学步园

    命令模式是一个数据驱动设计模式，它归属于行为型模式的范畴。一个请求作为命令的形式被封装成一个对象，并且传递给调用对象。调用对象查找能够处理这个命令的恰当对象，传递命令给相应的对象，那个对象就执行这个命令。

    在这个实例中，我们创建了一个充当一个命令的接口Order，创建一个股票类Stock充当一个请求。具体的命令类BuyStock和SellStock实现Order接口，它们将做实际上的命令处理。经纪人类Broker充当一个调用对象，它负责收集和发送命令。UML图如下所示：

接口order.java

public interface Order {
   void execute();
}

股票类Stock.java

public class Stock {
          
   private String name = "ABC";
   private int quantity = 10;
 
   public void buy(){
      System.out.println("Stock [ Name: "+name+", 
         Quantity: " + quantity +" ] bought");
   }
   public void sell(){
      System.out.println("Stock [ Name: "+name+", 
         Quantity: " + quantity +" ] sold");
   }
}

实现订单接口的实现类，购买股票BuyStock.java

public class BuyStock implements Order {
   private Stock abcStock;
 
   public BuyStock(Stock abcStock){
      this.abcStock = abcStock;
   }
 
   public void execute() {
      abcStock.buy();
   }

卖出股票类SellStock.java

public class SellStock implements Order {
   private Stock abcStock;
 
   public SellStock(Stock abcStock){
      this.abcStock = abcStock;
   }
 
   public void execute() {
      abcStock.sell();
   }
}
调用命令类Broker.java
import java.util.ArrayList;
import java.util.List;
 
   public class Broker {
   private List<Order> orderList = new ArrayList<Order>(); 
 
   public void takeOrder(Order order){
      orderList.add(order);              
   }
 
   public void placeOrders(){
      for (Order order : orderList) {
         order.execute();
      }
      orderList.clear();
   }
}
测试类CommandPatternDemo.java
public class CommandPatternDemo {
   public static void main(String[] args) {
      Stock abcStock = new Stock();
 
      BuyStock buyStockOrder = new BuyStock(abcStock);
      SellStock sellStockOrder = new SellStock(abcStock);
 
      Broker broker = new Broker();
      broker.takeOrder(buyStockOrder);
      broker.takeOrder(sellStockOrder);
 
      broker.placeOrders();
   }
}

输出结果：

Stock [ Name: ABC, Quantity: 10 ] bought
Stock [ Name: ABC, Quantity: 10 ] sold

你会发现，这里面包含了组合模式。