I have never heard of any menu system that will display a sub menu when ALT + A + B is pressed. The way I have seen menus work, ALT + A will bring up the A menu. Pressing a B will bring up the B Sub-menu.
Recently working with menus I found that Listeners are the way to implement most items associated with the clicking of menu items. Check them out.
As you can see I am new to Java Also and I hope I am helping here.