Design Patterns - Command Pattern

Command pattern is a data driven design pattern and falls under behavioral pattern category. A request is wrapped under an object as command and passed to invoker object. Invoker object looks for the appropriate object which can handle this command and passes the command to the corresponding object which executes the command.

Implementation

We have created an interface Order which is acting as a command. We have created a Stock class which acts as a request. We have concrete command classes BuyStock and SellStock implementing 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 Broker class to demonstrate command pattern.

Step 1

Create a command interface.

Order.java

pubpc interface Order {
   void execute();
}

Step 2

Create a request class.

Stock.java

pubpc class Stock {
	
   private String name = "ABC";
   private int quantity = 10;

   pubpc void buy(){
      System.out.println("Stock [ Name: "+name+", 
         Quantity: " + quantity +" ] bought");
   }
   pubpc void sell(){
      System.out.println("Stock [ Name: "+name+", 
         Quantity: " + quantity +" ] sold");
   }
}

Step 3

Create concrete classes implementing the Order interface.

BuyStock.java

pubpc class BuyStock implements Order {
   private Stock abcStock;

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

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

SellStock.java

pubpc class SellStock implements Order {
   private Stock abcStock;

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

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

Step 4

Create command invoker class.

Broker.java

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

   pubpc class Broker {
   private List<Order> orderList = new ArrayList<Order>(); 

   pubpc void takeOrder(Order order){
      orderList.add(order);		
   }

   pubpc void placeOrders(){
   
      for (Order order : orderList) {
         order.execute();
      }
      orderList.clear();
   }
}

Step 5

Use the Broker class to take and execute commands.

CommandPatternDemo.java

pubpc class CommandPatternDemo {
   pubpc 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