Design Patterns - Observer

Design Patterns | Observer

References
Example

References
- Definition: (source: http://en.wikipedia.org/wiki/Observer_pattern)
  
  The Observer pattern defines an object, called the subject, that maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
- Class diagram: (source: http://en.wikipedia.org/wiki/Observer_pattern)

Example

The Observer:

public interface Observer {
    void notifyObservers(final Observable observable);
}

The Concrete Observer:

public class Client implements Observer {
    private final String id;

    private Server server;

    public Client(final String id, final Server server) {
        this.id = id;

        setServer(server);
    }

    public String getId() {
        return id;
    }

    public Server getServer() {
        return server;
    }

    public void setServer(final Server server) {
        this.server = server;

        server.add(this);
    }

    public void notifyObservers(final Observable observable) {
        System.out.println("[Client '" + id + "'] Server '" + observable.getId() + "' - State: " + getServer().getState());
    }
}

The Abstract Subject:

public abstract class Observable {
    final List<Observer> observers = new ArrayList<>();

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

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

    public void notifyObservers() {
        for (final Observer observer : observers) {
            observer.notifyObservers(this);
        }
    }

    public abstract String getId();
}

The Concrete Subject:

public class Server extends Observable {
    private final String id;

    private Boolean state;

    public Server(final String id, final Boolean state) {
        this.id = id;
        this.state = state;
    }

    public String getId() {
        return id;
    }

    public boolean getState() {
        return state;
    }

    public void setState(final boolean state) {
        this.state = state;

        notifyObservers();
    }
}

A simple class to test the Observer design pattern:

public class ObserverPatternTest {
    public static void main(String[] args) {
        final Server server1 = new Server("server1", Boolean.TRUE);

        final Client client1 = new Client("client1", server1);
        final Client client2 = new Client("client2", server1);

        // notify observers: client1, client2
        System.out.println("-- Notify observers:");
        server1.setState(Boolean.FALSE);

        // remove observer: client2
        System.out.println("-- Remove observers: client2");
        server1.remove(client2);

        // notify observers: client1
        System.out.println("-- Notify observers:");
        server1.setState(Boolean.TRUE);
    }
}