Observer Example

[Nikhil Vasaikar]

Hi,

I am posting an example of Observer Pattern. Please feel free to comment. This might also help people who are looking for Java Examples for Patterns.

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

 
import java.util.*;
 
abstract class AbstractSubject {
 
private Vector observers;
private String name;
private double price;

AbstractSubject(String name){
observers = new Vector();
}

public void attach (AbstractObserver obs){
observers.add(obs);
}

public void remove (AbstractObserver obs)
{
observers.remove(obs);
}
 
public void notifyObservers(){
int noOfObservers = observers.size();
for(int i = 0; i < noOfObservers; i++){
((AbstractObserver)observers.get(i)).update(this);
}
}
 
public void setStockPrice(double price){
this.price = price;
notifyObservers();
}
 
public double getStockPrice(){
return this.price;
}
}
 
public class ConcreteSubject extends AbstractSubject{
 
ConcreteSubject(String name){
super(name);
}
}
 
interface AbstractObserver {
 
public void update(AbstractSubject subject);
}
 
class ConcreteObserver implements AbstractObserver{
 
private String name;

ConcreteObserver(String name){
this.name = name;
}

public void update(AbstractSubject subject){

System.out.println("Notifying Observer " + name + "....");
System.out.println("Price - " + subject.getStockPrice());
}
 
}
 
public class ObserverDemo {
 
public static void main(String[] args) {
ConcreteObserver obs1 = new ConcreteObserver("Jack"); 
ConcreteObserver obs2 = new ConcreteObserver("Smith");

ConcreteSubject sub = new ConcreteSubject("Sun"); 
sub.setStockPrice(125.00);
sub.attach(obs1);
sub.attach(obs2);
sub.setStockPrice(150.00);

}
}

The Output
Notifying Observer Jack....
Price - 150.0
Notifying Observer Smith....
Price - 150.0

Regards,
Nikhil

[Vladas Razas]

There is standard implementation of this pattern in JDK:

java.util.Observable
java.util.Observer

[Stan James]

Another example ... for my own use I made an abstract publisher class. It provides methods to add and remove subscribers based on string keys but no publish method. I make an interface for each message type that is to be published. The concrete publisher implements the interface, as do the subscribers. So a concrete publisher has something like this for each method defined in the interface:

public void someMethod(some args) 
{
   get subscribers for "key"
   for each subscriber
      cast subscriber to the interface
      call someMethod( some args )
   next
}

This is more hand coding than you'll find using Observable and Observer - usually almost exactly the 6 lines you see above - but has some neat advantages. I can publish any method name with any arguments. One publisher can have many methods. I can put logic in the publisher to do things like accumulate responses from subscribers for voting or polling, stop after any subscriber returns a success or failure flag, return values from the publish method, etc.

Another interesting variation has a queue per subscriber and puts message objects into the queue instead of calling the subscriber directly. That lets the publisher and subscriber work asynchronously and works even if the subscriber doesn't exist at the moment.

There is lots of fun to be had in pub-sub for sure.