Command Pattern

We have created an interface Order which is acting as a command. We have created a Stockclass which acts as a request. We have concrete command classes BuyStock and SellStockimplementing Order interface which will do actual command processing. A class Broker is created which acts as an invoker object. It can take and place orders.

Broker object uses command pattern to identify which object will execute which command based on the type of command. CommandPatternDemo, our demo class, will use Brokerclass to demonstrate command pattern.

command_pattern_uml_diagram

Step 1

Create a command interface.

Order.java

public interface Order {
   void execute();
}

Step 2

Create a request class.

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");
   }
}

Step 3

Create concrete classes implementing the Order interface.

BuyStock.java

public class BuyStock implements Order {
   private Stock abcStock;

   public BuyStock(Stock abcStock){
      this.abcStock = abcStock;
   }

   public void execute() {
      abcStock.buy();
   }
}

SellStock.java

public class SellStock implements Order {
   private Stock abcStock;

   public SellStock(Stock abcStock){
      this.abcStock = abcStock;
   }

   public void execute() {
      abcStock.sell();
   }
}

Step 4

Create command invoker class.

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();
   }
}

Step 5

Use the Broker class to take and execute commands.

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();
   }
}

Step 6

Verify the output.

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