Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/observer/StockMarket.java

@@ -0,0 +1,36 @@
+package observer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Subject (Observable) — maintains a list of observers and notifies them
+ * when the stock price changes. Observers register and deregister freely.
+ */
+public class StockMarket {
+    private final String ticker;
+    private double price;
+    private final List<StockObserver> observers = new ArrayList<>();
+
+    public StockMarket(String ticker, double initialPrice) {
+        this.ticker = ticker;
+        this.price  = initialPrice;
+        System.out.println("[Market] " + ticker + " initialised at $" + initialPrice);
+    }
+
+    public void subscribe(StockObserver observer)   { observers.add(observer); }
+    public void unsubscribe(StockObserver observer) { observers.remove(observer); }
+
+    public void setPrice(double newPrice) {
+        double old = this.price;
+        this.price = newPrice;
+        System.out.printf("[Market] %s price: $%.2f -> $%.2f%n", ticker, old, newPrice);
+        notifyObservers(old, newPrice);
+    }
+
+    private void notifyObservers(double old, double newPrice) {
+        for (StockObserver o : observers) {
+            o.onPriceChanged(ticker, old, newPrice);
+        }
+    }
+}