By using the command pattern you are seperating the operation from the invoking object. And just because of that it becomes easier to change the command without chagning the caller/s.
This means that you could use Command pattern when you might have the following situation

You want to parameterize objects to perform an action
You want to specify, execute and queue requests at different times.

Just to quickly start you need a command object, An interface will keep it easy going in this case, thus providing you with the option of extending other classes e.g. Swing MenuItem or Button.
Below the execute Method is the one invoked to do something when this command is called or asked to do its stuff.
Where as the getCommandName is assumed as a unique name how ever I am sure we can always come up with a better implementation for uniqueness.

  1. public interface Command {
  3.     public void execute();
  4.     public String getCommandName();
  6. }

And example implementation of the Command should look as follows
A Command Name, and and execute Method to tell what happens when this command is called.

  1. public class ForwardCmd implements Command {
  3.    private String COMMAND_NAME = "Back";
  5.    public BackCmd() {
  6.        super();
  7.    }
  9.    public String getCommandName() {
  10.        return COMMAND_NAME;
  11.     }
  13.     public void execute() {
  14.         System.out.println("Your wish, my command");
  15.     }
  16. }

The command manager is the controller in this case. It registers command objects. the “registerCommand” will simply take a command and store it in a list or something alike. This means you could load it out of a jar file, or an xml or path and just pass the object to the “registerCommand” AS a command offcourse.

the “execute” Command will simply execute the Command passed to it.

And the “getCommand” returns a command by looking up a COMMAND_NAME. So if you provide a name to it through you system it should give you an object of type Command and simple pass it to execute. Again this would be a controller logic and not the client one.

  1. public abstract class AbstractCommandManager {
  3.     public abstract void registerCommand(Command command);
  4.     public abstract Collection getAllCommands();
  5.     public abstract void execute(Command command);
  6.     public abstract Command getCommand(String name);
  7. }

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