设计模式——观察者

观察者模式定义了对象之间的一对多依赖，当一个对象改变状态时，他的所有的依赖者都会收到通知并自动更新；

定义主题，主题的功能是持有观察者的引用，提供注册、取消注册和通知，并且有状态；观察者实现接口中的update方法，必须注册具体主题；

 

JDK自带了对观察者的支持功能

被观察者Observable对象，提供add/delete/notifyObserver（）和setChanged（）设置状态方法

观察者Observer接口，提供update（）方法进行相关的更新操作。

 

观察者类图：

 

 

以下程序模拟股票的状态更新功能（使用jdk对观察者支持，参考网上实现）

 

1.定义Observable 接口实现类，该类是设置股票的信息，并且在数据更新后会通知所有的观察者进行相关状态的更新。

 

package com.observer;

import java.util.Observable;

public class StockData extends Observable {
	private String symbol;
	private float close;
	private float high;
	private float low;
	private long volume;

	public StockData() {
	}

	public String getSymbol() {
		return symbol;
	}

	public float getClose() {
		return close;
	}

	public float getHigh() {
		return high;
	}

	public float getLow() {
		return low;
	}

	public long getVolume() {
		return volume;
	}

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

	public void setStockData(String symbol, float close, float high, float low,
			long volume) {
		this.symbol = symbol;
		this.close = close;
		this.high = high;
		this.low = low;
		this.volume = volume;
		sendStockData();
	}
}

　

2.实现观察者Observer实现类，模拟股票的大买家

package com.observer;

import java.text.DecimalFormat;
import java.text.DecimalFormatSymbols;
import java.util.Observable;
import java.util.Observer;

public class BigBuyer implements Observer {
	private String symbol;
	private float close;
	private float high;
	private float low;
	private long volume;

	public BigBuyer(Observable observable) {
		observable.addObserver(this); // 注册关系
	}

	public void update(Observable observable, Object args) {
		if (observable instanceof StockData) {
			StockData stockData = (StockData) observable;
			this.symbol = stockData.getSymbol();
			this.close = stockData.getClose();
			this.high = stockData.getHigh();
			this.low = stockData.getLow();
			this.volume = stockData.getVolume();
			display();
		}
	}

	public void display() {
		DecimalFormatSymbols dfs = new DecimalFormatSymbols();
		DecimalFormat volumeFormat = new DecimalFormat("###,###,###,###", dfs);
		DecimalFormat priceFormat = new DecimalFormat("###.00", dfs);
		System.out.println("Big Buyer reports... ");
		System.out.println("	The lastest stock quote for " + symbol + " is:");
		System.out.println("	$" + priceFormat.format(close)
				+ " per share (close).");
		System.out.println("	$" + priceFormat.format(high)
				+ " per share (high).");
		System.out.println("	$" + priceFormat.format(low)
				+ " per share (low).");
		System.out.println("	" + volumeFormat.format(volume)
				+ " shares traded.");
		System.out.println();
	}
}

3.实现观察者Observer实现类，作为第二个观察者

package com.observer;

import java.text.DecimalFormat;
import java.text.DecimalFormatSymbols;
import java.util.Observable;
import java.util.Observer;

public class TradingFool implements Observer
{
private String symbol;
private float close;

public TradingFool(Observable observable)
	{
	observable.addObserver(this);//注册关系
	}

public void update(Observable observable,Object args)
	{
	if(observable instanceof StockData)
		{
		StockData stockData = (StockData)observable;
		this.symbol = stockData.getSymbol();
		this.close = stockData.getClose();
		display();
		}
	}

public void display()
	{
	DecimalFormatSymbols dfs = new DecimalFormatSymbols();
	DecimalFormat priceFormat = new DecimalFormat("###.00",dfs);
	System.out.println("Trading Fool says... ");
	System.out.println("	" + symbol + " is currently trading at $" + priceFormat.format(close) + " per share.");
	System.out.println();
	}
}

　　

4.测试

package com.observer;
public class StockQuotes
{
public static void main(String[] args)
	{
	System.out.println();
	System.out.println("-- Stock Quote Application --");
	System.out.println();

	StockData stockData = new StockData();

	new TradingFool(stockData);
	new BigBuyer(stockData);

	stockData.setStockData("JUPM",16.10f,16.15f,15.34f,(long)481172);
	stockData.setStockData("SUNW",4.84f,4.90f,4.79f,(long)68870233);
	stockData.setStockData("MSFT",23.17f,23.37f,23.05f,(long)75091400);
	}
}