38 lines
1.3 KiB
Markdown
38 lines
1.3 KiB
Markdown
---
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layout: pattern
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title: Observer
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folder: observer
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permalink: /patterns/observer/
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categories: Behavioral
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tags:
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- Java
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- Difficulty-Beginner
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- Gang Of Four
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---
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**Also known as:** Dependents, Publish-Subscribe
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**Intent:** Define a one-to-many dependency between objects so that when one
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object changes state, all its dependents are notified and updated
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automatically.
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**Applicability:** Use the Observer pattern in any of the following situations
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* when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
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* when a change to one object requires changing others, and you don't know how many objects need to be changed
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* when an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
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**Typical Use Case:**
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* changing in one object leads to a change in other objects
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**Real world examples:**
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* [java.util.Observer](http://docs.oracle.com/javase/8/docs/api/java/util/Observer.html)
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**Credits**
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* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
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