观察者模式(observer)

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();