# Observer Design Pattern — Java Example

**Pattern:** Behavioral → Observer
**Article:** https://ankurm.com/observer-design-pattern-java/

## What this example shows

A stock market subject that notifies subscribers without knowing what they do with the notification. `StockObserver` is the one-method interface every subscriber implements. `StockMarket` is the subject: it holds the observer list and pushes both the old and new price to every subscriber whenever `setPrice()` is called. `AlertObserver` and `PortfolioObserver` are two concrete observers that react completely differently to the same notification — one watches for threshold breaches, the other computes a P&L delta. `Main` subscribes all three, moves the price twice, then unsubscribes Bob before a third move so his portfolio observer never sees it.

## How to run

```bash
javac observer/*.java -d out/observer
java -cp out/observer observer.Main
```

Requires Java 25.

## Post Section ↔ File Mapping

| Post Section | File(s) |
|---|---|
| Implementation: Stock Price Monitoring — the Observer interface | `StockObserver.java` |
| Implementation: Stock Price Monitoring — the Subject | `StockMarket.java` |
| Implementation: Stock Price Monitoring — AlertObserver | `AlertObserver.java` |
| Implementation: Stock Price Monitoring — PortfolioObserver | `PortfolioObserver.java` |
| Implementation: Stock Price Monitoring — wiring it together | `Main.java` |

Note: the "Push vs Pull Notification Models" pull-model snippet (`PullAlertObserver`) and the "Thread Safety" `CopyOnWriteArrayList` snippet are illustrative only — they are not part of this repository's runnable example.

Article: https://ankurm.com/observer-design-pattern-java/
All patterns: https://ankurm.com/design-patterns-java/