观察者模式（Observer Pattern）

观察者模式：定义了对象之间的一对多依赖，这样依赖，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。

下面举一个具体例子：

两个观察者，BinaryObserver的update将setState传入的整数转换成二进制，OctaObserver则是转成八进制

interface Subject {
    
   private List<Observer> observers;

    public void registerObserver(Observer o);

    public void removeObserver(Observer o);  

   public void notifyAllObservers();
}

public class ConcreteSubject implements Subject{
    
   private List<Observer> observers 
      = new ArrayList<Observer>();
   private int state;

   public int getState() {
      return state;
   }

   public void setState(int state) {
      this.state = state;
      notifyAllObservers();
   }

   public void registerObserver(Observer observer){
      observers.add(observer);        
   }

    public void removeObserver(Observer observer){
       int i = observers.indexOf(observer);
       if(i >= 0)
              observers.remove(i);
    }

   public void notifyAllObservers(){
      for (Observer observer : observers) {
         observer.update();
      }
   }     
}

abstract Observer {
   public abstract void update();
}

public class BinaryObserver implements Observer{
　 Subject subject;
   public BinaryObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
      System.out.println( "Binary String: " 
      + Integer.toBinaryString( subject.getState() ) ); 
   }
}

public class OctalObserver implements Observer{
   Subject subject;
   public OctalObserver(Subject subject){
      this.subject = subject;
      this.subject.attach(this);
   }

   @Override
   public void update() {
     System.out.println( "Octal String: " 
     + Integer.toOctalString( subject.getState() ) ); 
   }
}

测试代码：

Subject subject = new ConcreteSubject();
Observer o = new OctalObserver(subject);
Observer b = new BinaryObserver(subject);

subject.registerObserver(o);
subject.registerObserver(b);
subject.setState(15);
subject.setState(10);

在Swing中有使用到观察者模式，比如JButton的addActionListener方法。