new ApplicationMenuItemDefinition("Playground", 0) {

                               @Override
                               protected MenuItem internalCreateItem(ApplicationMenu menu) {
                                   return menu.addItem(getName(), item -> LinkkiUi.getCurrentNavigator()
                                           .navigateTo(PlaygroundView.NAME));
                               }

                           }

                           new ApplicationMenuItemDefinition("Playground", PlaygroundApplicationView.class)

public class CustomMenuItemDefinition extends ApplicationMenuItemDefinition {

    private List<MySubSubMenuItem> subSubMenuItems;

    public CustomMenuItemDefinition(String name, int position, List<MySubSubMenuItem> subSubMenuItems) {
        super(name, position);
        this.subSubMenuItems = subSubMenuItems;
    }

    private List<MySubSubMenuItem> getSubSubMenuItems() {
        return subSubMenuItems;
    }

    @Override
    protected MenuItem internalCreateItem(ApplicationMenu menu) {
        MenuItem newApplicationMenuItem = menu.addItem(getName(), null, null);

        MenuItem newMenuItem = newApplicationMenuItem.addItem("New", null);

        if (getSubSubMenuItems().size() > 1) {
            getSubSubMenuItems().forEach(i -> newMenuItem.addItem(i.getName(), i.getCommand()));
        } else if (getSubSubMenuItems().size() == 1) {
            MySubSubMenuItem onlyItem = getSubSubMenuItems().get(0);
            newMenuItem.addItem(onlyItem.getName(), onlyItem.getCommand());
        }
        return newApplicationMenuItem;
    }

    public static class MySubSubMenuItem {
        private String name;

        public MySubSubMenuItem(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public Command getCommand() {
            return i -> Notification.show(name);
        }
    }
}

public class CustomMenuItemDefinitionCreator {

    public static ApplicationMenuItemDefinition createMenuItem(String name, List<MySubSubMenuItem> subSubMenuItems) {
        return new ApplicationMenuItemDefinition(name, Collections.singletonList(createSubMenuItem(subSubMenuItems)));
    }

    private static ApplicationMenuItemDefinition createSubMenuItem(List<MySubSubMenuItem> subSubMenuItems) {
        String subMenuName = "New";
        if (subSubMenuItems.size() > 1) {
            return new ApplicationMenuItemDefinition(subMenuName,
                    subSubMenuItems.stream()
                            .map(i -> new ApplicationMenuItemDefinition(i.getName(), () -> i.getCommand().apply()))
                            .collect(Collectors.toList()));
        } else if (subSubMenuItems.size() == 1) {
            MySubSubMenuItem onlyItem = subSubMenuItems.get(0);
            return new ApplicationMenuItemDefinition(onlyItem.getName(), onlyItem.getCommand());
        } else {
            return new ApplicationMenuItemDefinition(subMenuName, Handler.NOP_HANDLER);
        }
    }

    public static class MySubSubMenuItem {
        private String name;

        public MySubSubMenuItem(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public Handler getCommand() {
            return () -> Notification.show(name);
        }
    }
}