How to Implement the Observer Design Pattern in Java


In software development, design patterns offer reusable solutions to common problems. One such pattern is the Observer design pattern, also known as the Publish-Subscribe pattern. This pattern allows an object, called the subject, to maintain a list of its dependents, known as observers, and notifies them of any internal changes. In this tutorial, we’ll discuss how to implement the Observer pattern in Java using a real-world example.


To follow along with this tutorial, you’ll need to have a basic understanding of Java programming and object-oriented principles.

Setting Up the Project

Before we dive into the implementation, let’s set up a new Java project. Open your preferred Java IDE and create a new project with the name “ObserverPatternExample”. Once the project is created, create a new Java class called “Subject” and another class called “Observer”.

Implementing the Subject Class

The Subject class will act as the subject in the Observer pattern. It will maintain the list of observers and provide methods to register and notify them. Here’s the initial implementation of the Subject class:

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

public class Subject {
    private List<Observer> observers = new ArrayList<>();
    private int state;

    public int getState() {
        return state;

    public void setState(int state) {
        this.state = state;

    public void attach(Observer observer) {

    public void detach(Observer observer) {

    public void notifyObservers() {
        for (Observer observer : observers) {

In the above code, we define a private list of observers and a state variable. The getState() and setState() methods enable reading and updating the state. When the setState() method is called, it updates the state and triggers the notifyObservers() method.

Implementing the Observer Class

The Observer class represents an observer in the Observer pattern. It will define an update() method that will be called by the subject whenever a change occurs. Here’s the initial implementation of the Observer class:

public interface Observer {
    void update();

We create an interface called Observer with a single method update(). This method will be implemented by any class that wants to observe changes in the subject.

Implementing the Concrete Observers

Now that we have defined the interfaces, let’s create two concrete observer classes, BinaryObserver and HexadecimalObserver, which will observe changes in the subject and print the updated state in binary and hexadecimal format, respectively. Here’s the initial implementation of these classes:

public class BinaryObserver implements Observer {
    private Subject subject;

    public BinaryObserver(Subject subject) {
        this.subject = subject;

    public void update() {
        System.out.println("Binary String: " + Integer.toBinaryString(subject.getState()));

public class HexadecimalObserver implements Observer {
    private Subject subject;

    public HexadecimalObserver(Subject subject) {
        this.subject = subject;

    public void update() {
        System.out.println("Hexadecimal String: " + Integer.toHexString(subject.getState()).toUpperCase());

In the above code, each observer class implements the Observer interface and registers itself with the subject in their constructors. The update() method prints the state of the subject in binary and hexadecimal formats, respectively.

Testing the Implementation

To test our implementation, let’s create a Main class with a main method. In the main method, we’ll create an instance of the Subject class, along with instances of the BinaryObserver and HexadecimalObserver classes. We’ll then update the state of the subject and observe the output. Here’s the implementation:

public class Main {
    public static void main(String[] args) {
        Subject subject = new Subject();
        new BinaryObserver(subject);
        new HexadecimalObserver(subject);

        System.out.println("Setting state to 15");

        System.out.println("Setting state to 10");

Upon running the Main class, you should see the following output:

Setting state to 15
Binary String: 1111
Hexadecimal String: F
Setting state to 10
Binary String: 1010
Hexadecimal String: A

As you can see, the observers correctly receive and display the updated state in their respective formats.


In this tutorial, you’ve learned how to implement the Observer design pattern in Java. The Observer pattern is a powerful tool for decoupling objects and achieving loose coupling, thus enhancing the maintainability and extensibility of your software. Try experimenting with more observers and subjects to deepen your understanding of this pattern. Happy coding!

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