Home/Design Patterns/Observer Pattern

Observer Pattern

🎯 Explain Like I'm 5...

Imagine you LOVE a YouTube channel and you want to know every time they post a new video! 📺

😟 The Problem:

• You: Want to watch new videos immediately when posted
• YouTube Channel: Posts videos at random times
• You have to keep checking the channel all day long! 😢

💡 The Solution - Subscribe with Notifications!

• Click the SUBSCRIBE button! 🔔
• Turn ON notifications
• When channel posts a video, YOU get notified automatically!
• No more checking! You're AUTOMATICALLY told when something new happens! 🎉

🌟 The Key Idea:

The channel doesn't know who you are specifically - it just notifies ALL subscribers! When something changes (new video), everyone watching (subscribed) gets updated automatically! It's a ONE-TO-MANY relationship! 🌍

🚀 Why Is This Pattern Useful?

✨Automatically notify many objects when something changes!
✨No need for objects to constantly check for updates!
✨Loose coupling - the subject doesn't need to know details about observers!

📋 Pattern Purpose

The Observer pattern defines a one-to-many dependency between objects so that when one object (Subject) changes state, all its dependents (Observers) are notified and updated automatically. It's like a subscription service!

⚡ When to Use Observer Pattern

✓When a change to one object requires changing many others
✓When an object should notify others without knowing who they are
✓When you need a publish-subscribe mechanism
✓When changes in one object need to trigger updates in multiple objects

🧩 Components

Subject (Publisher):

Knows its observers, provides interface to attach/detach observers

Observer (Subscriber):

Defines an update interface for objects that should be notified

ConcreteSubject:

Stores state, sends notifications to observers when state changes

ConcreteObserver:

Maintains reference to subject, implements update interface

💻 Java Implementations

Example 1: Weather Station

Weather station notifies all displays when weather data changes.

Observer.java

java

// Observer Interface - all observers must implement this
public interface Observer {
    void update(float temperature, float humidity, float pressure);
}

Subject.java

java

// Subject Interface - subject must implement this
public interface Subject {
    void registerObserver(Observer o);
    void removeObserver(Observer o);
    void notifyObservers();
}

WeatherStation.java

java

import java.util.ArrayList;
import java.util.List;
// Concrete Subject - the weather station that observers watch
public class WeatherStation implements Subject {
    private List<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;
    public WeatherStation() {
        observers = new ArrayList<>();
    }
    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
        System.out.println("New observer registered!");
    }
    @Override
    public void removeObserver(Observer o) {
        observers.remove(o);
        System.out.println("Observer removed!");
    }
    @Override
    public void notifyObservers() {
        // Notify ALL observers about the change
        for (Observer observer : observers) {
            observer.update(temperature, humidity, pressure);
        }
    }
    // When measurements change, notify all observers!
    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        notifyObservers(); // Automatic notification!
    }
}

CurrentConditionsDisplay.java

java

// Concrete Observer 1 - displays current conditions
public class CurrentConditionsDisplay implements Observer {
    private float temperature;
    private float humidity;
    private Subject weatherStation;
    public CurrentConditionsDisplay(Subject weatherStation) {
        this.weatherStation = weatherStation;
        weatherStation.registerObserver(this);
    }
    @Override
    public void update(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        display();
    }
    public void display() {
        System.out.println("Current conditions: " + temperature
            + "°F and " + humidity + "% humidity");
    }
}

StatisticsDisplay.java

java

// Concrete Observer 2 - displays statistics
public class StatisticsDisplay implements Observer {
    private float maxTemp = 0.0f;
    private float minTemp = 200.0f;
    private float tempSum = 0.0f;
    private int numReadings = 0;
    public StatisticsDisplay(Subject weatherStation) {
        weatherStation.registerObserver(this);
    }
    @Override
    public void update(float temperature, float humidity, float pressure) {
        tempSum += temperature;
        numReadings++;
        if (temperature > maxTemp) {
            maxTemp = temperature;
        }
        if (temperature < minTemp) {
            minTemp = temperature;
        }
        display();
    }
    public void display() {
        System.out.println("Avg/Max/Min temperature = "
            + (tempSum / numReadings) + "/" + maxTemp + "/" + minTemp);
    }
}

