文章目录
一、问题描述
Weather-O-Rama气象站计划建立下一代的Internet气象观察站，该气象站必须建立在WeatherData对象的基础上，WeatherData对象提供天气数据，有三种布告板，分别显示目前的状况、气象统计及简单的预报。并且以后可以方便地增加布告板进行扩展。
使用观察者模式进行设计，WeatherData对象即观察者模式中的主题对象，三个布告板即观察者。
使用java内置的对观察者模式的支持实现该系统，java.util.Observable为抽象主题（可观察者），java.util.Observer接口为抽象观察者。
二、类图
三、实现代码
1.具体主题WeatherData
public class WeatherData extends Observable {
private float temperature;
private float humidity;
private float pressure;
public WeatherData() { }
public void measurementsChanged() {
setChanged();
notifyObservers();
}
public void setMeasurements(float temperature, float humidity, float pressure) {
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}
public float getTemperature() {
return temperature;
}
public float getHumidity() {
return humidity;
}
public float getPressure() {
return pressure;
}
}
2.辅助接口DisplayElement
public interface DisplayElement {
public void display();
}
3.具体观察者：各种布告板
(1)布告板CurrentConditionsDisplay：显示当前温度和湿度
import java.util.Observable;
import java.util.Observer;
public class CurrentConditionsDisplay implements Observer, DisplayElement {
Observable observable;//维护一个Observable类型的引用
private float temperature;
private float humidity;
public CurrentConditionsDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}
public void update(Observable obs, Object arg) {
if (obs instanceof WeatherData) {
WeatherData weatherData = (WeatherData)obs;
//拉数据
this.temperature = weatherData.getTemperature();
this.humidity = weatherData.getHumidity();
display();
}
}
public void display() {
System.out.println("Current conditions: " + temperature
+ "F degrees and " + humidity + "% humidity");
}
}
(2)布告板StatisticsDisplay：显示温度的最大、最小及平均值。
public class StatisticsDisplay implements Observer, DisplayElement {
Observable observable;//维护一个Observable类型的引用
private float maxTemp = 0.0f;
private float minTemp = 200;
private float tempSum= 0.0f;
private int numReadings;
public StatisticsDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}
public void update(Observable observable, Object arg) {
if (observable instanceof WeatherData) {
WeatherData weatherData = (WeatherData)observable;
//拉数据
float temp = weatherData.getTemperature();
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);
}
}
(3)布告板ForecastDisplay：天气预报
public class ForecastDisplay implements Observer, DisplayElement {
Observable observable;//维护一个Observable类型的引用
private float currentPressure = 29.92f;
private float lastPressure;
public ForecastDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}
public void update(Observable observable, Object arg) {
if (observable instanceof WeatherData) {
WeatherData weatherData = (WeatherData)observable;
lastPressure = currentPressure;
//拉数据
currentPressure = weatherData.getPressure();
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");
}
}
}
(4)增加“酷热指数”布告板
public class HeatIndexDisplay implements Observer, DisplayElement {
Observable observable;//维护一个Observable类型的引用
float heatIndex = 0.0f;
public HeatIndexDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}
public void update(Observable observable, Object arg) {
if (observable instanceof WeatherData) {
WeatherData weatherData = (WeatherData)observable;
float t = weatherData.getTemperature();
float rh = weatherData.getHumidity();
heatIndex = (float)
(
(16.923 + (0.185212 * t)) +
(5.37941 * rh) -
(0.100254 * t * rh) +
(0.00941695 * (t * t)) +
(0.00728898 * (rh * rh)) +
(0.000345372 * (t * t * rh)) -
(0.000814971 * (t * rh * rh)) +
(0.0000102102 * (t * t * rh * rh)) -
(0.000038646 * (t * t * t)) +
(0.0000291583 * (rh * rh * rh)) +
(0.00000142721 * (t * t * t * rh)) +
(0.000000197483 * (t * rh * rh * rh)) -
(0.0000000218429 * (t * t * t * rh * rh)) +
(0.000000000843296 * (t * t * rh * rh * rh)) -
(0.0000000000481975 * (t * t * t * rh * rh * rh)));
display();
}
}
public void display() {
System.out.println("Heat index is " + heatIndex);
}
}
4.测试
public class WeatherStationHeatIndex {
public static void main(String[] args) {
WeatherData weatherData = new WeatherData();
CurrentConditionsDisplay currentConditions = new CurrentConditionsDisplay(weatherData);
StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);
HeatIndexDisplay heatIndexDisplay = new HeatIndexDisplay(weatherData);
weatherData.setMeasurements(80, 65, 30.4f);
weatherData.setMeasurements(82, 70, 29.2f);
weatherData.setMeasurements(78, 90, 29.2f);
}
}
四、说明
1.各个具体观察者（即布告板）实现了Observer接口的update(Observable observable, Object arg)方法，该方法支持推拉数据。
Observable类中的方法notifyObservers()就是用“拉”数据的方式进行主题和观察者数据的传递。
public void notifyObservers() {
notifyObservers(null);
}
Observable类中的方法notifyObservers(Object arg)在参数arg不为null时就是“推”数据，arg就是要“推”的数据（状态）。
2.具体主题WeatherData中使用到了超类Observable中的方法setChanged()，其作用如下：
抽象超类Observable中有一个属性changed属性用来表示状态，初始值为false，只有changed为true时，主题才会通知各个观察者，代码如下：
protected synchronized void setChanged() {
changed = true;
}
public void notifyObservers(Object arg) {
Object[] arrLocal;
synchronized (this) {
if (!changed)//changed为false时直接返回
return;
arrLocal = obs.toArray();//获得所有已注册的观察者
clearChanged();//将changed属性设为false
}
for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}
setChanged()方法可以让我们在更新观察者时具有更多的弹性，我们可以适当地通知观察者。比如，如果不使用setChanged()方法，我们的气象站测量是如此敏锐，以至于温度计读数每十分之一度就会更新，这造成WeatherData对象持续不断地通知观察者，我们并不希望看到这样的事情发生。我们希望半度以上才更新，就可以在温度差距达到半度时，调用setChanged()方法，进行有效地更新。
3.以上代码中，具体观察者（即布告板）中都持有一个Observable类型的引用，在上面的需求中该引用并没有用到，但是以后可能会用得到，比如以后可能让观察者自行取消对主题的注册，或者调用主题的其它方法时，这个引用就有用处了，所以我们要应付变化的需求
4.因为java是单继承的，这里的Observable是个抽象类，并且它的方法有些是protected的，会导致产生一些弊端。
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