观察者模式

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

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

public interface DisplayElement {
	public void display();
}

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;
	}
}

	
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");
	}
}

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);
	}
}

使用JAVA内置的观察者模式支持:

	
import java.util.Observable;
import java.util.Observer;

public class WeatherData extends Observable {
	private float temperature;
	private float humidity;
	private float pressure;

	public WeatherData() { }

	public void measurementsChanged() {
		setChanged();
		notifyObservers();
	}

	public void setMeasurements(float temperature, float humidity, float pressure) {
		this.temperature = temperature;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}

	public float getTemperature() {
		return temperature;
	}

	public float getHumidity() {
		return humidity;
	}

	public float getPressure() {
		return pressure;
	}
}

import java.util.Observable;
import java.util.Observer;

public class CurrentConditionsDisplay implements Observer, DisplayElement {
	Observable observable;
	private float temperature;
	private float humidity;

	public CurrentConditionsDisplay(Observable observable) {
		this.observable = observable;
		observable.addObserver(this);
	}

	public void update(Observable obs, Object arg) {
		if (obs instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)obs;
			this.temperature = weatherData.getTemperature();
			this.humidity = weatherData.getHumidity();
			display();
		}
	}

	public void display() {
		System.out.println("Current conditions: " + temperature 
			+ "F degrees and " + humidity + "% humidity");
	}
}

另外,其实在JDK中有很多观察者模式,最常用的是Swing设计中,如:
Button.addActionListener(new XXXListener());
实际上是添加一个观察者.