ForecastDisplay.java

java

// Concrete Observer 3 - displays forecast
public class ForecastDisplay implements Observer {
    private float currentPressure = 29.92f;
    private float lastPressure;
    public ForecastDisplay(Subject weatherStation) {
        weatherStation.registerObserver(this);
    }
    @Override
    public void update(float temperature, float humidity, float pressure) {
        lastPressure = currentPressure;
        currentPressure = pressure;
        display();
    }
    public void display() {
        System.out.print("Forecast: ");
        if (currentPressure > lastPressure) {
            System.out.println("Improving weather on the way!");
        } else if (currentPressure == lastPressure) {
            System.out.println("More of the same");
        } else {
            System.out.println("Watch out for cooler, rainy weather");
        }
    }
}

WeatherStationDemo.java

java

// Demo - see the Observer pattern in action!
public class WeatherStationDemo {
    public static void main(String[] args) {
        // Create the subject
        WeatherStation weatherStation = new WeatherStation();
        // Create observers and register them
        CurrentConditionsDisplay currentDisplay =
            new CurrentConditionsDisplay(weatherStation);
        StatisticsDisplay statisticsDisplay =
            new StatisticsDisplay(weatherStation);
        ForecastDisplay forecastDisplay =
            new ForecastDisplay(weatherStation);
        System.out.println("\n--- First weather update ---");
        // When weather changes, ALL displays are notified automatically!
        weatherStation.setMeasurements(80, 65, 30.4f);
        System.out.println("\n--- Second weather update ---");
        weatherStation.setMeasurements(82, 70, 29.2f);
        System.out.println("\n--- Third weather update ---");
        weatherStation.setMeasurements(78, 90, 29.2f);
    }
}
/* Output:
New observer registered!
New observer registered!
New observer registered!
--- First weather update ---
Current conditions: 80.0°F and 65.0% humidity
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Watch out for cooler, rainy weather
--- Second weather update ---
Current conditions: 82.0°F and 70.0% humidity
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Watch out for cooler, rainy weather
--- Third weather update ---
Current conditions: 78.0°F and 90.0% humidity
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same
*/

Example 2: News Agency

News agency notifies all news channels when breaking news arrives.

NewsSubscriber.java

java

// Observer Interface for news channels
public interface NewsSubscriber {
    void update(String news);
}

NewsAgency.java

java

import java.util.ArrayList;
import java.util.List;
// Subject - News Agency that publishes news
public class NewsAgency {
    private List<NewsSubscriber> subscribers;
    private String latestNews;
    public NewsAgency() {
        subscribers = new ArrayList<>();
    }
    public void subscribe(NewsSubscriber subscriber) {
        subscribers.add(subscriber);
        System.out.println("New subscriber added!");
    }
    public void unsubscribe(NewsSubscriber subscriber) {
        subscribers.remove(subscriber);
        System.out.println("Subscriber removed!");
    }
    public void publishNews(String news) {
        this.latestNews = news;
        System.out.println("\n📰 BREAKING NEWS PUBLISHED!");
        notifySubscribers();
    }
    private void notifySubscribers() {
        System.out.println("Notifying all " + subscribers.size() + " subscribers...");
        for (NewsSubscriber subscriber : subscribers) {
            subscriber.update(latestNews);
        }
    }
    public String getLatestNews() {
        return latestNews;
    }
}

TVNewsChannel.java

java

// Concrete Observer 1 - TV News Channel
public class TVNewsChannel implements NewsSubscriber {
    private String channelName;
    public TVNewsChannel(String channelName) {
        this.channelName = channelName;
    }
    @Override
    public void update(String news) {
        System.out.println("📺 [" + channelName + " TV] Broadcasting: " + news);
    }
}

RadioChannel.java

java

// Concrete Observer 2 - Radio Channel
public class RadioChannel implements NewsSubscriber {
    private String stationName;
    public RadioChannel(String stationName) {
        this.stationName = stationName;
    }
    @Override
    public void update(String news) {
        System.out.println("📻 [" + stationName + " Radio] Announcing: " + news);
    }
}

