NotifyObservers

NotifyObservers is a method associated with the Observer design pattern, used by a subject to inform its registered observers about a state change. The subject maintains a collection of observers and, when appropriate, calls each observer’s update method, often passing a data object that describes the change.

In typical implementations, observers register with the subject via an addObserver method and are notified when the subject’s state changes. The notifyObservers method iterates over the registered observers and invokes their update method, usually providing the subject itself and an optional data argument. Many designs gate the notification behind a changed flag and clear that flag after dispatch.

In Java’s classic API, the java.util.Observable class provides addObserver, deleteObserver, and notifyObservers, which call update on each registered Observer with the Observable and an optional argument. However, since Java 9, Observable and notifyObservers are deprecated in favor of alternatives such as PropertyChangeSupport or custom event listener patterns.

Considerations for notifyObservers include thread safety, as observers may be added or removed concurrently; exception handling within observers to avoid breaking notification; and the potential for memory leaks if observers are not properly deregistered. The pattern is widely used across languages; some ecosystems replace explicit notifyObservers with events, signals, or callback registries.

Example use cases include user interface components reacting to model changes, or domain objects broadcasting state updates to interested subsystems. The mechanism fosters loose coupling between subjects and observers while enabling dynamic, event-driven behavior.