Posted on 2008-01-15 10:07
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DesignPattern
【Head First設計模式】-Observer模式
一、要完成的任務
此系統中的三個部分是氣象站（獲取實際氣象數據的物理裝置）、WeatherData對象（追蹤來自氣象站的數據，并更新布告板）和布告板（顯示目前天氣狀況給用戶看）。
二、Observer模式
1、定義觀察者模式
觀察者模式定義了對象之間的一對多依賴，這樣一來，當一個對象改變狀態時，它的所有依賴者都會收到通知并自動更新。
2.設計氣象站
三、代碼實現
1．定義接口
（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．實現接口
（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");
}
}
（3）StatisticsDisplay
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．實現氣象站
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);
}
}