观察者模式（observer）-优快云博客

本文介绍两种观察者模式的实现方式，一种是通过自定义接口实现，另一种是利用Java内置的Observable类实现。这两种方式分别展示了如何让多个观察者对象同时监听某一个主题对象，并在主题对象状态发生变化时得到通知。

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