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，它的所有依赖都会收到通知并自动更新。
类图：
设计气象站：
代码：
三个接口：Subject、Observer、DisplayElement
/**
* Created by zyzhmily on 2018/5/20.
* 主题
*/
public interface Subject {
public void registerObserver(Observer observer);
public void removeObserver(Observer observer);
public void notifyObserver();
}
/**
* Created by zyzhmily on 2018/5/20.
*/
public interface Observer {
public void update(float temperature,float humidity,float pressure);
}
/**
* Created by zyzhmily on 2018/5/20.
*/
public interface DisplayElement {
public void display();
}
WeatherData：
/**
* Created by zyzhmily on 2018/5/20.
*/
public class WeatherData implements Subject {
private ArrayList observers;
private float temperature;
private float humidity;
private float pressure;
public WeatherData(){
observers=new ArrayList();
}
@Override
public void registerObserver(Observer observer) {
observers.add(observer);
}
@Override
public void removeObserver(Observer observer) {
int i=observers.indexOf(observer);
if (i>=0){
observers.remove(i);
}
}
@Override
public void notifyObserver() {
for (int i=0;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();
}
}
CurrentConditionsDisplay：
/**
* Created by zyzhmily on 2018/5/20.
*/
public class CurrentConditionsDisplay implements Observer,DisplayElement{
private float temperature;
private float humidity;
private Subject weatherData;
public CurrentConditionsDisplay(Subject subject) {
this.weatherData=subject;
weatherData.registerObserver(this);
}
@Override
public void update(float temperature, float humidity, float pressure) {
this.temperature=temperature;
this.humidity=humidity;
display();
}
@Override
public void display() {
System.out.println("Current conditions: "+temperature+" F degree and "+humidity+" % humidity");
}
}
测试类：
/**
* Created by zyzhmily on 2018/5/19.
*/
public class PatternObserver {
public static void main(String[] args){
WeatherData weatherData=new WeatherData();
CurrentConditionsDisplay currentConditionsDisplay=new CurrentConditionsDisplay(weatherData);
weatherData.setMeasurements(80,65,30.4f);
}
}
本文来自 《Head First 设计模式》