Which scenario best illustrates the Observer pattern?

Which scenario best illustrates the Observer pattern?

• A. Notify registered observers about changes, commonly in event systems or UI.
• B. Directly manage state without notifying others.
• C. Create new types by combining existing ones.
• D. Reduce memory usage.

Answer: (A)

Notifying a set of interested observers when a subject changes is the key idea being tested. In the Observer pattern, a subject keeps a list of observers that have registered to be informed. When the subject’s state changes, it notifies all registered observers—typically by calling a common update method—so they can react accordingly. This enables automatic updates in things like user interfaces or other dependent components while keeping the subject decoupled from the concrete observers. You can add or remove observers at runtime without altering the subject’s code, which is the main advantage of this approach. The other scenarios describe different concepts. Managing state without notifying others doesn’t enable automatic reactions or decoupled communication. Creating new types by combining existing ones points to composition or a structural pattern like Decorator, not Observer. Reducing memory usage is an optimization goal, not a pattern for how changes are communicated.

Notifying a set of interested observers when a subject changes is the key idea being tested. In the Observer pattern, a subject keeps a list of observers that have registered to be informed. When the subject’s state changes, it notifies all registered observers—typically by calling a common update method—so they can react accordingly. This enables automatic updates in things like user interfaces or other dependent components while keeping the subject decoupled from the concrete observers. You can add or remove observers at runtime without altering the subject’s code, which is the main advantage of this approach.

The other scenarios describe different concepts. Managing state without notifying others doesn’t enable automatic reactions or decoupled communication. Creating new types by combining existing ones points to composition or a structural pattern like Decorator, not Observer. Reducing memory usage is an optimization goal, not a pattern for how changes are communicated.