observer pattern
                     Observer Design 
Pattern in Java
"Define a one-to-many dependency between objects so that when one object 
changes state, all its dependents are notified and updated automatically".
You can think of observer design pattern in two ways
Subject-Observer relationship:Object which is being observed is refereed as 
Subject and classes which observe subject are called Observer 
Publisher-Subscriber relationship:A publisher is one who publish data and 
notifies it to the list of subscribers who have subscribed for the same to that 
publisher. A simple example is Newspaper. Whenever a new edition is 
published by the publisher,it will be circulated among subscribers whom have 
subscribed to publisher.
The observers will not monitor every time whether there is any change in 
state of subject or not, since they will be notified for every state change of 
subject, until they stop observing subject.
Some real life examples:
You might have surfed "Flipkart.com-Online megastore".So when you 
search for any product and it is unavailable then there is option called 
"Notify me when product is available".If you subscribe to that option 
then when state of product changes i.e. it is available,you will get 
notification mail "Product is available now you can buy it".In this 
case,Product is subject and You are an observer.
• Lets say,your permanent address is changed then you need to notify 
passport authority and pan card authority.So here passport authority 
and pan card authority are observers and You are a subject.
• On facebook also,If you subscribe someone then whenever new 
updates happen then you will be notified.
When to use it:
• When one object changes its state,then all other dependents object 
must automatically change their state to maintain consistency
• When subject doesn't know about number of observers it has.
• When an object should be able to notify other objects without 
knowing who objects are.
 UML diagram for observer design pattern:
Components:
Subject
Knows its observers
Has any number of observer
Provides an interface to attach and detaching observer object at run 
time
Observer
Provides an update interface to receive signal from subject
ConcreteSubject
Stores state of interest to ConcreteObserver objects.
Send notification to it's observer 
ConcreteObserver
Maintains reference to a ConcreteSubject object
Maintains observer state consistent with subjects.
Implements update operation
Java in-built API for observer pattern: 
• The java API provides one class and one inteface for implementing 
observer pattern.
• 1. java.util.Observable-class
• 2. java.util.Observer-interfac
java.util.Observable: 
For being observed,class must extend this class. The subclass becomes 
observable and override methods of  java.util.Observable and other objects 
can "observe" changes in state of this object.
Methods:
addObserver(Observer o) :add Observer to the list of observers for this 
subject.
deleteObserver(Observer o) :delete Observers from the list of observers .
notifyObservers() : notify all the observers if object has changed.
hasChanged() :return true if object has changed.
setChanged() :This method marks object has changed
clearChanged() :this method will indicate that subject has no changes or all 
the observers has been notified.
java.util.Observer: 
The class that performs the "observing" must implement the 
java.util.Observer interface. There is a single method: 
public void update(Observable obj, Object arg) :This method is called 
whenever the observed object is changed. An application calls an 
Observable object's notifyObservers method for notifying to all the 
observers of change.
Example: 
You might have surfed "Flipkart.com-Online megastore".So 
when you search for any product and it is unavailable then 
there is option called "Notify me when product is 
available".If you subscribe to that option then when state of 
product changes i.e. it is available,you will get notification 
mail "Product is available now you can buy it".