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

03-behavioral/observer/AlertObserver.java

@@ -0,0 +1,22 @@
+package observer;
+
+/** Fires an alert when the price changes by more than a threshold % */
+public class AlertObserver implements StockObserver {
+    private final String name;
+    private final double thresholdPercent;
+
+    public AlertObserver(String name, double thresholdPercent) {
+        this.name = name;
+        this.thresholdPercent = thresholdPercent;
+    }
+
+    @Override
+    public void onPriceChanged(String ticker, double oldPrice, double newPrice) {
+        double change = Math.abs((newPrice - oldPrice) / oldPrice * 100);
+        if (change >= thresholdPercent) {
+            System.out.printf("  [ALERT:%s] %s moved %.1f%% — ALERT TRIGGERED%n", name, ticker, change);
+        } else {
+            System.out.printf("  [Alert:%s] %s moved %.1f%% — within threshold%n", name, ticker, change);
+        }
+    }
+}