Observer Pattern

// ─── EXAMPLE 1 ──────────────────────────────────────────────────────────────
// WHAT WE ARE IMPLEMENTING:
// An online store inventory monitor alerting waiting buyers immediately when
// an out-of-stock item is replenished.
//
// WHERE THE OBSERVER FITS IN:
// Subject is the Observable (e.g., Product). Observer is the Subscriber
// interface. Customer classes act as Concrete Observers.
// ────────────────────────────────────────────────────────────────────────────
import java.util.concurrent.CopyOnWriteArrayList;

// --- Observer interface ---
interface StockObserver {
    void update(String symbol, double price);
}

// --- Subject ---
class StockMarket {
    private final java.util.List<StockObserver> observers = new CopyOnWriteArrayList<>();
    private final java.util.Map<String, Double> prices = new java.util.HashMap<>();

    public void addObserver(StockObserver obs) { observers.add(obs); }
    public void removeObserver(StockObserver obs) { observers.remove(obs); }

    public void setPrice(String symbol, double price) {
        prices.put(symbol, price);
        notifyObservers(symbol, price);
    }

    private void notifyObservers(String symbol, double price) {
        for (StockObserver obs : observers) {
            obs.update(symbol, price);
        }
    }
}

// --- Concrete observers ---
class MobileApp implements StockObserver {
    private final String name;
    public MobileApp(String name) { this.name = name; }
    public void update(String symbol, double price) {
        System.out.println("  [MobileApp " + name + "] " + symbol + " now $" + price);
    }
}

class EmailAlertService implements StockObserver {
    public void update(String symbol, double price) {
        if (price > 500) {
            System.out.println("  [EMAIL ALERT] " + symbol + " crossed $500 at $" + price);
        }
    }
}

class TradingBot implements StockObserver {
    public void update(String symbol, double price) {
        if (price < 50) {
            System.out.println("  [TradingBot] Buying " + symbol + " at $" + price);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        StockMarket market = new StockMarket();

        market.addObserver(new MobileApp("Alice"));
        market.addObserver(new MobileApp("Bob"));
        market.addObserver(new EmailAlertService());
        market.addObserver(new TradingBot());

        System.out.println("--- Price Update: AAPL ---");
        market.setPrice("AAPL", 150.0);

        System.out.println("\n--- Price Update: TSLA ---");
        market.setPrice("TSLA", 520.0);
    }
}