PUBLISHER & SUBSCRIBER PATTERN

Using the UML Drawing Tool by implementing the code in Java demonstrate the Observer 

Design Pattern.

The Publisher-Subscriber design pattern helps to keep the state of cooperating components synchronized. To achieve this it enables one-way propagation of changes: one publisher notifies any number of subscribers about changes to its state.

Structure:

Participant Classes:

Subject: Keeps track of its observers. Provides an interface for attaching and detaching Observer objects

Observer: Defines an interface for update notification

ConcreteSubject: The object being observed. stores state of interest to ConcreteObserver objects. Sends a notification to its observers when its state changes

ConcreteObserver: The observing object. Stores state that should stay consistent with the subject's. Implements the Observer update interface to keep its state consistent with the ConcreteSubject

Problem Statement:

A situation often arises in which data changes in one place, but many other components depend on this data. The classical example is user interface elements:  when some internal data element changes all views that depend on this data have to be updated. We could solve the problem by  introducing direct calling dependencies  along which  to propagate  the  changes,  but  this  solution  is  inflexible  and  not reusable. We are looking for a more general change-propagation mechanism that is applicable in many contexts.

Solution:

One dedicated component takes the role of the publisher (called subject). All components dependent on changes in the publisher are its subscribers (called observers).

Source code

import java.util.Observable;

import java.util.Observer;         

import java.io.*;

class Publisher extends Observable 

{

private String name;

public Publisher(String name)

{

                        this.name=name;

                        System.out.println("Subject is Created with the name "+name);

}

public void setobserver()

    {

           BufferedReader br = new BufferedReader(new InputStreamReader( System.in));

            System.out.print("Enter new name> ");

            try

            {

                        name = br.readLine();

                        setChanged();

                        notifyObservers(name);

            }

            catch(Exception e)

            {

            }       

    }

}

class Subscriber1 implements Observer

{

    private String resp;

 public void update (Observable obj, Object arg)

    {

        if (arg instanceof String)

        {

            resp = (String) arg;

           System.out.println("\nReceived Response in Subscriber1 and name changed to "+resp);

        }

    }

}

class Subscriber2 implements Observer

{

    private String resp;

    public void update (Observable obj, Object arg)

    {

        if (arg instanceof String)

        {

            resp = (String) arg;

            System.out.println("\nReceived Response in Subscriber2 and name changed to "+resp);

        }

    }

}

public class Publisher_Subscriber

{

    public static void main(String args[])

    {

        Publisher pub = new Publisher("Summer");

        Subscriber1 sub1 = new  Subscriber1(); 

        Subscriber2 sub2 = new  Subscriber2();

        pub.addObserver(sub1);

        pub.addObserver(sub2);

        pub.setobserver();

    }

}

Output

Subject is Created with the name Summer

Enter new name> Winter

Received Response in Subscriber2 and name changed to Winter

Received Response in Subscriber1 and name changed to Winter