Command, Singleton, JMenuItem, JButton, AbstractButton – One Listener for the app

Here I would like to demonstrate a simple use of JMenuItems being used with Single Listener for the entire system.
A simple sample of use would probably be SingleInstance Desktop Application.

Lets see how that is done here.

1. First lets create a OneListener class that should be able to listen to ActionEvents and also be able to add Commands to itself. Please refer to my previous post on Command,Singleton if you would like to see more about this patterns and there usage.

  1. package com.shaafshah.jmenus;
  3. import java.awt.event.ActionEvent;
  4. import java.awt.event.ActionListener;
  5. import java.util.ArrayList;
  7. import javax.swing.AbstractButton;
  9. // Implements the ActionListener and is a Singleton also.
  11. public class OneListener implements ActionListener{
  13. 	private static OneListener oneListener = null;
  15. 	// Holds the list of all commands registered to this listener
  16. 	private ArrayList<Command> commandList = null;
  18. 	// A private constructor
  19. 	private OneListener(){
  20. 		commandList = new ArrayList<Command>();
  21. 	}
  23. 	// Ensuring one instance.
  24. 	public static OneListener getInstance(){
  25. 		if(oneListener != null)	
  26. 			return oneListener;
  27. 		else return oneListener = new OneListener();
  28. 	}
  30. 	// Add the command and add myself as the listener
  31. 	public void addCommand(Command command){
  32. 			commandList.add(command);
  33. 		    ((AbstractButton)command).addActionListener(this);
  34. 	}
  37. 	// All Events hit here.
  38. 	@Override
  39. 	public void actionPerformed(ActionEvent e) {
  40. 		((Command)e.getSource()).execute();
  41. 	}
  43. }

In the above code, the addCommand method adds the command Object and adds a listener to it.
Now how is that possible.
Basically because I am categorizing my UI objects as Commands with in the system having some UI. And I am also assuming that these commands are Currently AbstractButton i.e. JMenuItem, JButton. Lets have a look at the Command Interface and its Implementation.

  1. public interface Command {
  2. 	public void execute();	
  3. }

And the implementation, note that the Command is an interface where as the class is still extending a UI object.

  1. import javax.swing.JMenuItem;
  3. public class TestCmd extends JMenuItem implements Command{
  5. 	public TestCmd() {
  6. 		super("Test");
  7. 		OneListener.getInstance().addCommand(this);
  8. 	}
  10. 	@Override
  11. 	public void execute() {
  12. 		System.out.println("HelloWorld");
  13. 	}
  14. }

Personally don’t like calling the OneListener in the constructor of the class but just for the sake of simplicity of this post I have left it that way. There are many different ways to omit it.

So the TestCmd is a JMenuItem but is also a Command and thats what the OneListener understands.
As this commads Listener is also OneListener all ActionEvents are thrown there and from there only the Command.execute is called on.

So now you dont have to worry about what listeners are you in. The only thing you know is that when execute is called you need to do your stuff.

You can download the code from Here.

Hope this helps.


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. }