Observer

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.



Observer or Publish/Subscribe in Java recognizable by behavioral methods which invokes a method on an instance of another abstract/interface type, depending on own state

• java.util.Observer/java.util.Observable (rarely used in real world though)
• All implementations of java.util.EventListener (practically all over Swing thus)
• javax.servlet.http.HttpSessionBindingListener
• javax.servlet.http.HttpSessionAttributeListener
• javax.faces.event.PhaseListener

import java.util.Observable;
import java.util.Scanner;

class EventSource extends Observable implements Runnable {
	public void run() {
		while (true) {
			String response = new Scanner(System.in).next();
			setChanged();
			notifyObservers(response);
		}
	}
}

import java.util.Observable;
import java.util.Observer;

public class MyApp {
	public static void main(String[] args) {
		System.out.println("Enter Text: ");
		EventSource eventSource = new EventSource();

		eventSource.addObserver(new Observer() {
			public void update(Observable obj, Object arg) {
				System.out.println("Received response: " + arg);
			}
		});

		new Thread(eventSource).start();
	}
}
/*
interface Observer{
	public void update(Observable obj, Object arg);
}
*/