Definition

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

UML class diagram

Participants

• Command
  • declares an interface for executing an operation
• ConcreteCommand
  • defines a binding between a Receiver object and an action
  • implements Execute by invoking the corresponding operation(s) on Receiver
• Client
  • creates a ConcreteCommand object and sets its receiver
• Invoker
  • asks the command to carry out the request
• Receiver
  • knows how to perform the operations associated with carrying out the request.

Sample code in Java

package com.hong.command;

abstract class Command
{
    protected Receiver receiver; 
    public Command(Receiver receiver)
    {
        this.receiver = receiver; 
    }
    public abstract void execute();
}

class ConcreteCommand extends Command
{
    public ConcreteCommand(Receiver recever) {
        super(recever);
    }
    @Override
    public void execute() {
        System.out.println("execute command from ConcreteCommand object.");
        if(receiver != null)
            receiver.action(); 
    }
}

class Receiver
{
    public void action()
    {
        System.out.println("Action from receiver class");
    }
}

class Invoker 
{
    private Command command; 
    public void setCommand(Command command)
    {
        this.command = command;
    }
    public void executeCommand()
    {
        System.out.println("execute command from Invoker"); 
        command.execute();
    }
    
}


public class Command_structure {
    
    public static void main(String[] args)
    {
    Command command1 = new ConcreteCommand(new Receiver());
    Command command2 = new ConcreteCommand(new Receiver());
    
 
    Invoker invoker = new Invoker();
    //Invoke command1.
    invoker.setCommand(command1);
    invoker.executeCommand();
    
    //Invoke command2.
    invoker.setCommand(command2); 
    invoker.executeCommand();
    
    }
    

}