OnlineNewsPortal.java

java

// Concrete Observer 3 - Online News Portal
public class OnlineNewsPortal implements NewsSubscriber {
    private String websiteName;
    public OnlineNewsPortal(String websiteName) {
        this.websiteName = websiteName;
    }
    @Override
    public void update(String news) {
        System.out.println("💻 [" + websiteName + ".com] Publishing: " + news);
    }
}

NewsAgencyDemo.java

java

// Demo - News agency notifying all channels
public class NewsAgencyDemo {
    public static void main(String[] args) {
        // Create the news agency (subject)
        NewsAgency agency = new NewsAgency();
        // Create subscribers (observers)
        TVNewsChannel cnn = new TVNewsChannel("CNN");
        TVNewsChannel bbc = new TVNewsChannel("BBC");
        RadioChannel npr = new RadioChannel("NPR");
        OnlineNewsPortal huffpost = new OnlineNewsPortal("HuffPost");
        // Subscribe to the news agency
        agency.subscribe(cnn);
        agency.subscribe(bbc);
        agency.subscribe(npr);
        agency.subscribe(huffpost);
        // Publish news - ALL subscribers get notified!
        agency.publishNews("Scientists discover water on Mars!");
        System.out.println("\n--- BBC unsubscribes ---");
        agency.unsubscribe(bbc);
        // Next news update - only remaining subscribers notified
        agency.publishNews("Stock market hits all-time high!");
        // Add new subscriber
        System.out.println("\n--- New subscriber joins ---");
        OnlineNewsPortal buzzfeed = new OnlineNewsPortal("BuzzFeed");
        agency.subscribe(buzzfeed);
        agency.publishNews("New programming language announced!");
    }
}
/* Output:
New subscriber added!
New subscriber added!
New subscriber added!
New subscriber added!
📰 BREAKING NEWS PUBLISHED!
Notifying all 4 subscribers...
📺 [CNN TV] Broadcasting: Scientists discover water on Mars!
📺 [BBC TV] Broadcasting: Scientists discover water on Mars!
📻 [NPR Radio] Announcing: Scientists discover water on Mars!
💻 [HuffPost.com] Publishing: Scientists discover water on Mars!
--- BBC unsubscribes ---
Subscriber removed!
📰 BREAKING NEWS PUBLISHED!
Notifying all 3 subscribers...
📺 [CNN TV] Broadcasting: Stock market hits all-time high!
📻 [NPR Radio] Announcing: Stock market hits all-time high!
💻 [HuffPost.com] Publishing: Stock market hits all-time high!
--- New subscriber joins ---
New subscriber added!
📰 BREAKING NEWS PUBLISHED!
Notifying all 4 subscribers...
📺 [CNN TV] Broadcasting: New programming language announced!
📻 [NPR Radio] Announcing: New programming language announced!
💻 [HuffPost.com] Publishing: New programming language announced!
💻 [BuzzFeed.com] Publishing: New programming language announced!
*/

🌍 Real-World Examples

📺YouTube Subscriptions: Channel notifies subscribers of new videos
📈Stock Market: Stock price changes notify all watching traders
📱Social Media: Posts notify all followers
🖱️Event Listeners: UI events notify all registered listeners
📡RSS Feeds: Blog updates notify all feed subscribers

✅ Benefits

✅Loose Coupling: Subject and observers are loosely coupled
✅Dynamic Relationships: Can add/remove observers at runtime
✅Broadcast Communication: One change notifies multiple objects
✅Open/Closed Principle: Add new observers without modifying subject

⚠️ Drawbacks

⚠️Unexpected Updates: Observers notified in random order
⚠️Memory Leaks: Forgetting to unregister observers can cause leaks
⚠️Performance: Many observers can slow down notifications

🔑 Key Points to Remember

1️⃣Observer defines ONE-TO-MANY dependency between objects
2️⃣Subject maintains list of observers and notifies them of changes
3️⃣Observers subscribe/unsubscribe to subject at runtime
4️⃣Subject doesn't know concrete details about observers
5️⃣Also known as Publish-Subscribe or Pub-Sub pattern

💪 Practice Scenarios

• Create a temperature monitoring system with multiple display types
• Build a chat application where messages notify all participants
• Implement a stock trading system that updates multiple portfolio views
• Design an event system for a game (player actions notify UI, sound, etc.)
• Create a notification system for a shopping cart (updates total, tax, shipping)