设计模式（观察者模式）-蒲公英云

观察者模式=发布者+订阅者

Subject.java

public interface Subject{
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

Observer.java

public interface Observer{
   public void update(float temp,float humidity,float pressure);
}

DisplayElement.java

public interface DisplayElement{
    public void display();
}

WeatherData.java

import java.util.ArrayList;
public class WeatherData implements Subject{
     private ArrayList observers;
     private float temperature;
     private float humidity;
     private float pressure;
     public WeatherData(){
         observers = new ArrayList();
     }
     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 notifyObservers(){
        for(int i=0;i<observers.size();i++){
            Observer observer=(Observer)observers.get(i);
            observer.update(temperature,humidity,pressure);
        }
    }
    public void measurementsChanged(){
        notifyObservers();
    } 
    public void setMeasurements(float temperature,float humidity,float pressure){
         this.temperature = temperature;
         this.humidity = humidity;
         this.pressure = pressure;
         measurementsChanged();
    }
}

CurrentConditionDisplay.java

import java.io.*;
public class CurrentConditionsDisplay implements Observer ,DisplayElement{
    private float temperature;
    private float humidity;
    private WeatherData weatherData;
    public CurrentConditionsDisplay(WeatherData weatherData_){
          this.weatherData= weatherData_;
          weatherData.registerObserver(this);
    }
    public void update(float temperature,float humidity,float pressure){
        this.temperature = temperature;
        this.humidity = humidity;
        display();
    }
    public void display(){
         System.out.println("Current condition :"+temperature+" F degrees and "+ humidity + "%humidity");
    }
}

WeatherStation.java

public class WeatherStation{
     public static void main(String[] args){
         WeatherData weatherData = new WeatherData();
         CurrentConditionsDisplay currentDisplay= new CurrentConditionsDisplay(weatherData);
         weatherData.setMeasurements(80,65,30.4f);
    }
}

这种编码方式不是最好的解决方法，有其他可以改进的N种方法。比如说采用MVC混合模式来设计。而且上面设计的代码有个弊端就是，当客户端的一个发布站点要更新数据时，服务器端就需要遍历更新全部站点的数据并发布，其实只有客户端的数据一个数据变化了，其他的

仍然是原先的状态（值未变），既然值未变，服务端仍用原先值来做下更新的动作，是不是显得多此一举了呢。

下面是改进的一个方法：