Action a1 = new Action("XXX") {};
Action a2 = new Action("YYY") {};
Action a3 = new Action("ZZZ") {};

final MenuManager mgr = new MenuManager();


mgr.add(a1);
mgr.add(a2);
mgr.add(a3);

mgr.addMenuListener(new IMenuListener() {
        public void menuAboutToShow(IMenuManager manager) {
            IStructuredSelection selection = (IStructuredSelection) tree
                    .getSelection();
            if (!selection.isEmpty()) {
                String str = ((MyModel) selection.getFirstElement())
                        .toString();
                if (str.equals("Item1")) {
                    for (int i = 0; i < 3; i++) {
                        manager.getItems()[i].setVisible(true);
                    }
                } else {
                    for (int i = 0; i < 3; i++) {
                        manager.getItems()[i].setVisible(false);
                    }
                }
            }
        }
    });