1 /**
   2  * ObserverPattern example
   3  * NumberGenerator observer
   4  */
   5 
   6 import java.util.Vector;
   7 import java.util.Iterator;
   8 import java.util.Random;
   9 
  10 interface Observer {
  11     public abstract void update(NumberGenerator generator);
  12 }
  13 
  14 abstract class NumberGenerator {
  15     private Vector observers = new Vector();
  16     public void addObserver(Observer observer) {
  17         observers.add(observer);
  18     }
  19     public void deleteObserver(Observer observer) {
  20         observers.remove(observer);
  21     }
  22     public void notifyObservers() {
  23         Iterator it = observers.iterator();
  24         while(it.hasNext()) {
  25             Observer o = (Observer)it.next();
  26             o.update(this);
  27         }
  28     }
  29     public abstract int getNumber();
  30     public abstract void execute();
  31 }
  32 
  33 class RandomNumberGenerator extends NumberGenerator {
  34     private Random random = new Random();
  35     private int number;
  36     public int getNumber() {
  37         return number;
  38     }
  39     public void execute() {
  40         for(int i=0; i < 20; i++) {
  41             number = random.nextInt(50);
  42             notifyObservers();
  43         }
  44     }
  45 }
  46 
  47 class DigitObserver implements Observer {
  48     public void update(NumberGenerator generator) {
  49         System.out.println("DigitObserver:"+generator.getNumber());
  50         try {
  51             Thread.sleep(100);
  52         } catch(InterruptedException e) {
  53         }
  54     }
  55 }
  56 
  57 class GraphObserver implements Observer {
  58     public void update(NumberGenerator generator) {
  59         System.out.print("GraphObserver:");
  60         int count = generator.getNumber();
  61         for(int i=0; i < count; i++) {
  62             System.out.print('*');
  63         }
  64         System.out.println("");
  65         try {
  66             Thread.sleep(100);
  67         } catch(InterruptedException e) {
  68         }
  69     }
  70 }
  71 
  72 public class ObserverGenerator {
  73     public static void main(String[] args) {
  74         NumberGenerator generator = new RandomNumberGenerator();
  75         Observer observer1 = new DigitObserver();
  76         Observer observer2 = new GraphObserver();
  77         generator.addObserver(observer1);
  78         generator.addObserver(observer2);
  79         generator.execute();
  80     }
  81 }