Java: Observer Pattern

The Observer Pattern is a behavioral design pattern used in Java to establish a one-to-many dependency between objects. When one object (called the Subject) changes its state, all dependent objects (Observers) are notified and updated automatically.

When to Use Observer Pattern

• When changes to one object require notifying and updating multiple dependent objects.

• In event-driven architectures or pub-sub systems.

• To implement features like notifications, UI updates, or reactive programming.

Basic Observer Pattern Implementation

Define the Observer Interface

package com.agiledcoders.observer;

public interface Observer<T> {
    void update(T data);
}

The update method will receive the updates from the Subject.

Define the Subject Interface

package com.agiledcoders.observer;

public interface Subject<T> {
    void register(Observer<T> observer);
    void unregister(Observer<T> observer);
    void notify(T data);
}

• The register method allows one to register an Observer.

• The unregister method allows one to unregister an Observer.

• The notify method notifies the Observer(s)

Implement Subject

package com.agiledcoders.observer;

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

public class SubjectImpl implements Subject<String> {
    private final List<Observer<String>> observers = new ArrayList<>();

    @Override
    public void register(Observer<String> observer) {
        if (Objects.isNull(observer)) {
            throw new IllegalArgumentException("Observer must be non-null");
        }
        this.observers.add(observer);
    }

    @Override
    public void unregister(Observer<String> observer) {
        this.observers.remove(observer);
    }

    @Override
    public void notify(String data) {
        this.observers.forEach(stringObserver -> stringObserver.update(data));
    }
}

Implement Observer

package com.agiledcoders.observer;

public class ObserverImpl implements Observer<String> {
    private static int id;

    public ObserverImpl() {
        id = id + 1;
    }

    @Override
    public void update(String data) {
        System.out.println("----------------------------------------");
        System.out.println("Observer Id: " + id + " received update.");
        System.out.println(data);
        System.out.println("----------------------------------------");
    }
}

Create a Runner to test the pattern

package com.agiledcoders.observer;

public class Runner {
    public static void main(String[] args) {
        Observer<String> first = new ObserverImpl();
        Observer<String> second = new ObserverImpl();

        Subject<String> subject = new SubjectImpl();

        subject.register(first);
        subject.register(second);

        subject.notify("First iteration");

        subject.unregister(first);
        subject.notify("Second iteration");
    }
}

Output

----------------------------------------
Observer Id: 2 received update.
First iteration
----------------------------------------
----------------------------------------
Observer Id: 2 received update.
First iteration
----------------------------------------
----------------------------------------
Observer Id: 2 received update.
Second iteration
----------------------------------------