Question in design

Question

Hi,
I am writing SMTP server in java as final project in programming course, the server uses the state and command patterns.
in each state the server can only process specific SMTP commands which are encapsulated in a Command object.
(Not a code,rather a pseudocode):
public interface ICommand
     public ISMTPSession getSession();
     public String Execute();
class SMTPCommand implements ICommand
some common stuff for all commands
class HELOCommand extends SMTPCommand
class InitialState extends SMTPState
     InitialState()
     //Can handle HELO only
     String ExecuteCommand(ICommand command)
          //Must be HELO only
          if(command is HELOCommand){
               return "error";
          return command.Execute();
The state must know what kind of command it deal with so I am querying the interface for the concrete class type.the problem is that other user can implement their commands, for example
Class HELOCommandEx implements ICommand
This will cause the following line to always fail
if(command is of HELOCommand){
               return "error";
The simplest solution is to add a function to the ICommand interface which
Will return the SMTP command name
public interface ICommand
     Public String getDescription();//return the SMTP command name
     public ISMTPSession getSession();
     public String Execute();
Is there a better way to do it in java?
Thanks in advance

Answer 1

You could replace ICommand by an abstract class with package-private constructor, so users can't override it. If you make your basic Commands (like HELOCommand) accessible via public classes or interfaces, all users implementations will inherit from these basic types, not only from ICommand - and your type checking will work.