1 public class Guanchazhe {
 2 
 3     public static void main(String[] args) {
 4         Watched watched = new Watched();
 5         Watcher watcher = new Watcher(watched);
 6         watched.setData("start");
 7         watched.setData("run");
 8         watched.setData("stop");
 9     }
10 }
11 
12 // 被观察者
13 class Watched extends Observable {
14     private String data;
15 
16     public Watched() {
17 
18     }
19 
20     public String getData() {
21         return data;
22     }
23 
24     public void setData(String data) {
25         this.data = data;
26         setChanged();
27         this.notifyObservers();
28 
29     }
30 
31 }
32 
33 class Watcher implements Observer {
34 
35     public Watcher(Observable observable) {
36 
37         observable.addObserver(this);
38     }
39 
40     public void update(Observable o, Object arg) {
41         Watched watched = (Watched) o;
42         System.out.println(watched.getData());
43 
44     }
45 
46 }

 1 public void notifyObservers() {
 2 notifyObservers(null);
 3 }
 4 
 5 public void notifyObservers(Object arg) {
 6 /*
 7 * a temporary array buffer, used as a snapshot of the state of
 8 * current Observers.
 9 */
10 Object[] arrLocal;
11 
12 synchronized (this) {
13 
14 if (!changed)
15 return;
16 arrLocal = obs.toArray();
17 clearChanged();
18 }
19 
20 for (int i = arrLocal.length-1; i>=0; i--)
21 ((Observer)arrLocal[i]).update(this, arg);
22 }

 1 public interface Observer {
 2     /**
 3      * This method is called whenever the observed object is changed. An
 4      * application calls an <tt>Observable</tt> object‘s
 5      * <code>notifyObservers</code> method to have all the object‘s
 6      * observers notified of the change.
 7      *
 8      * @param   o     the observable object.
 9      * @param   arg   an argument passed to the <code>notifyObservers</code>
10      *                 method.
11      */
12     void update(Observable o, Object arg);
13 }

 1 public class Observable {
 2     private boolean changed = false;
 3     private Vector obs;
 4    
 5  
 6     public Observable() {
 7     obs = new Vector();
 8     }
 9 
10    
11     public synchronized void addObserver(Observer o) {
12         if (o == null)
13             throw new NullPointerException();
14     if (!obs.contains(o)) {
15         obs.addElement(o);
16     }
17     }
18 
19     
20     public synchronized void deleteObserver(Observer o) {
21         obs.removeElement(o);
22     }
23 
24    
25     public void notifyObservers() {
26     notifyObservers(null);
27     }
28 
29     
30     public void notifyObservers(Object arg) {
31     
32         Object[] arrLocal;
33 
34     synchronized (this) {
35         
36         if (!changed)
37                 return;
38             arrLocal = obs.toArray();
39             clearChanged();
40         }
41 
42         for (int i = arrLocal.length-1; i>=0; i--)
43             ((Observer)arrLocal[i]).update(this, arg);
44     }
45 
46    
47     public synchronized void deleteObservers() {
48     obs.removeAllElements();
49     }
50 
51    
52     protected synchronized void setChanged() {
53     changed = true;
54     }
55 
56    
57     protected synchronized void clearChanged() {
58     changed = false;
59     }
60 
61    
62     public synchronized boolean hasChanged() {
63     return changed;
64     }
65 
66   
67     public synchronized int countObservers() {
68     return obs.size();
69     }
70 }