push sends data proactively, which can increase bandwidth usage. Pull requires observers to request data, adding complexity.

❓ component

expert

Identifying the Correct Sequence of Observer Notification

Given the following steps in the Observer pattern, what is the correct order of operations when the Subject's state changes?

A 1, 3, 2, 4

B 2, 1, 3, 4

C 1, 2, 3, 4

D 3, 1, 2, 4

💡 Hint

Think about what must happen before notifications and what observers do after receiving them.

The Subject first updates its state, then notifies observers, who then receive the notification and update themselves.