From 03c93b503718bceb73c223a5810a263c62744b2f Mon Sep 17 00:00:00 2001
From: Ankur <asmhatre@gmail.com>
Date: Wed, 24 Jun 2026 16:52:38 +0530
Subject: [PATCH] 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 | 32 ++++++++++++++++++++++++++++++++
 README.md                        | 18 ++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 03-behavioral/observer/README.md

diff --git a/03-behavioral/observer/README.md b/03-behavioral/observer/README.md
new file mode 100644
index 0000000..d354296
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+++ b/03-behavioral/observer/README.md
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+# 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/
diff --git a/README.md b/README.md
index 59f67ad..1ccb34b 100644
--- a/README.md
+++ b/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