观察者模式

package com.weather;

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

package com.weather;

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

package com.weather;

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 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();
	}
	// WeatherData的其他方法
}

package com.weather;

public interface DisplayElement {

	public void display();
}

package com.weather;

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");
	}
}