设计模式之二观察者模式
?通俗的说：我们向报社订阅报纸，这个就是观察者模式的实例，我们是观察者(Observer)，报社就是主题(Subject)。
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观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。
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?现在我们设计一个气象监测站系统，此系统中的三个部分分别是气象站(获取实际气象数据)、布告板(显示当前的天气状况)
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、WeatherData(跟踪来自气象站的数据，并更新布告板)，布告板可以提供扩展。
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系统设计类图如下：
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/** * 主题 * @author wengn * */public interface Subject {public void registerObserver(Observer o);public void removeObserver(Observer o);public void notifyObserver();}/** * 观察者 * @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();}/** * 主题实现类 * @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();}}/** * 布告板实现类 * @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);}}/** * 测试类 * @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);}}?
? 附上java awt中观察者模式的模拟代码：
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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;}}?