Dependents, Publish-Subscribe

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

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

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

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

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