Java設計模式——觀察者模式

轉自：http://www.cnblogs.com/shamgod/p/4588557.html

 一、概述

childe類中的是關聯監聽者dad的，若要再增加監聽者，會很不方便，且要修改代碼。好的方法是封裝監聽者類，用addListener()方法動態添加監聽者

 二、代碼

1.Test.java

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class WakenUpEvent{           private long time;     private String location;     private Child source;           public WakenUpEvent(long time, String location, Child source) {         super();         this.time = time;         this.location = location;         this.source = source;     }       public long getTime() {         return time;     }       public void setTime(long time) {         this.time = time;     }       public String getLocation() {         return location;     }       public void setLocation(String location) {         this.location = location;     }       public Child getSource() {         return source;     }       public void setSource(Child source) {         this.source = source;     }             }   class Child implements Runnable {           private List<WakenUpListener> wakenUpListeners = new ArrayList<WakenUpListener>();           public void addWakenUpListener(WakenUpListener wul){         wakenUpListeners.add(wul);     }     public void wakeUp(){         for(int i = 0; i < wakenUpListeners.size(); i++){             WakenUpListener l = wakenUpListeners.get(i);             l.actionToWakenUp(new WakenUpEvent(System.currentTimeMillis(), "bed", this));         }     }       @Override     public void run() {         try {             Thread.sleep(3000);         } catch (Exception e) {             e.printStackTrace();         }         wakeUp();     } }     interface WakenUpListener {     public void actionToWakenUp(WakenUpEvent e); }   class Dad implements WakenUpListener {       public void actionToWakenUp(WakenUpEvent e) {         System.out.println("Fedd the child");     }       }   class GrandFather implements WakenUpListener {       public void actionToWakenUp(WakenUpEvent e) {         System.out.println("抱孩子");     }       }     public class Test {       public static void main(String[] args) {         Child c = new Child();         c.addWakenUpListener(new Dad());         c.addWakenUpListener(new GrandFather());         new Thread(c).start();     } }

　　

三、運行結果

轉自：http://blog.csdn.net/zhangerqing/article/details/8243942

典型應用：事件監聽器

轉自：http://www.cnblogs.com/java-my-life/archive/2012/05/16/2502279.html

在閻宏博士的《JAVA與模式》一書中開頭是這樣描述觀察者（Observer）模式的：

　　觀察者模式是對象的行爲模式，又叫發佈-訂閱(Publish/Subscribe)模式、模型-視圖(Model/View)模式、源-監聽器(Source/Listener)模式或從屬者(Dependents)模式。

　　觀察者模式定義了一種一對多的依賴關係，讓多個觀察者對象同時監聽某一個主題對象。這個主題對象在狀態上發生變化時，會通知所有觀察者對象，使它們能夠自動更新自己。

---

15、觀察者模式（Observer）

包括這個模式在內的接下來的四個模式，都是類和類之間的關係，不涉及到繼承，學的時候應該 記得歸納，記得本文最開始的那個圖。觀察者模式很好理解，類似於郵件訂閱和RSS訂閱，當我們瀏覽一些博客或wiki時，經常會看到RSS圖標，就這的意思是，當你訂閱了該文章，如果後續有更新，會及時通知你。其實，簡單來講就一句話：當一個對象變化時，其它依賴該對象的對象都會收到通知，並且隨着變化！對象之間是一種一對多的關係。先來看看關係圖：

我解釋下這些類的作用：MySubject類就是我們的主對象，Observer1和Observer2是依賴於MySubject的對象，當MySubject變化時，Observer1和Observer2必然變化。AbstractSubject類中定義着需要監控的對象列表，可以對其進行修改：增加或刪除被監控對象，且當MySubject變化時，負責通知在列表內存在的對象。我們看實現代碼：

一個Observer接口：

public interface Observer {
    public void update();
}

兩個實現類：

public class Observer1 implements Observer {

    @Override
    public void update() {
        System.out.println("observer1 has received!");
    }
}

public class Observer2 implements Observer {

    @Override
    public void update() {
        System.out.println("observer2 has received!");
    }

}

Subject接口及實現類：

public interface Subject {

    /*增加觀察者*/
    public void add(Observer observer);

    /*刪除觀察者*/
    public void del(Observer observer);

    /*通知所有的觀察者*/
    public void notifyObservers();

    /*自身的操作*/
    public void operation();
}

public abstract class AbstractSubject implements Subject {

    private Vector<Observer> vector = new Vector<Observer>();
    @Override
    public void add(Observer observer) {
        vector.add(observer);
    }

    @Override
    public void del(Observer observer) {
        vector.remove(observer);
    }

    @Override
    public void notifyObservers() {
        Enumeration<Observer> enumo = vector.elements();
        while(enumo.hasMoreElements()){
            enumo.nextElement().update();
        }
    }
}

public class MySubject extends AbstractSubject {

    @Override
    public void operation() {
        System.out.println("update self!");
        notifyObservers();
    }

}

測試類：

public class ObserverTest {

    public static void main(String[] args) {
        Subject sub = new MySubject();
        sub.add(new Observer1());
        sub.add(new Observer2());

        sub.operation();
    }

}

輸出：

update self!
observer1 has received!
observer2 has received!

