CSC/ECE 517 Fall 2012/ch2b 2w40 sn

Introduction to Command Pattern

The command pattern is one of the most used behavioral design patterns. The main concept of the pattern is an object which can be used to represent and encapsulate all the information needed to call a method at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

The command pattern has been often associated with these terms client, invoker and receiver. The client instantiates the command object and provides the information required to call the method at a later time. The invoker decides when the method should be called. The receiver is an instance of the class that contains the method's code.

The intent of the Command pattern can be listed as:

1. encapsulate a request in an object
2. allows the parametrization of clients with different requests
3. allows saving the requests in a queue

A Java Example

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

//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();
    }
    
}

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

//Invoker
public class RemoteControl
{
    private Command command;

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

//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();
    
    }

}