layout	title	folder	permalink	categories	tags
pattern	Observer	observer	/patterns/observer/	Behavioral	Java Difficulty-Beginner Gang Of Four Reactive

Also known as

Dependents, Publish-Subscribe

Intent

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Applicability

Use the Observer pattern in any of the following situations

• when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
• when a change to one object requires changing others, and you don't know how many objects need to be changed
• 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

Typical Use Case

• changing in one object leads to a change in other objects

Real world examples

• java.util.Observer
• java.util.EventListener
• javax.servlet.http.HttpSessionBindingListener
• RxJava

Credits

• Design Patterns: Elements of Reusable Object-Oriented Software
• Java Generics and Collections