How to access global variable $menu inside a class function

You need to hook into some action that happens after the global $menu has been set.

as an example:

class adminMenus {
   public function getWPdefaultmenus(){
      global $menu;
      return $menu;
   }
 add_action( 'admin_menu', array( $this, 'getWPdefaultmenus' ) );
}