public static ArrayList<Categorias> sqlSelectTodos(){
        ArrayList<Categorias> cat = new ArrayList();
        if(!BD.getInstance().sqlSelect("select * from categorias ")){
            return cat;
        }        
        while(BD.getInstance().sqlFetch()){
            cat.add(new Categorias(BD.getInstance().getInt("idcategorias")
                                        ,BD.getInstance().getString("nombre") ));
        }
        return cat;
    }