Java Design Patterns Practice Test & Interview Questions

Q#1. In a chat application, you want users to receive updates about messages in a chat room only when they are subscribed to that room. The system should also manage the communication between different chat rooms. Which combination of patterns is suitable? (Scenario-based, Multi-select)

• A) Observer Pattern
• B) Adapter Pattern
• C) Chain of Responsibility Pattern
• D) Mediator Pattern

Answer: A) Observer Pattern and D) Mediator Pattern

Explanation:

• A) Observer Pattern: Correct. The Observer pattern suits the need for notifying subscribers of updates in a chat room.
• B) Adapter Pattern: Incorrect. Given scenario doesn’t satisfy the features of Adapter pattern.
• C) Chain of Responsibility Pattern: Incorrect. Given scenario doesn’t satisfy the features of the Chain of Responsibility pattern.
• D) Mediator Pattern: Correct. Mediator pattern facilitates coordinated communication among different chat rooms.

Q#2. What technique should be used in a Singleton class to prevent multiple instances from being created when the object is serialized and then deserialized? (Concept-based, Single-select)