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，他的所有依赖者都会受到通知并自动更新。
1，定义事件源接口
package com.pattern.observer;
public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObserver();
}
2，定义事件源
package com.pattern.observer; import java.util.ArrayList; /**
* 事件源
*/
public class WeatherData implements Subject { private ArrayList observers;
private float temperature;
private float humidity;
private float pressure; public WeatherData() {
observers = new ArrayList();
} /**
* @see com.pattern.observer.Subject#registerObserver(com.pattern.observer.Observer)
*/
@Override
public void registerObserver(Observer o) {
observers.add(o);
} /**
* @see com.pattern.observer.Subject#removeObserver(com.pattern.observer.Observer)
*/
@Override
public void removeObserver(Observer o) {
int i = observers.indexOf(o);
if (i >= 0) {
observers.remove(i);
}
} /**
* @see com.pattern.observer.Subject#notifyObserver()
*/
@Override
public void notifyObserver() {
for(int i=0;i<observers.size();i++){
Observer observer = (Observer)observers.get(i);
observer.update(temperature, humidity, pressure);
}
} public void measurementsChanged(){
notifyObserver();
} public void setMeasurements(float temperature,float humidity,float pressure){
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
} }
3，定义观察者接口
package com.pattern.observer;
public interface Observer {
public void update(float temp,float humidity,float pressure);
}
4，定义显示接口
package com.pattern.observer;
public interface DisplayElement {
public void display();
}
5，观察者实现类
package com.pattern.observer;
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);
}
/**
* @see com.pattern.observer.DisplayElement#display()
*/
@Override
public void display() {
System.out.println("Current conditions: "+temperature +"F degrees and "+humidity+"% humidity");
}
/**
* @see com.pattern.observer.Observer#update(float, float, float)
*/
@Override
public void update(float temperature, float humidity, float pressure) {
this.temperature = temperature;
this.humidity = humidity;
display();
}
}
6,测试
package com.pattern.observer;
public class WeatherStation {
public static void main(String[] args){
WeatherData weatherData = new WeatherData();
CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
weatherData.setMeasurements(80, 65, 30.4f);
}
}
类图如下所示：