 這些東西，其實不難，只是有些抽象，不太容易整體理解，建議讀者：根據關係圖，新建項目，自己寫代碼（或者參考我的代碼）,按照總體思路走一遍，這樣才能體會它的思想，理解起來容易！

天氣站廣播天氣（傳智播客）

1）問題描述：編寫一個氣象站、一個工人or學生兩個類；當氣象站更新添加的時候，要通知人做出相應的處理；

實現方法：通過事件監聽方式的實現

2）類結構：

廣播類：WeatherStation

聽衆類：Emp  &&  Student

訂閱天氣預報的接口：Weather

測試類：WeatherMain

3）代碼

WeatherStation.java

package com.dp.ood;

import java.util.ArrayList;
import java.util.Random;

/***
 * 觀察者模式：事件監聽
 * @author admin
 * 需求：編寫一個氣象站、一個工人兩個類；當氣象站更新添加的時候，
 * 要通知人做出相應的處理
 * 
 * 問題1：不能    出現 氣象站更新了多次天氣，然後人才做一次的處理
 * 
 * 問題2：不能  氣象站只能夠通知一個人
 *
 * 問題3：不能  只是服務於一種羣體
 * 
 * 觀察者設計模式的步驟：
 * 	1.當目前對象發生指定動作時，要通知另外一個對象作出相應的處理，
 * 這時候應該把對方的相應處理方法定義在接口上；
 *  2.在當前對象維護的接口的引用，噹噹前對象發生指定的動作時即可調用接口中的方法
 * 
 */
public class WeatherStation {
	String[] weathers={"Sun","Flog","Wind","Snow"};
	
	//Current weather
	String currentWeather;
	
	//人
	/*Emp e;
	public WeatherStation(Emp e){
		this.e=e;
	}*/
	ArrayList<Weather> list=new ArrayList<Weather>();
	public void addListener(Weather e){
		list.add(e);
	}
	
	//Start
	public void startWork() throws InterruptedException{
		Random random=new Random();
		
		new Thread(){
			@Override
			public void run(){
				while(true){//Update cycle: 1-1.5s
		
					updateWeather();
					
					//e.notifyWeather();
					for(Weather e:list){
						e.notifyWeather(currentWeather);
					}
					
					int s=random.nextInt(501)+1000;
					try {
						Thread.sleep(s);
					} catch (InterruptedException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
				}
			}
		}.start();
		
	}
	
	//Update weather
	public void updateWeather(){
		Random random=new Random();
		int index=random.nextInt(weathers.length);
		currentWeather=weathers[index];
		System.out.println("Current weather: "+currentWeather);
	}
	
}

Weather.java

package com.dp.ood;

//訂閱天氣預報的接口
public interface Weather {
	void notifyWeather(String weather);
}

Emp.java

package com.dp.ood;

//人根據天氣做出相應的處理
public class Emp implements Weather{
	String name;

	public Emp(String name) {
		super();
		this.name = name;
	}
	
	//weathers={"Sun","Flog","Wind","Snow"};
	public void notifyWeather(String weather){
		if(weather.equals("Sun")){
			System.out.println(name+"go for job Sunny.");
		}else if(weather.equals("Snow")){
			System.out.println(name+"go for job snowly.");
		}else{
			System.out.println(name+"go for job others.");
		}
		
	}
}

Student.java

package com.dp.ood;

public class Student implements Weather{
	String name;

	public Student(String name) {
		super();
		this.name = name;
	}
	
	//weathers={"Sun","Flog","Wind","Snow"};
	public void notifyWeather(String weather){
		if(weather.equals("Sun")){
			System.out.println(name+"go to school Sunny.");
		}else if(weather.equals("Snow")){
			System.out.println(name+"go to school snowly.");
		}else{
			System.out.println(name+"go to school others.");
		}
		
	}
}

WeatherMain.java

package com.dp.ood;

public class WeatherMain {
	public static void main(String[] args) throws InterruptedException {
		//工人
		Emp e=new Emp("Daniel");
		Emp e2=new Emp("Lili");
		
		Student s1=new Student("Ergou");
		Student s2=new Student("Sanya");
		
		WeatherStation station=new WeatherStation();
		station.addListener(e);
		station.addListener(e2);
		station.addListener(s1);
		station.addListener(s2);
		
		station.startWork();
		
	}
}

4)測試結果

Current weather: Snow
Danielgo for job snowly.
Liligo for job snowly.
Ergougo to school snowly.
Sanyago to school snowly.
Current weather: Flog
Danielgo for job others.
Liligo for job others.
Ergougo to school others.
Sanyago to school others.
Current weather: Snow
Danielgo for job snowly.
Liligo for job snowly.
Ergougo to school snowly.
Sanyago to school snowly.
Current weather: Snow
Danielgo for job snowly.
Liligo for job snowly.
Ergougo to school snowly.
Sanyago to school snowly.
Current weather: Snow
Danielgo for job snowly.
Liligo for job snowly.
Ergougo to school snowly.
Sanyago to school snowly.