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:

观察者模式实例股票通知_java

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！！