I don't get it how it work - java help on CodeGym

We disabled undo and redo function from edit menu.

public class UndoMenuListener implements MenuListener {
    private View view;
    private JMenuItem undoMenuItem;
    private JMenuItem redoMenuItem;

    public UndoMenuListener(View view, JMenuItem undoMenuItem, JMenuItem redoMenuItem) {
        this.view = view;
        this.undoMenuItem = undoMenuItem;
        this.redoMenuItem = redoMenuItem;
    }

    @Override
    public void menuSelected(MenuEvent e) {
        undoMenuItem.setEnabled(view.canUndo());
        redoMenuItem.setEnabled(view.canRedo());
    }

But I don't get it why it work? In class UndoAction is only:

public class UndoAction extends AbstractAction {
    private View view;

    @Override
    public void actionPerformed(ActionEvent e) {

    }

    public UndoAction(View view) {
        this.view = view;
    }
}

In MenuHelper we initialize new Undo and RedoAction classes:

public static void initEditMenu(View view, JMenuBar menuBar) {
       JMenu editMenu = new JMenu("Edit");
       menuBar.add(editMenu);

       JMenuItem undoItem = addMenuItem(editMenu, "Undo", new UndoAction(view));
       JMenuItem redoItem = addMenuItem(editMenu, "Redo", new RedoAction(view));
       addMenuItem(editMenu, "Cut", new DefaultEditorKit.CutAction());
       addMenuItem(editMenu, "Copy", new DefaultEditorKit.CopyAction());
       addMenuItem(editMenu, "Paste", new DefaultEditorKit.PasteAction());

       editMenu.addMenuListener(new UndoMenuListener(view, undoItem, redoItem));
   }

I don't see connection between UndoMenuListener and UndoAction, RedoAction classes. Why it working when it can't :D How it possible, where is connection which causes this succesful action?

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Thomas

Level 41 , Bayreuth , Germany

8 July 2023, 12:07

in initEditMenu() you add eg. the 'Undo' Button. Here you use addMenuItem to add the Undo menuItem to the editMenu and the passed ActionListener to the undo menuItem object. When the menu item 'Undo' is selected (and not greyed out -> done in UndoMenuListener and that listener is added to the editMenu object), then the UndoActions actionPerformed method is executed and view.undo() called. A little bit confusing if you do not examine the helper methods.

matemate123

Level 50 , Kraków , Poland

9 July 2023, 07:55

OK thanks now I see where magic was occured :D

editMenu.addMenuListener(new UndoMenuListener(view, undoItem, redoItem));