Design Patterns - Command

design-patterns

Design Patterns - Behavioral - 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.

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.

// Step 1: Create a command interface.
public interface Order {
   void execute();
}

// Step 2: Create a request class.
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.
public class BuyStock implements Order {
   private Stock abcStock;

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

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

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.
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.
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();
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