Observer Design pattern

Observer Pattern is one of the behavioral Design pattern. Observer design pattern is useful when you are interested in the state of an object and want to get notified whenever there is any change. In observer pattern, the object that watch on the state of another object are called Observer and the object that is being watched is called Subject.

Intent:
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Subject contains a list of observers to notify of any change in it’s state, so it should provide methods using which observers can register and unregistered themselves. Subject also contain a method to notify all the observers of any change and either it can send the update while notifying the observer or it can provide another method to get the update.

Observer should have a method to set the object to watch and another method that will be used by Subject to notify them of any updates.

Java Message Service (JMS) uses Observer design pattern along with Mediator Pattern to allow applications to subscribe and publish data to other applications.

Java provides inbuilt java.util.observer interface and java.util.obserable class but its not widely used. Because most of the times we don't wanna end up extending a class just for implementing observer pattern, since java don't support multiple inheritances in class. 

Java.util.Observer :

Public Interface Observer:

Any class who implements this interface must be notified when subject or observable object change its status.

Update (Observable Ob, Object arg): This method is called when subject is changed.

Class Observable:

It’s a subject to whom observer wants to observe.

Some Important Method:

addObserver(Observer o):add Observers in the set of observers for this subject or observable object.

deleteObserver(Observer o): delete Observers in the set of observers .

hasChanged():check if object has changed.

clearChanged():this method will indicate that subject has no changes or all the observers has been notified when changes is made.

notifyObservers(): notify all the observers if object has changed .

Example: 

import java.util.ArrayList;

interface Observer {

       public void update(float 

}

interface Subject {

       public void registerObserver(Observer observer);

       public void removeObserver(Observer observer);

       public void notifyObservers();

}

class Loan implements Subject {

       private ArrayList<Observer> observers = new ArrayList<Observer>();

       private String type;

       private float interest;

       private String bank;

       public Loan(String type, float interest, String bank) {

              this.type = type;

              this.interest = interest;

              this.bank = bank;

       }

       public float getInterest() {

              return interest;

       }

       public void setInterest(float interest) {

              this.interest = interest;

              notifyObservers();

       }

       public String getBank() {

              return this.bank;

       }

       public String getType() {

              return this.type;

       }

       @Override

       public void registerObserver(Observer observer) {

              observers.add(observer);

       }

       @Override

       public void removeObserver(Observer observer) {

              observers.remove(observer);

       }

       @Override

       public void notifyObservers() {

              for (Observer ob : observers) {

                     System.out.println("Notifying Observers on change in Loan interest rate");

                     ob.update(this.interest);

              }

       }

}

class Newspaper implements Observer {

       @Override

       public void update(float interest) {

              System.out.println("Newspaper: Interest Rate updated, new Rate is: "

                           + interest);

       }

}

class Internet implements Observer {

       @Override

       public void update(float interest) {

              System.out.println("Internet: Interest Rate updated, new Rate is: "

                           + interest);

       }

}

public class ObserverTest {

       public static void main(String args[]) {

              // this will maintain all loans information

              Newspaper printMedia = new Newspaper();

              Internet onlineMedia = new Internet();

              Loan personalLoan = new Loan("Personal Loan", 12.5f,

                           "Standard Charterd");

              personalLoan.registerObserver(printMedia);

              personalLoan.registerObserver(onlineMedia);

              personalLoan.setInterest(3.5f);

       }

}