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Observer Pattern - Four Objects

publish date: 2026/06/12 10:18:11.914328 UTC

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The Observer pattern involves two abstract objects and two concrete objects. Match each object to its role.

Subject Attach (Observer) Detach (Observer) Notify () Observer Update () ConcreteSubject GetState () subjectState ConcreteObserver Update () observerState for all o in observers o -> Update () return subjectState observerState = subject -> GetState ()

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Object

Observer
Subject
ConcreteObserver
ConcreteSubject

Role

Abstract object that maintains state and provides Attach, Detach, and Notify operations; does not know details of the concrete class
Abstract object that defines the Update() interface that all observers must implement
Inherits from Subject; maintains subjectState and provides GetState(); notifies observers when state changes
Inherits from Observer; maintains a copy of subjectState and implements Update() to keep its copy in step with the subject

Correct Answer

(1) Subject,Abstract object that maintains state and provides Attach, Detach, and Notify operations; does not know details of the concrete class
(2) Observer,Abstract object that defines the Update() interface that all observers must implement
(3) ConcreteSubject,Inherits from Subject; maintains subjectState and provides GetState(); notifies observers when state changes
(4) ConcreteObserver,Inherits from Observer; maintains a copy of subjectState and implements Update() to keep its copy in step with the subject

Explanation

The Observer pattern's four objects: Subject (abstract) - manages state and the observer list (Attach/Detach/Notify); Observer (abstract) - defines the Update() interface; ConcreteSubject - the actual object whose state is tracked (GetState, subjectState); ConcreteObserver - each observer that maintains a copy of the state and implements Update(). The Subject only knows the abstract Observer, providing minimal coupling.

Reference

Software Engineering, Ian Sommerville, 10th edition

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