观察者模式实例股票通知

演示注册的投资者在股票市场发生变化时，可以自动得到通知。
该例子仍然使用的是传统的Observer处理手段。
UML:

首先是C#环境下的代码：

// Observer pattern -- Real World example  
using System;
using System.Collections;

// "Subject"
abstract class Stock
{
  // Fields
  protected string symbol;
  protected double price;
  private ArrayList investors = new ArrayList();

  // Constructor
  public Stock( string symbol, double price )
  {
    this.symbol = symbol;
    this.price = price;
  }

  // Methods
  public void Attach( Investor investor )
  {
    investors.Add( investor );
  }

  public void Detach( Investor investor )
  {
    investors.Remove( investor );
  }

  public void Notify()
  {
    foreach( Investor i in investors )
      i.Update( this );
  }

  // Properties
  public double Price
  {
    get{ return price; }
    set
    {
      price = value;
      Notify(); 
    }
  }

  public string Symbol
  {
    get{ return symbol; }
    set{ symbol = value; }
  }
}

// "ConcreteSubject"
class IBM : Stock
{
  // Constructor
  public IBM( string symbol, double price )
    : base( symbol, price ) {}
}

// "Observer"
interface IInvestor
{
  // Methods
  void Update( Stock stock );
}

// "ConcreteObserver"
class Investor : IInvestor
{
  // Fields
  private string name;
  private string observerState;
  private Stock stock;

  // Constructors
  public Investor( string name )
  {
    this.name = name;
  }

  // Methods
  public void Update( Stock stock )
  {
    Console.WriteLine( "Notified investor {0} of {1}'s change to {2:C}", 
      name, stock.Symbol, stock.Price );
  }

  // Properties
  public Stock Stock
  {
    get{ return stock; }
    set{ stock = value; }
  }
}

/// <summary>
/// ObserverApp test
/// </summary>
public class ObserverApp
{
  public static void Main( string[] args )
  {
    // Create investors
    Investor s = new Investor( "Sorros" );
    Investor b = new Investor( "Berkshire" );

    // Create IBM stock and attach investors
    IBM ibm = new IBM( "IBM", 120.00 );
    ibm.Attach( s );
    ibm.Attach( b );

    // Change price, which notifies investors
    ibm.Price = 120.10;
    ibm.Price = 121.00;
    ibm.Price = 120.50;
    ibm.Price = 120.75;
  }
}

Java环境下相同的代码：

//Stock
package 观察者模式;

import java.util.ArrayList;
import java.util.List;


public abstract class Stock {
protected String symbol;
protected double price;
private List<Observer> observers = new ArrayList<Observer>();
public Stock() {}
public Stock(String symbol, double price) {
    this.symbol = symbol;
    this.price = price;
}
public void Attach(Observer observer) {
    observers.add(observer);
}
public void Detch(Observer observer) {
    observers.remove(observer);
}
public void Notify() {
    for(Observer o : observers){
        o.update( this);;
    }
}
public String getSymbol() {
    return symbol;
}
public void setSymbol(String symbol) {
    this.symbol = symbol;
}
public double getPrice() {
    return price;
}
public void setPrice(double price) {
    this.price = price;
}
}

//Observer
public interface Observer {
 void update(Stock stock);
}


//IBM
public class IBM extends Stock {

    public IBM() {
        super();
    }

    public IBM(String symbol, double price) {
        super(symbol, price);
    }
    
}


//Oobserver
public class Oobserver implements Observer {
private String name;
private String observerState;
private Stock stock;

    
    public Oobserver(String name) {
    this.name = name;
}




    public Stock getStock() {
        return stock;
    }




    public void setStock(Stock stock) {
        this.stock = stock;
    }




    @Override
    public void update(Stock stock) {
        System.out.print("Motified inverstor"+name+"of"+stock.symbol+"'s change to"+stock.price);

    }

}


//ObserverApp
public class ObserverApp {

    public static void main(String[] args) {
        Oobserver s=new Oobserver("tom");
        Oobserver b=new Oobserver("jary");
        
        IBM ibm=new IBM("IBM",120.00);
        ibm.Attach(s);
        ibm.Attach(b);
        
        ibm.price=120.10;
        ibm.price=121.00;
        ibm.price=120.50;
        ibm.price=120.75;
    }

}

以股民为观察着的java代码：
股民类，包含对股票的走势做出反应的方法：

class Observer {//观察者类，股民观察股票走势，所以就是股民类
    private String name;
    public Observer(String name) {
        this.name = name;
    }
    public void update(Stock s){
        System.out.println(s.getStockState()+" "+this.name+"已对股票"+s.getName()+"的变化做出反应！！");
    }
}

股票类，每个股票可以被多个股民关注，股票变化后必须通知每个股民，所以股票和股民是一对多的聚合关系：

import java.util.ArrayList;
import java.util.List;

public class Stock {
    private List<Observer> observers = new ArrayList<Observer>();
    private String action;
    private String name;
    public Stock(String name) {
        super();
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void Attach(Observer observer) {
        observers.add(observer);
    }

    public void Detch(Observer observer) {
        observers.remove(observer);
    }

    public void Notify() {
        for(Observer o : observers){
            o.update(this);
        }
    }

    public String getStockState() {
        return this.action;
    }

    public void setStockState(String action) {
        this.action=action;
    }
}

客户端测试：

public class Client {

    public static void main(String[] args) {
        Stock s1 = new Stock("股票一");
        Stock s2 = new Stock("股票二");
        Observer o1 = new Observer("张三");
        Observer o2 = new Observer("李斯");
        Observer o3 = new Observer("王五");
        Observer o4 = new Observer("小花");
        s1.setStockState("股票一大跌！！");
        s1.Attach(o1);
        s1.Attach(o2);
        s1.Attach(o3);
        s1.Notify();
        s2.setStockState("股票二连涨50%！！！");
        s2.Attach(o4);
        s2.Notify();
    }

}

输出：

股票一大跌！！ 张三已对股票股票一的变化做出反应！！
股票一大跌！！ 李斯已对股票股票一的变化做出反应！！
股票一大跌！！ 王五已对股票股票一的变化做出反应！！
股票二连涨50%！！！ 小花已对股票股票二的变化做出反应！！