Design Patterns Uncovered: The Command Pattern

Today's pattern is the Command, which allows the requester of a particular action to be decoupled from the object that performs the action. Where the Chain of Responsibility pattern forwarded requests along a chain, the Command pattern forwards the request to a specific module.

Command in the Real World 

One example of the command pattern being executed in the real world is the idea of a table order at a restaurant: the waiter takes the order, which is a command from the customer.This order is then queued for the kitchen staff.  The waiter tells the chef that the a new order has come in, and the chef has enough information to cook the meal.

 

Design Patterns Refcard
For a great overview of the most popular design patterns, DZone's Design Patterns Refcard is the best place to start. 

The Command Pattern

The Command pattern is known as a behavioural pattern, as it's used to manage algorithms, relationships and responsibilities between objects. The definition of Command provided in the original Gang of Four book on Design Patterns states: 

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations

So what does this mean in a class diagram? 

Command declares an interface for all commands, providing a simple execute() method which asks the Receiver of the command to carry out an operation. The Receiver has the knowledge of what to do to carry out the request.  The Invoker holds a command and can get the Command to execute a request by calling the execute method. The Client creates ConcreteCommands and sets a Receiver for the command. The ConcreteCommand defines a binding between the action and the receiver. When the Invoker calls execute the ConcreteCommand will run one or more actions on the Receiver.

The following sequence diagram shows the relationship in a clearer way: 

If this all seems a bit confusing right now, hang on for the code example later on in the article.

When Would I Use This Pattern?

The Command Pattern is useful when:

• A history of requests is needed
• You need callback functionality
• Requests need to be handled at variant times or in variant orders
• The invoker should be decoupled from the object handling the invocation.

You'll see command being used a lot when you need to have multiple undo operations, where a stack of the recently executed commands are maintained. To implement the undo, all you need to do is get the last Command in the stack and execute it's undo() method.

You'll also find Command useful for wizards, progress bars, GUI buttons and menu actions, and other transactional behaviour. 

So How Does It Work In Java?

Let's use a remote control as the example. Our remote is the center of home automation and can control everything. We'll just use a light as an example, that we can switch on or off, but we could add many more commands.

First we'll create our command interface:

//Command
public interface Command
{
    public void execute();
}

Now let's create two concrete commands. One will turn on the lights, another turns off lights:

//Concrete Command
public class LightOnCommand implementsCommand
{
    //reference to the light
    Light light;
    
    public LightOnCommand(Light light)
    {
        this.light = light;
    }
    
    public void execute()
    {
        light.switchOn();
    }
    
}
 

 //Concrete Command
public class LightOffCommand implementsCommand
{
    //reference to the light
    Light light;
    
    public LightOffCommand(Light light)
    {
        this.light = light;
    }
    
    public void execute()
    {
        light.switchOff();
    }
    
}

Light is our receiver class, so let's set that up now:

//Receiver
public class Light
{
   private boolean on;
  
   public void switchOn()
   {
      on = true;
   }
  
   public void switchOff()
   {
      on = false;
   }
  
}

Our invoker in this case is the remote control.

//Invoker
public class RemoteControl
{
    private Command command;

    public void setCommand(Command command)
    {
        this.command = command;
    }
    
   
    public void pressButton()
    {
        command.execute();
    }
    
}

Finally we'll set up a client to use the invoker

//Client
public class Client
{
    public static void main(String[] args)
    {
        RemoteControl control = new RemoteControl();
        
        Light light = new Light();
        
        Command lightsOn = new LightsOnCommand(light);
        Command lightsOff = new LightsOffCommand(light);
        
        //switch on
        control.setCommand(lightsOn);
        control.pressButton();
        
        //switch off
        control.setCommand(lightsOff);
        control.pressButton();
    
    }

}

Watch Out for the Downsides

This pattern ends up forcing a lot of Command classes that will make your design look cluttered - more operations being made possible leads to more command classes. Intelligence required of which Command to use and when leads to possible maintainence issues for the central controller.

Other Articles in This Series
The Observer Pattern
The Adapter Pattern
The Facade Pattern
The Factory Method Pattern
The Abstract Factory Pattern
The Singleton Pattern
The Strategy Pattern The Visitor Pattern
The Decorator Pattern
The Proxy Pattern

Next Up

We're going to look at the Template pattern early next week.