观察者模式

1.定义
观察者模式在对象之间定义一对多的依赖，这样一来，当一个对象改变状态，依赖他的对象都会收到通知，并自动更新。
2. 案例分析
假设现有以气象系统，我们所需要完成的是当气象站数据更新时，同步更新我们的布告板（即显示装置）。

我们的需要利用WeatherData对象取得数据，并更新三个布告板：目前状况，气象统计，天气预报。

下面我们用类图定义观察者模式

3. 实现气象站
Subject接口类

package sxd.learn.dp;public interface Subject{    public void registerObserver(Observer o);    public void removeObserver(Observer o);    public void notifyObservers();}

Observer接口类

package sxd.learn.dp;public interface Observer{    public void update(float temp, float humidity, float pressure);}

DisplayElement接口类

package sxd.learn.dp;public interface DisplayElement{    public void display();}

用观察者模式实现WeatherData类

package sxd.learn.dp;import java.util.ArrayList;public class WeatherData implements Subject{    private ArrayList<Observer> observers;    private float temperture;    private float humidity;    private float pressure;    public WeatherData(){        observers = new ArrayList<Observer>();    }    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(temperture, humidity , pressure);        }    }    public void measurementsChanged(){        notifyObservers();    }    public void setMeasurements(float temperture, float humidity, float pressure){        this.temperture = temperture;        this.humidity = humidity;        this.pressure = pressure;        measurementsChanged();    }}

布告板类CurrentConditionsDisplay

package sxd.learn.dp;public class CurrentConditionsDisplay implements Observer, DisplayElement{    private float temperature;    private float humidity;    private Subject weatherData;    public CurrentConditionsDisplay(Subject weatherData){        this.weatherData = weatherData;        weatherData.registerObserver(this);    }    @Override    public void display() {        // TODO Auto-generated method stub        System.out.println("Current conditions:" + temperature                + "F degrees and " + humidity + "% humidity");    }    @Override    public void update(float temp, float humidity, float pressure) {        // TODO Auto-generated method stub        this.temperature = temp;        this.humidity = humidity;        display();    }}

测试程序类

package sxd.learn.dp;public class WeatherStation {    public static void main(String[] args){        WeatherData weatherData = new WeatherData();        CurrentConditionsDisplay currentDisplay =                 new CurrentConditionsDisplay(weatherData);//      StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);//      ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);        weatherData.setMeasurements(80, 65, 30.4f);        weatherData.setMeasurements(82, 70, 30.3f);        weatherData.setMeasurements(78, 90, 29.2f);    }}