Observer pattern in Java

A classic real-life example of the Event Listener (or Observer) pattern in Java is a Stock Market Notifier.

The Producer is the Stock Exchange (the stock price), and the Subscribers are the individual investors who want to be notified when the price changes.

Real-Life Example: Stock Price Notifier 📈

Imagine a system where traders need immediate alerts when a specific stock hits a target price.

• Producer (The Subject): The StockExchange class. It holds the current price and a list of all traders watching that price.
• Subscriber (The Observer): The Trader interface/class. It implements the method that gets called when the price changes.

Java Code Implementation

1. The Listener Interface (The Contract)

First, we define the interface that all Subscribers must implement.

// 1. The Listener Interface (The contract for all Observers)
public interface PriceChangeListener {
    void priceChanged(String stockSymbol, double newPrice);
}

2. The Subscriber (The Trader)

A Trader implements the listener interface and defines their reaction to the event.

// 2. The Concrete Subscriber (The Observer/Listener)
public class Trader implements PriceChangeListener {
    private String name;
    private double targetPrice;

    public Trader(String name, double targetPrice) {
        this.name = name;
        this.targetPrice = targetPrice;
    }

    // This method is invoked by the Producer when the event occurs
    @Override
    public void priceChanged(String stockSymbol, double newPrice) {
        System.out.println(name + " received alert: " + stockSymbol + " is now $" + newPrice);
        
        if (newPrice >= targetPrice) {
            System.out.println("  >> " + name + " ALERT: Price hit your target of $" + targetPrice + "!");
        }
    }
}

3. The Producer (The Subject)

The StockExchange holds the list of subscribers and manages the notification logic.

import java.util.ArrayList;
import java.util.List;

// 3. The Producer (The Subject/Observable)
public class StockExchange {
    
    // Holds the list of registered Subscribers
    private List<PriceChangeListener> subscribers = new ArrayList<>();
    private double currentPrice;
    private String symbol;

    public StockExchange(String symbol, double initialPrice) {
        this.symbol = symbol;
        this.currentPrice = initialPrice;
    }

    // Method for Subscribers to register
    public void addListener(PriceChangeListener listener) {
        subscribers.add(listener);
        System.out.println("-> New listener registered.");
    }

    // The core event-generation method
    public void updatePrice(double newPrice) {
        if (newPrice != currentPrice) {
            this.currentPrice = newPrice;
            System.out.println("\n*** EVENT: Price of " + symbol + " changed to $" + newPrice + " ***");
            
            // Invoke methods of ALL registered Subscribers (The Notification)
            notifyListeners();
        }
    }

    // Iterates through the subscribers and calls the notification method
    private void notifyListeners() {
        for (PriceChangeListener listener : subscribers) {
            listener.priceChanged(symbol, currentPrice);
        }
    }
}

4. Running the Example

public class ObserverDemo {
    public static void main(String[] args) {
        
        StockExchange aaplStock = new StockExchange("AAPL", 150.00);
        
        // 1. Multiple Subscribers register with the Producer
        Trader investorA = new Trader("Investor Alice", 155.00);
        Trader investorB = new Trader("Investor Bob", 160.00);

        aaplStock.addListener(investorA);
        aaplStock.addListener(investorB);
        
        // 2. Event 1: Price changes, but is below targets
        aaplStock.updatePrice(152.50);

        // 3. Event 2: Price changes, hitting Alice's target
        aaplStock.updatePrice(155.00);
        
        // 4. Event 3: Price changes, hitting Bob's target
        aaplStock.updatePrice(161.00);
    }
}

Output

-> New listener registered.
-> New listener registered.

*** EVENT: Price of AAPL changed to $152.5 ***
Investor Alice received alert: AAPL is now $152.5
Investor Bob received alert: AAPL is now $152.5

*** EVENT: Price of AAPL changed to $155.0 ***
Investor Alice received alert: AAPL is now $155.0
  >> Investor Alice ALERT: Price hit your target of $155.0!
Investor Bob received alert: AAPL is now $155.0

*** EVENT: Price of AAPL changed to $161.0 ***
Investor Alice received alert: AAPL is now $161.0
  >> Investor Alice ALERT: Price hit your target of $155.0!
Investor Bob received alert: AAPL is now $161.0
  >> Investor Bob ALERT: Price hit your target of $160.0!