平时的运用中，我们一般用到Observer pattern的实现方式主要是：自定义主题(Subject)和Observer(观察者)和JDK自己封装的java.util.Observer和java.util.Observable(类似subject)来实现的。在这里，我主要讲述自定义的Observer pattern的实现吧。
以下以气象预报站(WeatherStation)进行天气的预报为例子。气象站的需求是建立三块公布版，一块是当前气象预报(CurrentConditionDisplay)公布版、静态预报(StatisticDisplay)公布版、预报(ForeCastDisplay)公布版。其中主题的实现是气象站的数据提供(WeatherData)实现Subject(主题)，观察者为Observer和DisplayElement两个接口。其中观察者的实现有三个，分别对应三块公布版。CurrentConditionDisplay、StatisticDisplay、ForestCastDisplay,散者均需要实现Observer和DisplayElement三个接口。从而，主题的实现类WeatherData通过调用观察者的接口通知观察者及时更新，观察者通过主题接口来向主题注册，在设计的过程中，会变的只是主题的状态和观察者的数目和类型。此模式可以改变依赖主题状态的对象而不该变主题的状态。
以下是代码的实现
package com.clark.observerpattern.subject;
import com.clark.observerpattern.observer.Observer;
/**
* 气象站主题，其对应着多个Observer对象
* @author Administrator
*
*/
public interface Subject {
//注册一个Observer
public void registerObserver(Observer obj);
//remove一个Observer
public void removeObserver(Observer obj);
//当主题对象发生改变的时候，通知所有的Observer
public void notifyAllObserver();
}
package com.clark.observerpattern.observer;
/**
* defination much observer object
* @author Administrator
*
*/
public interface Observer {
//defined a update method,parameter include temp,humidity,pressure
public void update(float temp,float humidity,float pressure);
}
package com.clark.observerpattern.observer;
/**
* display 布告板的信息
* @author Administrator
*
*/
public interface DisplayElement {
void display();
}
package com.clark.observerpattern;
import com.clark.observerpattern.observer.DisplayElement;
import com.clark.observerpattern.observer.Observer;
import com.clark.observerpattern.subject.Subject;
class CurrentConditionsDisplay implements Observer, DisplayElement {
private float temp;
private float humidity;
private Subject weatherData;
@Override
public void display() {
System.out.println("current conditions:"+temp+" F degrees and "+humidity+"% humidity");
}
@Override
public void update(float temp, float humidity, float pressure) {
this.temp=temp;
this.humidity=humidity;
display();
}
public CurrentConditionsDisplay(Subject weatherData){
this.weatherData=weatherData;
//注册改Observer，从而在后面移除的时候方便关联
weatherData.registerObserver(this);
}
}
package com.clark.observerpattern.observer;
import java.util.*;
import com.clark.observerpattern.subject.Subject;
import com.clark.observerpattern.subject.WeatherData;
/**
* 统计布告板
* @author Administrator
*
*/
public class StatisticsDisplay implements Observer, DisplayElement {
private float maxTemp = 0.0f;
private float minTemp = 200;
private float tempSum= 0.0f;
private int numReadings;
private Subject weatherData;
public StatisticsDisplay(WeatherData weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this);
}
public void update(float temp, float humidity, float pressure) {
tempSum += temp;
numReadings++;
if (temp > maxTemp) {
maxTemp = temp;
}
if (temp < minTemp) {
minTemp = temp;
}
display();
}
public void display() {
System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
+ "/" + maxTemp + "/" + minTemp);
}
}
package com.clark.observerpattern.observer;
import com.clark.observerpattern.subject.Subject;
import com.clark.observerpattern.subject.WeatherData;
/**
* 天气预报布告板的现场预报板块
* @author Administrator
*
*/
public class ForecastDisplay implements DisplayElement, Observer {
private float currentPressure = 29.92f;
private float lastPressure;
private Subject weatherData;
public ForecastDisplay(WeatherData weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this);
}
public void update(float temp, float humidity, float pressure) {
lastPressure = currentPressure;
currentPressure = pressure;
display();
}
public void display() {
System.out.print("Forecast: ");
if (currentPressure > lastPressure) {
System.out.println("Improving weather on the way!");
} else if (currentPressure == lastPressure) {
System.out.println("More of the same");
} else if (currentPressure < lastPressure) {
System.out.println("Watch out for cooler, rainy weather");
}
}
}
package com.clark.observerpattern.subject;
import java.util.ArrayList;
import com.clark.observerpattern.observer.Observer;
/**
* 模拟气象站的具体主题Subject
* @author Administrator
*
*/
public class WeatherData implements Subject {
private ArrayList observers;
private float temp;
private float humidity;
private float pressure;
public WeatherData() {
observers=new ArrayList();
}
@Override
public void registerObserver(Observer obj) {
observers.add(obj);
}
@Override
public void removeObserver(Observer obj) {
int i=observers.indexOf(obj);
if(i>=0){
observers.remove(i);
}
}
@Override
public void notifyAllObserver() {
for (int i = 0; i < observers.size(); i++) {
Observer o=(Observer) observers.get(i);
o.update(temp, humidity, pressure);
}
}
//当从气象站获取更新数据的时候，我们通知观察着
public void measurementsChange(){
notifyAllObserver();
}
//读取气象站数据
public void setMeasurements(float temp,float humidity,float pressure){
this.temp=temp;
this.humidity=humidity;
this.pressure=pressure;
measurementsChange();
}
}
package com.clark.observerpattern;
import com.clark.observerpattern.observer.ForecastDisplay;
import com.clark.observerpattern.observer.StatisticsDisplay;
import com.clark.observerpattern.subject.WeatherData;
/**
* 气象站
* @author Administrator
*
*/
public class WeatherStation {
public static void main(String[] args) {
WeatherData weatherData=new WeatherData();
//先往Subject中添加所有注册的主题
CurrentConditionsDisplay currentDisplay=new CurrentConditionsDisplay(weatherData);
StatisticsDisplay statistic=new StatisticsDisplay(weatherData);
ForecastDisplay foreCast=new ForecastDisplay(weatherData);
//再调用方法从主题中获取数据，从而在布告板中及时显示
weatherData.setMeasurements(50, 80, 66);
weatherData.setMeasurements(48, 80, 67);
weatherData.setMeasurements(52, 80, 65);
}
}