Which description best describes the Observer design pattern?

• A. Singleton ensures a single instance.
• B. Observer defines a one-to-many relationship where observers are notified of changes.
• C. Prototype creates new objects by cloning.
• D. Strategy encapsulates algorithms.

Answer: (B)

The Observer pattern describes a one-to-many relationship in which a subject keeps track of dependents (observers) and automatically notifies them when its state changes. This setup lets multiple observers react to changes without the subject needing to know details about each observer, enabling loose coupling. Observers can register themselves to start receiving updates or unregister if they no longer need them, and notifications can be pushed to observers as events occur, which is common in GUI updates and event-driven systems.

That’s why this description is the best fit: it centers on the mechanism of notifying multiple dependents about changes, which is the core idea of the Observer pattern. The other descriptions describe different patterns: a singleton ensures only one instance exists, which is about instance control rather than notifying dependents; a prototype focuses on creating new objects by cloning, which is about object creation; a strategy encapsulates a family of algorithms, focusing on choosing algorithms at runtime rather than notifying observers about state changes.