Pattern · Core

Observer

Define a one-to-many dependency: when one object changes, all its subscribers are notified automatically — without it knowing who they are.

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step 1 / 7
1/2The wiring
observers*
NewsAgency
- subs: List
+ subscribe(s)
+ publish(news)
«interface»
Subscriber
+ update(news)
EmailSub
+ update()
SmsSub
+ update()
AppSub
+ update()
The wiring
 1interface Subscriber { void update(String news); }
 2class NewsAgency { List<Subscriber> subs; void publish(String n){...} }
 3class EmailSub implements Subscriber { ... }   // + SmsSub, AppSub
State
knowsSubscriber interface only
✦

The NewsAgency (Subject) keeps a list of Subscribers and exposes subscribe() + publish(). It holds them as the interface, never concrete types.