Observer Pattern:

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Frequency of use: high

Participants：

1.Subject

2.Observer

3.ConcreteSubject

4.ConcreteObserver

 

（1）Subject

import java.util.*;public abstract class Subject{	protected ArrayList observers = new ArrayList();	public abstract void attach(Observer observer);	public abstract void detach(Observer observer);	public abstract void notify();}

（2）Observer

public interface Observer{	public void update();}

（3）ConcreteSubject

public class ConcreteSubject extends Subject{	public void attach(Observer observer)	{		observers.add(observer);	}		public void detach(Observer observer)	{		observers.remove(observer);	}		public void notify()	{		for(Object obs:observers)		{			((Observer)obs).update();		}	}	}

 

（4）ConcreteObserver

public class ConcreteObserver implements Observer{	public void update()	{		//具体更新代码	}}

（5）Test

Subject subject = new ConcreteSubject();Observer observer = new ConcreteObserver();subject.attach(observer);subject.notify();