观察者模式定义了一系列对象之间的一对多关系，当一个对象(主题，也称被观察者)改变状态，其他依赖者(观察者)都会收到通知.
1.实例描述
现有负责追踪天气状况(温度，湿度，气压)的WeatherData对象，利用该对象建立一个气象观测站应用，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，当WeatherData里面有最新的数据测量时，三种布告板必段实时更新。并要求我们能灵活的增加观察者，移除观察者。同时在布告板的块数可能增加，如增加酷热指数布告板。
2.分析思路
根据定义和描述，我们可以将WeatherDate作为主题，三块布告板作为观察者。首先创建Subjec接口，里面定义添加，移除，通知观察者功能，实现该接口的WeatherData通过ArrayList字段来模拟所有观察者，还有温度，湿度，气压字段，实现接口的方法。然后创建Observer接口，定义更新功能，由于每个布告板还有不同的展示方式，我们还需创建一个DispayElement接口，里面定义展示行为。所有的布告板都实现了Observer和DispayElement接口，并且定义了私有的Subject和相关私有字段。
WeatherData里面实现了注册和移除观察者的方法。当要增加一个布告板时，我们只需要实现Observer,DisplayElement两个接口，并且注册。
3.类图实现
主题：
观察者：
4.代码实现
Subject:主题接口
public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObservers();
}
WeatherData：主题的具体实现
import java.util.*;
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();
}
// other WeatherData methods here
public float getTemperature() {
return temperature;
}
public float getHumidity() {
return humidity;
}
public float getPressure() {
return pressure;
}
}
Observer：观察者核心接口
public interface Observer {
public void update(float temp, float humidity, float pressure);
}
** DisplayElement：**布告板展示行为的接口
public interface DisplayElement {
public void display();
}
CurrentConditionsDisplay：观察者1，即提供目前状况的布告板
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);
}
public void update(float temperature, float humidity, float pressure) {
this.temperature = temperature;
this.humidity = humidity;
display();
}
public void display() {
System.out.println("Current conditions: " + temperature
+ "F degrees and " + humidity + "% humidity");
}
}
StatisticsDisplay：观察者2，即提供气象统计的布告板
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);
}
}
**ForecastDisplay： **观察者3,即提供简单预报功能的布告板
public class ForecastDisplay implements Observer, DisplayElement {
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");
}
}
}
HeatIndexDisplay：观察者4，即新增加的提供酷热指数的布告板
public class HeatIndexDisplay implements Observer, DisplayElement {
float heatIndex = 0.0f;
private Subject weatherData;
public HeatIndexDisplay(WeatherData weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this);
}
public void update(float t, float rh, float pressure) {
heatIndex = computeHeatIndex(t, rh);
display();
}
private float computeHeatIndex(float t, float rh) {
float index = (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)));
return index;
}
public void display() {
System.out.println("Heat index is " + heatIndex);
}
}