观察者模式（observer）

设计模式中观察者模式的实现有很多方式，这里列出两种实现方式
1、主题实现Subject接口，观察者实现Observer、DisplayElement接口，其中，Subject、Observer、DisplayElement接口都是用户自定义的接口。

package com.interfaces;

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

package com.interfaces;

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

}

package com.interfaces;

public interface DisplayElement {
	
	public void display();

}

package com.theme;

import java.util.ArrayList;

import com.interfaces.Observer;
import com.interfaces.Subject;

@SuppressWarnings("unchecked")
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 notifyObservers() {
		for(int i = 0; i < observers.size();i++){
			Observer observer = (Observer)observers.get(i);
			observer.update(temperature, humidity, pressure);
		}
	}

	@Override
	public void registerObserver(Observer o) {
		observers.add(o);
	}

	@Override
	public void removeObserver(Observer o) {

		int i = observers.indexOf(o);
		if(i>=0){
			observers.remove(i);
		}
	}

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

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

package com.observers;

import com.interfaces.DisplayElement;
import com.interfaces.Observer;
import com.interfaces.Subject;

public class CurrentConditionsDisplay implements Observer, DisplayElement {

	private float temperature;

	private float humidity;

	private float pressure;

	private Subject weatherData;
	
	public CurrentConditionsDisplay(Subject weatherData){
		this.weatherData = weatherData;
		weatherData.registerObserver(this);
	}

	@Override
	public void update(float temp, float humidity, float pressure) {
		this.temperature = temp;
		this.humidity = humidity;
		this.pressure = pressure;
		display();
	}

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

package com.observers;

import com.interfaces.DisplayElement;
import com.interfaces.Observer;
import com.theme.WeatherData;

public class ForecastDisplay implements DisplayElement, Observer {
	private float currentPressure = 29.92f;  
	private float lastPressure;
	private WeatherData weatherData;

	public ForecastDisplay(WeatherData weatherData) {
		this.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");
		}
	}

}

package com.observers;

import com.interfaces.DisplayElement;
import com.interfaces.Observer;
import com.theme.WeatherData;

public class StatisticsDisplay implements Observer, DisplayElement {

	private float maxTemp = 0.0f;
	private float minTemp = 200;
	private float tempSum= 0.0f;
	private int numReadings;
	private WeatherData weatherData;

	public StatisticsDisplay(WeatherData weatherData) {
		this.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);
	}

}

package com.test;

import com.observers.CurrentConditionsDisplay;
import com.observers.ForecastDisplay;
import com.observers.StatisticsDisplay;
import com.theme.WeatherData;

public class WeatherStation {
	
	public static void main(String[] args) {
		
		WeatherData weatherData = new WeatherData();
		
		new CurrentConditionsDisplay(weatherData);
		new StatisticsDisplay(weatherData);
		new ForecastDisplay(weatherData);

		weatherData.setMeasurements(80, 65, 30.4f);
		weatherData.setMeasurements(82, 70, 29.2f);
		weatherData.setMeasurements(78, 90, 29.2f);
	}

}

测试结果为：

Current conditions : 80.0F degrees and 65.0% humidity and pressure is :30.4
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 and pressure is :29.2
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 and pressure is :29.2
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same

运行方式：每当主题的状态有所变化时，就会通知每一个观察者。观察者可以动态的来注册主题或者移除对主题的关注，主题根本不知道有哪些观察者。这个实现方式的优点是：主题会每一次状态的改变都会通知所有的观察者，这也会带来缺点，因为并不是主题每一次的状态改变的信息都是每个观察者所需要的，主题是将每次的改变推给观察者们的，而不是观察者们去拿的。

2、主题继承Observale类，Observable类封装了一些方法，观察者实现了Observer、DisplayElement接口

package com.interfaces;

public interface DisplayElement {
	public void display();
}

package com.theme;

import java.util.Observable;

public class WeatherData extends Observable {

	private float temperature;

	private float humidity;

	private float pressure;

	public WeatherData(){

	}

	public void measurementsChanged(){
		this.setChanged();
		this.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;
	}
}

package com.observers;

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

import com.interfaces.DisplayElement;
import com.theme.WeatherData;

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

	public CurrentConditionsDisplay(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");
	}
}

package com.observers;

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

import com.interfaces.DisplayElement;
import com.theme.WeatherData;

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

	public ForecastDisplay(Observable observable) {
		observable.addObserver(this);
	}

	public void update(Observable observable, Object arg) {
		if (observable instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)observable;
			lastPressure = currentPressure;
			currentPressure = weatherData.getPressure();
			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");
		}
	}
}

package com.observers;

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

import com.interfaces.DisplayElement;
import com.theme.WeatherData;

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(Observable observable) {
		observable.addObserver(this);
	}

	public void update(Observable observable, Object arg) {
		if (observable instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)observable;
			float temp = weatherData.getTemperature();
			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);
	}
}

package com.test;

import com.observers.CurrentConditionsDisplay;
import com.observers.ForecastDisplay;
import com.observers.StatisticsDisplay;
import com.theme.WeatherData;

public class WeatherStation {
	public static void main(String[] args) {
		WeatherData weatherData = new WeatherData();
		new CurrentConditionsDisplay(weatherData);
		new StatisticsDisplay(weatherData);
		new ForecastDisplay(weatherData);

		weatherData.setMeasurements(80, 65, 30.4f);
		weatherData.setMeasurements(82, 70, 29.2f);
		weatherData.setMeasurements(78, 90, 29.2f);
	}

}

运行结果为：

Forecast: Improving weather on the way!
Avg/Max/Min temperature = 80.0/80.0/80.0
Current conditions: 80.0F degrees and 65.0% humidity
Forecast: Watch out for cooler, rainy weather
Avg/Max/Min temperature = 81.0/82.0/80.0
Current conditions: 82.0F degrees and 70.0% humidity
Forecast: More of the same
Avg/Max/Min temperature = 80.0/82.0/78.0
Current conditions: 78.0F degrees and 90.0% humidity

运行方式和第一种实现方式差不多，只不过Observable类帮主题类实现了很多方法，主题类直接用这些方法即可。优缺点：由于这里的主题是继承Observable类，而不是实现接口，这就缺少很多弹性，这限制了主题类的使用和复用