观察者模式

当对象间存在一对多关系时，则使用观察者模式（Observer Pattern）。比如，当一个对象被修改时，则会自动通知它的依赖对象。观察者模式属于行为型模式。

示例如下：

1. 创建 Subject 类

    import java.util.ArrayList;
    import java.util.List;
        
    public class 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 attach(Observer observer){
          observers.add(observer);      
       }
        
       public void notifyAllObservers(){
          for (Observer observer : observers) {
             observer.update();
          }
       }  
    }
   

2. 创建 Observer 类。

    public abstract class Observer {
       protected Subject subject;
       public abstract void update();
    }
   

3. 创建实体观察者类。

    public class BinaryObserver extends Observer{
        
       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 extends Observer{
    
       public OctalObserver(Subject subject){
          this.subject = subject;
          this.subject.attach(this);
       }
        
       @Override
       public void update() {
         System.out.println( "Octal String: " 
         + Integer.toOctalString( subject.getState() ) ); 
       }
    }
   

    public class HexaObserver extends Observer{
    
       public HexaObserver(Subject subject){
          this.subject = subject;
          this.subject.attach(this);
       }
        
       @Override
       public void update() {
          System.out.println( "Hex String: " 
          + Integer.toHexString( subject.getState() ).toUpperCase() ); 
       }
    }
   

4. 使用 Subject 和实体观察者对象

    public class ObserverPatternDemo {
       public static void main(String[] args) {
          Subject subject = new Subject();
        
          new HexaObserver(subject);
          new OctalObserver(subject);
          new BinaryObserver(subject);
        
          System.out.println("First state change: 15");   
          subject.setState(15);
          System.out.println("Second state change: 10");  
          subject.setState(10);
       }
    }