Creating an "Undo" method

Question

Our prof. gave us a photo editing program skeleton, mostly GUI, written in java which we had to implement a couple of features. That was a couple of days ago. I've been toying around with it trying to add some other things.
One of the things I wanted to add was an undo command, such that when you click the Undo button, the image reverts back to whatever it was previously.
When a button is clicked this code executes:
protected void applyCommand ()
             Pixmap p = myCanvas.getPixmap();//PixmapCanvas myCanvas, holds the canvas
             if(!(myCommand.getName().equals("Undo")))//my additions
                  myCanvas.store(p);
                  System.out.println("Creating Backup");
             }//end of my additions
             myCommand.execute(p);
                myCanvas.refresh();
        }Here is the store method:
public void store(Pixmap p)
         oneBack = new Pixmap(p);
    }Here is where I'm stuck, in the undo command, I'm not quite sure how to access oneBack in PixmapCanvas. I'm sure there are ways to do it but I'm trying to keep this as elegant as possible, I don't want to change the structure of all of the other commands.
public void execute(Pixmap target)
          target = new Pixmap("I need to get oneBack" );
}

Answer 1

Ok well here's something else I tried and it still doesn't work. I'll try and be more descriptive of my code.
Main:
// create GUI components
        PixmapCanvas canvas = new PixmapCanvas(frame);
        ButtonPanel commands = new ButtonPanel(canvas);
        // more commands
        commands.add(new Undo(canvas));ButtonPanel:
public class ButtonPanel extends JPanel
    private PixmapCanvas myView;
    public ButtonPanel (PixmapCanvas view)
        myView = view;
    public void add (Command c)
        add(new CommandButton(c, myView));
    class CommandButton extends JButton
        private PixmapCanvas myCanvas;
        private Command myCommand;
        public CommandButton (Command command, PixmapCanvas canvas)
            super(command.getName());
            myCanvas = canvas;
            myCommand = command;
            // perform command when button is clicked
            addActionListener(new ActionListener()
                    public void actionPerformed (ActionEvent e)
                        applyCommand();
        protected void applyCommand ()
            Pixmap p = myCanvas.getPixmap();
            if(!(myCommand.getName().equals("Undo")))
                 myCanvas.store(p);//So every command should store the current pixmap
            myCommand.execute(p);
            myCanvas.refresh();
}PixmapCanvas (important parts of it):
public class PixmapCanvas extends JPanel
    private Pixmap myPixmap;
    public Pixmap oneBack;
    public PixmapCanvas (JFrame container, String pixName)
        setBorder(BorderFactory.createLoweredBevelBorder());
        myContainer = container;
        myPixmap = new Pixmap(pixName);
        refresh();
    public void store(Pixmap p)
         oneBack = new Pixmap(p);
    public void refresh ()
        if (! myPixmap.getSize().equals(getSize()))
            setSize(myPixmap.getSize());
            myContainer.setTitle(myPixmap.getName());
            myContainer.pack();
        repaint();
    public void paintComponent (Graphics pen)
        super.paintComponent(pen);
        myPixmap.paint(pen);
}Undo:
public class Undo extends Command
     PixmapCanvas ref;
     public Undo(PixmapCanvas c)
          super("Undo");
          ref = c;
     public void execute(Pixmap target)
          target = new Pixmap(ref.oneBack);
}Edited by: JFactor2004 on Sep 20, 2007 7:26 PM