EventDemo.java

// publisher-subscriber demo based on an article from http://www.25hoursaday.com/CsharpVsJava.html
import java.util.EventObject;
import java.util.Random;
import java.util.Stack;


// example event object
class EvenNumberEvent extends EventObject {
 private int number;

 public EvenNumberEvent(Object source, int number) {
 // pass the trigger to parent constructor
 super(source);
 // store number
 this.number = number;
 }

 public int getNumber() { return this.number; }
}


// subscriber/listener interface to handle ALL listeners
// alternative - base class with a ready implementation
// or interface with a 'default' method
interface EvenNumberListener {
 void evenNumberFound(EvenNumberEvent ene);
}


class Publisher {
 // collection for tracking subscribers/listeners
 Stack<EvenNumberListener> subscribers = new Stack<EvenNumberListener>();

 // event trigger
 private void OnEvenNumberFound(int num) {
 // loop through subscribers/listeners and trigger callback
 for (int i = 0; i < subscribers.size(); i++) {
 EvenNumberListener listener = (EvenNumberListener) subscribers.get(i);
 listener.evenNumberFound(new EvenNumberEvent(this, num));
 }
 }

 // subscriber/listener adder
 public void addEvenNumberEventListener(EvenNumberListener enl) {
 subscribers.add(enl);
 }

 // subscriber/listener remover
 public void removeEvenNumberEventListener(EvenNumberListener enl) {
 subscribers.remove(enl);
 }

 // test runner
 public void runNumbers() {
 // create randomizer with seed
 Random rand = new Random(System.currentTimeMillis());

 // generate 20 numbers in the range 1-20
 // and trigger event if the number is even
 for (int i = 0; i < 20; i++) {
 int current = (int) rand.nextInt() % 20;
 System.out.println("Current number is: " + current);

 if ((current % 2) == 0 && current != 0) {
 OnEvenNumberFound(current);
 }
 }
 }
}


public class EventDemo implements EvenNumberListener {
 // callback function called when even number is found
 public void evenNumberFound(EvenNumberEvent ene) {
 System.out.println("Even number found: " + ene.getNumber());
 }

 public static void main(String[] args) {
 EventDemo ed = new EventDemo();
 Publisher pub = new Publisher();

 // add subscriber/listener
 pub.addEvenNumberEventListener(ed);

 // run test
 pub.runNumbers();

 // remove subscriber/listener
 pub.removeEvenNumberEventListener(ed);
 }
}