《Head First设计模式》学习-Observer模式

一、要完成的任务

此系统中的三个部分是气象站（获取实际气象数据的物理装置）、WeatherData对象（追踪来自气象站的数据，并更新布告板）和布告板（显示目前天气状况给用户看）。

二、Observer模式

1、定义观察者模式

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。

2.设计气象站

三、代码实现

1．定义接口

（1）Subject接口

Subject.java

package  com.sterning.ch2_observer;

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

（2）Observer接口

Observer.java

package  com.sterning.ch2_observer;

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

（3）Displayment接口

Displayment.java

package  com.sterning.ch2_observer;

public   interface  Displayment  {
    public void display();
}

2．实现接口

（1）WeatherData

WeatherData.java

package  com.sterning.ch2_observer;

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 notifyObservers() {
        for(int i=0;i<observers.size();i++){
            Observer observer=(Observer)observers.get(i);
            observer.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(){
        notifyObservers();
    }
    public void setMeasurements(float temperature,float humidity,float pressure){
        this.temperature=temperature;
        this.humidity=humidity;
        this.pressure=pressure;
        measurementsChanged();
    }
}

（2）CurrentConditionsDisplay

CurrentConditionsDisplay.java

package  com.sterning.ch2_observer;

public   class  CurrentConditionsDisplay  implements  Observer, Displayment  {
    private float temperature;
    private float humidity;
    private Subject weatherData;
    
    public CurrentConditionsDisplay(Subject weatherData){
        this.weatherData=weatherData;
        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("Current conditions:"+temperature+"F degrees and "+humidity+"% humidity");
    }

}

StatisticsDisplay.java

package  com.sterning.ch2_observer;

import  java.util. * ;

public   class  StatisticsDisplay  implements  Observer, Displayment  {
    private float maxTemp = 0.0f;
    private float minTemp = 200;
    private float tempSum= 0.0f;
    private int numReadings;
    private WeatherData 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);
    }
}

（4）ForecastDisplay

ForecastDisplay.java

package  com.sterning.ch2_observer;

import  java.util. * ;

public   class  ForecastDisplay  implements  Observer, Displayment  {
    private float currentPressure = 29.92f;  
    private float lastPressure;
    private WeatherData 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");
        }
    }
}

3．实现气象站

WeatherStation.java

package  com.sterning.ch2_observer;

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, 29.2f);
        weatherData.setMeasurements(78, 90, 29.2f);        
        
    }
}