观察者模式定义：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，它的所有依赖者都会收到通知并自动更新。

简单定义：让对象能够在状态改变时被通知。

OO原则：

为了交互对象之间的松耦合设计而努力。

以书中的WeatherStation为例：

Subject.java（定义接口）

package headfirst.observer.weather;

public interface Subject {
	public void registerObserver(Observer o);
	public void removeObserver(Observer o);
	public void notifyObservers();
}

Observer.java

package headfirst.observer.weather;

public interface Observer {
	public void update(float temp, float humidity, float pressure);
}

DisplayElement.java

package headfirst.observer.weather;

public interface DisplayElement {
	public void display();
}

WeatherData.java（天气数据类，布告板在此进行注册和删除观察者）

package headfirst.observer.weather;

import java.util.*;

public class WeatherData implements Subject {
	private ArrayList<Observer> observers;
	private float temperature;
	private float humidity;
	private float pressure;
	
	public WeatherData() {
		observers = new ArrayList<Observer>();
	}
	
	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;
	}
}

CurrentConditionsDisplay.java（布告板类）

package headfirst.observer.weather;
	
public class CurrentConditionsDisplay implements Observer, DisplayElement {
	private float temperature;
	private float humidity;
	public CurrentConditionsDisplay(Subject 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");
	}
}

package headfirst.observer.weather;


public class StatisticsDisplay implements Observer, DisplayElement {
	private float maxTemp = 0.0f;
	private float minTemp = 200;
	private float tempSum= 0.0f;
	private int numReadings;
	public StatisticsDisplay(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.java

package headfirst.observer.weather;

public class ForecastDisplay implements Observer, DisplayElement {
	private float currentPressure = 29.92f;  
	private float lastPressure;
	public ForecastDisplay(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");
		}
	}
}

WeatherStation.java

package headfirst.observer.weather;


public class WeatherStation {

	public static void main(String[] args) {
		WeatherData weatherData = http://www.mamicode.com/new WeatherData();>
输出：
Current conditions: 80.0F degrees and 65.0% humidity
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Improving weather on the way!
Current conditions: 82.0F degrees and 70.0% humidity
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Watch out for cooler, rainy weather
Current conditions: 78.0F degrees and 90.0% humidity
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same