Observer Design Pattern in a Nutshell

The Observer pattern, also known as the Publish-Subscriber pattern, is one of the behavioral design patterns that defines a one-to-many relationship between objects. For example, when the state of one object, Subject, changes, all its dependents, Observers, are notified and updated automatically by calling their methods.

Mainly, this pattern is used to implement distributed event handling systems. It is also a key part in the Model-View-Controller (MVC) architectural pattern.

Structure

The structure of the Observer pattern is shown below:

Figure: Observer Pattern Structure

Here, Subject has a list of Observers, e.g. ConcreteObserverA and ConcreteObserverB. Whenever there is a change in state of an object in Subject, all of the objects of the concrete implementation of the Observerclass will be updated via their update function.

Example

Here, we've presented a more practical example that will hopefully help you to understand this pattern a bit more.

The class diagram of the system is:

Figure: Observer Pattern Example - Stock Update Notification

Here, StockMarket is the subject and StockBroker is the Observer. StockBroker has two concrete implementations:StockBuyer andStockViewer. So, whenever the stockMarket object is changed, these two concrete observers get an update via their function update().

Let's look at the sequence diagram to make understanding more clear.

Figure: Sequence Diagram for Stock Market Add

Whenever a stock value is updated for a symbol, then subscribers StockBuyer  and StockViewer of the StockMarket will get updated information.

Java Implementation

Below is the Java implementation of the system described above.

Let's start by creating the interface StockBroker  as an interface for the observer.

import java.util.Map;
/**
 * Observer interface
 */
interface StockBroker {
    void update(Map<String, Double> stockList);
}

These are the concrete implementations of the abstract observer.

import java.util.Iterator;
import java.util.Map;

/**
 * Here, StockBuyer and StockViewer are concrete Observers
 */
public class StockBuyer implements StockBroker {

    public void update(Map<String, Double> stocks) {
        System.out.println("StockBuyer: stockList is changed:");
        stocks.forEach((symbol, value) -> System.out.println(symbol + " - $" + value));
    }
}
public class StockViewer implements StockBroker {

    public void update(Map<String, Double> stocks) {
        System.out.println("StockViewer: stockList is changed:");
        stocks.forEach((symbol, value) -> System.out.println(symbol + " - $" + value));
    }
}

Now, we've created a Subject class that notifies all the Observers upon the change in state.

/**
 * Subject
 */
public abstract class AbstractStockMarket {
    private List<StockBroker> stockBrokers = new ArrayList<StockBroker>();

    public void addStockBroker(StockBroker stockBroker) {
        stockBrokers.add(stockBroker);
    }

    public void notifyStockBroker(Map<String, Double> stockList) {
        for (StockBroker broker : stockBrokers) {
            broker.update(stockList);
        }
    }

    public abstract void addStock(String stockSymbol, Double price);

    public abstract void update(String stockSymbol, Double price);
}

We could have combined the two above Subject classes. However, the separation of them provides an extra layer and the extensibility on future classes.

The following code represents the concrete Subject.

/**
 * It is concrete Subject
 */
public class StockMarket extends AbstractStockMarket {

    private Map<String, Double> stockList = new HashMap<>();

    public void addStock(String stockSymbol, Double price) {
        stockList.put(stockSymbol, price);
    }

    public void update(String stockSymbol, Double price) {
        stockList.put(stockSymbol, price);
        notifyStockBroker(stockList);
    }
}

So finally, here is the application class used to implement the above classes.

/**
 * Client
 */
public class Application {
    public static void main(String[] args) {
        AbstractStockMarket market = new StockMarket();

        StockBroker buyer = new StockBuyer();
        StockBroker viewer = new StockViewer();

        market.addStockBroker(buyer);
        market.addStockBroker(viewer);

        market.addStock("ORC", 12.23);
        market.addStock("MSC", 45.78);
        System.out.println("===== Updating ORC =====");
        market.update("ORC", 12.34);
        System.out.println("===== Updating MSC =====");
        market.update("MSC", 44.68);
    }
}

The output of the program is:

===== Updating ORC =====
StockBuyer: stockList is changed:
ORC - $12.34
MSC - $45.78
StockViewer: stockList is changed:
ORC - $12.34
MSC - $45.78
===== Updating MSC =====
StockBuyer: stockList is changed:
ORC - $12.34
MSC - $44.68
StockViewer: stockList is changed:
ORC - $12.34
MSC - $44.68

You can see every subscriber gets an update notification when there is a change in the price of the stock.

Potential Issue

The Observer design pattern can cause memory leaks in the basic implementation and requires both explicit attachment and explicit detachment. This is because the subject holds strong references to the observers, keeping them alive. This is also referred to as the lapsed-listener problem. 

This issue can be eliminated by the Subject holding weak references to the Observers. 

Conclusion

This post talked about the summarized form of the Observer Design Pattern, as one of the GOF patterns, with a simple example. 

The source code for all example presented above is available on GitHub with an additional example of a banking application.

Happy coding!