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

03-behavioral/observer/AlertObserver.java

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+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);
+        }
+    }
+}

03-behavioral/observer/Main.java

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+package observer;
+
+/**
+ * Observer Design Pattern — Runnable Demo
+ * Run: javac observer/*.java -d out/observer && java -cp out/observer observer.Main
+ * Article: https://ankurm.com/observer-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Observer Design Pattern Demo ===\n");
+
+        StockMarket aapl = new StockMarket("AAPL", 175.00);
+
+        StockObserver alert    = new AlertObserver("RiskEngine", 2.0);
+        StockObserver alice    = new PortfolioObserver("Alice", 100);
+        StockObserver bob      = new PortfolioObserver("Bob", 50);
+
+        aapl.subscribe(alert);
+        aapl.subscribe(alice);
+        aapl.subscribe(bob);
+
+        System.out.println();
+        aapl.setPrice(178.50);  // +2% — should trigger alert
+
+        System.out.println();
+        aapl.setPrice(179.00);  // small move
+
+        System.out.println("\n-- Bob unsubscribes --");
+        aapl.unsubscribe(bob);
+
+        System.out.println();
+        aapl.setPrice(165.00);  // big drop — Bob doesn't hear it
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}

03-behavioral/observer/PortfolioObserver.java

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+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);
+    }
+}

03-behavioral/observer/StockMarket.java

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+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);
+        }
+    }
+}

03-behavioral/observer/StockObserver.java

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+package observer;
+
+/** Observer interface — all subscribers implement this */
+public interface StockObserver {
+    void onPriceChanged(String ticker, double oldPrice, double newPrice);
+}