?通俗的說：我們向報社訂閱報紙，這個就是觀察者模式的實例，我們是觀察者(Observer)，報社就是主題(Subject)。

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觀察者模式定義了對象之間的一對多依賴，這樣一來，當一個對象改變狀態時，它的所有依賴者都會收到通知并自動更新。

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?現在我們設計一個氣象監測站系統，此系統中的三個部分分別是氣象站(獲取實際氣象數據)、布告板(顯示當前的天氣狀況)

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、WeatherData(跟蹤來自氣象站的數據，并更新布告板)，布告板可以提供擴展。

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系統設計類圖如下：

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    /**
 * 主題
 * @author wengn
 *
 */
public interface Subject {
	public void registerObserver(Observer o);
	
	public void removeObserver(Observer o);
	
	public void notifyObserver();
}

/**
 * 觀察者
 * @author wengn
 *
 */
public interface Observer {
	public void update(float temp,float humidity,float pressure);
}

/**
 * 布告板接口
 * @author wengn
 *
 */
public interface DisplayElement {
	public void display();
}

/**
 * 主題實現類
 * @author Administrator
 *
 */
public class WeatherData implements Subject {
	private ArrayList observers;
	private float temperature;
	private float humidity;
	private float pressure;

	public WeatherData(){
		observers = new ArrayList();
	}
	
	public void notifyObserver() {
		for(int i=0;i<observers.size();i++){
			Observer o = (Observer)observers.get(i);
			o.update(temperature, humidity, pressure);
		}
	}

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

	public void removeObserver(Observer o) {
		int i = observers.indexOf(o);
		if(i>=0){
			observers.remove(i);
		}
	}
	
	public void measurementsChanged(){
		this.notifyObserver();
	}
	
	public void setMeasurements(float temperature,float humidity,float pressure){
		this.temperature = temperature;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}
}

/**
 * 布告板實現類
 * @author wengn
 *
 */
public class CurrentConditionsDisplay implements Observer, DisplayElement {
	private float temperature;
	private float humidity;
	private Subject weatherData;
	
	public CurrentConditionsDisplay(Subject s){
		this.weatherData = s;
		weatherData.registerObserver(this);
	}

	public void update(float temp, float humidity, float pressure) {
		this.temperature = temp;
		this.humidity = humidity;
		display();
	}

	public void display() {
		System.out.println("show:"+temperature+"||"+humidity);
	}
}

/**
 * 測試類
 * @author wengn
 *
 */
public class Test {

	public static void main(String[] args) {
		WeatherData wd = new WeatherData();
		CurrentConditionsDisplay ccd = new CurrentConditionsDisplay(wd);
		wd.setMeasurements(0, 0, 0);
		wd.setMeasurements(1, 1, 1);
		wd.setMeasurements(2, 2, 2);
	}
}

  

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? 附上java awt中觀察者模式的模擬代碼：

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    package com.hanbing.awt;

import java.util.ArrayList;
import java.util.List;

public class MyAwt {
	public static void main(String[] args) {
		Button bt = new Button();
		MyActionLisenter lisenter = new MyActionLisenter();
		MyActionLisenter2 lisenter2 = new MyActionLisenter2();
		bt.addActionLisenter(lisenter);
		bt.addActionLisenter(lisenter2);
		bt.buttonPressed();
	}
}


class Button {
	private List<ActionLisenter> list = new ArrayList<ActionLisenter>();

	public void buttonPressed() {
		ActionEvent e = new ActionEvent(System.currentTimeMillis(), this);
		for (ActionLisenter lisenter:list) {
			lisenter.actionPerformed(e);
		}
	}

	public void addActionLisenter(ActionLisenter lisenter) {
		list.add(lisenter);
	}
}


interface ActionLisenter {
	public void actionPerformed(ActionEvent e);
}


class MyActionLisenter implements ActionLisenter {
	public void actionPerformed(ActionEvent e) {
		System.out.println("oh my god!!!");
	}
}


class MyActionLisenter2 implements ActionLisenter {
	public void actionPerformed(ActionEvent e) {
		System.out.println("oh my god2!!!");
	}
}


class ActionEvent {
	private long when;

	private Object source;

	public ActionEvent(long when, Object source) {
		super();
		this.when = when;
		this.source = source;
	}

	public long getWhen() {
		return when;
	}
	
	public Object getSource(){
		return source;
	}
}
  

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設計模式之二觀察者模式