Observer pattern is behavioral pattern that is used when there is one-to-many relationship between objects such as if one object is modified, its dependent objects are to be notified automatically.

Observer pattern uses three actor classes – Subject, Observer and Client. Subject is an object having methods to attach and detach observers to a client object. Observer is an abstract class, its implementations are attached to Subject while creation for further notification. See scheme below :

Step 1 : Create Subject class

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;
        notifyAllObservers();
    }

    public void attach(Observer observer) {
        observers.add(observer);
    }

    public void remove(Observer observer) {
        observers.remove(observer);
    }

    public void notifyAllObservers() {
        for (Observer observer : observers) {
            observer.update();
        }
    }
}

Step 2 : Create Observer class

public abstract class Observer {
    protected Subject subject;

    public abstract void update();
}

Step 3 : Create concrete observer classes

public class BinaryObserver extends Observer {

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

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

public class OctalObserver extends Observer {

    public OctalObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

    @Override
    public void update() {
        System.out.println( "Octal String: " + Integer.toOctalString( subject.getState() ) );
    }
}

public class HexaObserver extends Observer {

    public HexaObserver(Subject subject) {
        this.subject = subject;
        this.subject.attach(this);
    }

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

Step 4 : Use Subject and concrete observer classes

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

        Observer binaryObserver = new BinaryObserver(subject);
        new OctalObserver(subject);
        new HexaObserver(subject);

        System.out.println("First state range: 15");
        subject.setState(15);

        subject.remove(binaryObserver);
        System.out.println("Second state range: 10");
        subject.setState(10);
    }
}

The output will be :

First state change: 15
Binary String: 1111
Octal String: 17
Hex String: F
Second state change: 10
Octal String: 12
Hex String: A