Quick Answer
Observer pattern defines a one-to-many dependency. When one object (subject/observable) changes state, all registered observers are notified automatically. Java built-in: Observable class (deprecated), PropertyChangeListener. Modern approach: event buses (Guava EventBus), reactive streams (RxJava, Project Reactor). Used in GUI event handling, MVC pattern (model notifies views), messaging systems.
Answer
Defines one-to-many relationship between objects. Observers are notified automatically on state changes. Useful for event-driven systems and GUIs.
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Verified Answer
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