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

03-behavioral/observer/PortfolioObserver.java

@@ -0,0 +1,19 @@
+package observer;
+
+/** Updates portfolio value whenever a held stock changes price */
+public class PortfolioObserver implements StockObserver {
+    private final String ownerName;
+    private final int    sharesHeld;
+
+    public PortfolioObserver(String ownerName, int sharesHeld) {
+        this.ownerName  = ownerName;
+        this.sharesHeld = sharesHeld;
+    }
+
+    @Override
+    public void onPriceChanged(String ticker, double oldPrice, double newPrice) {
+        double gain = (newPrice - oldPrice) * sharesHeld;
+        System.out.printf("  [Portfolio:%s] %s×%d  P&L change: %+.2f%n",
+                ownerName, ticker, sharesHeld, gain);
+    }
+}