Observer: sync post code blocks to repo exactly, split merged AlertObserver/PortfolioObserver block, Java 25, H6 file labels, fix stale console output, README mapping

03-behavioral/observer/README.md

@@ -0,0 +1,32 @@
+# 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/

README.md

@@ -389,6 +389,24 @@ javac 03-behavioral/memento/*.java -d out/memento
 java -cp out/memento memento.Main
 ```
 
+### Observer (`03-behavioral/observer/`)
+
+| 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 and the "Thread Safety" `CopyOnWriteArrayList` snippet are illustrative only — they are not part of this repository's runnable example.
+
+Run it:
+```bash
+javac 03-behavioral/observer/*.java -d out/observer
+java -cp out/observer observer.Main
+```
+
 ## Reference
 
 - *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides